# The wide-field, multiplexed, spectroscopic facility WEAVE: Survey design,
overview, and simulated implementation

Shoko Jin1,2,3,4⋆, Scott C. Trager2, Gavin B. Dalton1,3, J. Alfonso L.
Aguerri5,6, J. E. Drew7,8, Jesús Falcón-Barroso5,6, Boris T. Gänsicke9,
Vanessa Hill10, Angela Iovino11, Matthew M. Pieri12, Bianca M. Poggianti13, D.
J. B. Smith7, Antonella Vallenari13, Don Carlos Abrams14, David S.
Aguado15,16,17, Teresa Antoja18,19,20, Alfonso Aragón-Salamanca21, Yago
Ascasibar22, Carine Babusiaux23,24, Marc Balcells14,5,6, R. Barrena5,6,
Giuseppina Battaglia5,6, Vasily Belokurov17, Thomas Bensby25, Piercarlo
Bonifacio24, Angela Bragaglia26, Esperanza Carrasco27, Ricardo Carrera13,
Daniel J. Cornwell21, Lilian Domínguez-Palmero14,5,6, Kenneth J. Duncan28,
Benoit Famaey29, Cecilia Fariña14,5,6, Oscar A. Gonzalez30, Steve Guest3, Nina
A. Hatch21, Kelley M. Hess31,2,32, Matthew J. Hoskin9, Mike Irwin17, Johan H.
Knapen5,6,33, Sergey E. Koposov28,34,17, Ulrike Kuchner21, Clotilde Laigle35,
Jim Lewis17,†, Marcella Longhetti11, Sara Lucatello13,36,‡, Jairo Méndez-
Abreu5,6, Amata Mercurio88,37, Alireza Molaeinezhad1,5,6,17, Maria
Monguió7,18,19,20, Sean Morrison38,12, David N. A. Murphy17, Luis Peralta de
Arriba17, Isabel Pérez39,40, Ignasi Pérez-Ràfols41,12, Sergio Picó14, Roberto
Raddi42, Mercè Romero-Gómez18,19,20, Frédéric Royer24, Arnaud Siebert29,
George M. Seabroke43, Debopam Som44, David Terrett3, Guillaume Thomas5,6,
Roger Wesson8, C. Clare Worley17, Emilio J. Alfaro31, Carlos Allende
Prieto5,6, Javier Alonso-Santiago45, Nicholas J. Amos14,46, Richard P.
Ashley14, Lola Balaguer-Núñez18,19,20, Eduardo Balbinot2, Michele
Bellazzini26, Chris R. Benn14, Sara R. Berlanas47,48, Edouard J. Bernard10,
Philip Best28, Daniela Bettoni13, Andrea Bianco49, Georgia Bishop3, Michael
Blomqvist12, Corrado Boeche13, Micol Bolzonella26, Silvia Bonoli50,51, Albert
Bosma12, Nikolay Britavskiy5,6,97, Gianni Busarello37, Elisabetta Caffau24,
Tristan Cantat-Gaudin18,19,20, Alfred Castro-Ginard52,18,19,20, Guilherme
Couto53, Juan Carbajo-Hijarrubia18,19,20, David Carter33,†, Laia
Casamiquela24,54, Ana M. Conrado31, Pablo Corcho-Caballero55,56,57, Luca
Costantin58, Alis Deason59,60, Abel de Burgos5, Sabrina De Grandi49, Paola Di
Matteo24, Jesús Domínguez-Gómez61, Ricardo Dorda95, Alyssa Drake7, Rajeshwari
Dutta63,49, Denis Erkal64, Sofia Feltzing25, Anna Ferré-Mateu5, Diane
Feuillet25, Francesca Figueras18,19,20, Matteo Fossati11,63, Elena
Franciosini16, Antonio Frasca45, Michele Fumagalli65,63, Anna Gallazzi16,
Rubén García-Benito31, Nicola Gentile Fusillo9, Marwan Gebran66, James
Gilbert67,1, T. M. Gledhill7, Rosa M. González Delgado31, Robert Greimel68,
Mario Giuseppe Guarcello69, Jose Guerra70, Marco Gullieuszik13, Christopher P.
Haines39,11, Martin J. Hardcastle7, Amy Harris7, Misha Haywood24, Amina
Helmi2, Nauzet Hernandez70, Artemio Herrero5,6, Sarah Hughes1, Vid Irsic71,
Pascale Jablonka24, Matt J. Jarvis1,72, Carme Jordi18,19,20, Rohit
Kondapally28, Georges Kordopatis10, Jens-Kristian Krogager73, Francesco La
Barbera37, Man I Lam53, Søren S. Larsen74, Bertrand Lemasle75, Ian J. Lewis1,
Emilie Lhomé76,14, Karin Lind77,78,36 Marcello Lodi70, Alessia
Longobardi63,49, Ilaria Lonoce79,11, Laura Magrini16, Jesús Maíz Apellániz80,
Olivier Marchal29, Amparo Marco62, Nicolas F. Martin29,78, Tadafumi Matsuno2,
Sophie Maurogordato10, Paola Merluzzi37, Jordi Miralda-Escudé18,19,20,81,
Emilio Molinari82, Giacomo Monari29, Lorenzo Morelli39,11, Christopher J.
Mottram33, Tim Naylor83, Ignacio Negueruela47, Jose Oñorbe84, Elena Pancino16,
Sébastien Peirani10,35, Reynier F. Peletier2, Lucia Pozzetti26, Monica
Rainer49, Pau Ramos18,19,20,85, Shaun C. Read21, Elena Maria Rossi52, Huub J.
A. Röttgering52, Jose Alberto Rubiño-Martín5,6, Jose Sabater Montes28, José
San Juan70, Nicoletta Sanna16, Ellen Schallig1,86, Ricardo P. Schiavon33,
Mathias Schultheis10, Paolo Serra82, Timothy W. Shimwell32, Sergio Simón-
Díaz5,6, Russell J. Smith60, Rosanna Sordo13, Daniele Sorini59,28,87, Caroline
Soubiran54, Else Starkenburg2, Iain A. Steele33, John Stott46, Remko
Stuik86,52, Eline Tolstoy2, Crescenzo Tortora37, Maria Tsantaki16, Mathieu Van
der Swaelmen16, Reinout J. van Weeren52, Daniela Vergani26, Marc A. W.
Verheijen2, Kristiina Verro2, Jorick S. Vink89, Miguel Vioque90,91,92, C.
Jakob Walcher53, Nicholas A. Walton17, Christopher Wegg10, Anne-Marie
Weijmans93, Wendy L. Williams52, Andrew J. Wilson83, Nicholas J. Wright48,
Theodora Xylakis-Dornbusch94,78, Kris Youakim77, Stefano Zibetti16, Cristina
Zurita5,6
The authors’ affiliations are shown in Appendix B.

(Accepted XXX. Received YYY; in original form ZZZ)

###### Abstract

WEAVE, the new wide-field, massively multiplexed spectroscopic survey facility
for the William Herschel Telescope, will see first light in late 2022. WEAVE
comprises a new 2-degree field-of-view prime-focus corrector system, a nearly
1000-multiplex fibre positioner, 20 individually deployable ‘mini’ integral
field units (IFUs), and a single large IFU. These fibre systems feed a dual-
beam spectrograph covering the wavelength range 366$-$959 nm at $R\sim 5000$,
or two shorter ranges at $R\sim 20\,000$. After summarising the design and
implementation of WEAVE and its data systems, we present the organisation,
science drivers and design of a five- to seven-year programme of eight
individual surveys to: (i) study our Galaxy’s origins by completing Gaia’s
phase-space information, providing metallicities to its limiting magnitude for
$\sim$3 million stars and detailed abundances for $\sim 1.5$ million brighter
field and open-cluster stars; (ii) survey $\sim 0.4$ million Galactic-plane
OBA stars, young stellar objects and nearby gas to understand the evolution of
young stars and their environments; (iii) perform an extensive spectral survey
of white dwarfs; (iv) survey $\sim 400$ neutral-hydrogen-selected galaxies
with the IFUs; (v) study properties and kinematics of stellar populations and
ionised gas in $z<0.5$ cluster galaxies; (vi) survey stellar populations and
kinematics in $\sim 25\,000$ field galaxies at $0.3\lesssim z\lesssim 0.7$;
(vii) study the cosmic evolution of accretion and star formation using $>1$
million spectra of LOFAR-selected radio sources; (viii) trace structures using
intergalactic/circumgalactic gas at $z>2$. Finally, we describe the WEAVE
Operational Rehearsals using the WEAVE Simulator.

###### keywords:

surveys – instrumentation: spectrographs – Galaxy: general – stars: general –
galaxies: general – cosmology: observations

††pubyear: 2022††pagerange: The wide-field, multiplexed, spectroscopic
facility WEAVE: Survey design, overview, and simulated implementation–B

##  1 Introduction

The last decade has seen the opening of new windows on the Universe, made
possible by the development of new instrumentation. Two of these new windows
are ultra-high-precision astrometry, enabled by the Gaia mission (Gaia
Collaboration et al., 2016) and providing us with radically new views on the
structure, dynamics and evolution of our own Milky Way system, and low-
frequency radio astronomy, enabled by LOFAR (van Haarlem et al., 2013). The
latter is already revealing millions of active galactic nuclei and star-
forming galaxies across cosmic time, and their interactions with their local
circumgalactic and intergalactic media (e.g. Shimwell et al., 2019; Williams
et al., 2019; Duncan et al., 2019; Tasse et al., 2021; Sabater et al., 2021;
Kondapally et al., 2021; Duncan et al., 2021), while the former has provided
unprecedented insights into – or constraints on – subject matters such as star
formation in the Galactic disc (e.g. Ruiz-Lara et al., 2020), the dynamics of
the Milky Way disc (e.g. Antoja et al., 2018; Bennett & Bovy, 2019; Laporte et
al., 2019), the mass of the Milky Way (e.g. Posti & Helmi, 2019; Wegg et al.,
2019; Cautun et al., 2020), the dynamics of satellite galaxies (e.g. Battaglia
et al., 2022), the effect of the Large Magellanic Cloud on the dynamics of the
Milky Way and stellar streams, (e.g. Vasiliev et al., 2021), stellar streams
in the Galactic halo (e.g. Malhan et al., 2018), a major accretion event (Gaia
Enceladus Sausage, e.g. Belokurov et al., 2018; Helmi et al., 2018), the
chemodynamics of stellar streams (Mackereth et al., 2019) and determination of
stellar ages in the disc (Bland-Hawthorn et al., 2019) through combining Gaia
data with those from complementary surveys, and the Galactic disc as seen from
its component stellar clusters (Cantat-Gaudin et al., 2020).

At the start of the last decade, the European astronomical community, through
the European Commission-funded ASTRONET programme, laid out a set of
recommendations for solving pressing astrophysical problems in the following
decade and the infrastructure required for those solutions. A clear
recommendation from both the ASTRONET Wide-Field Spectroscopy Working
Group111https://www.astronet-eu.org/sites/default/files/d26-vsdef-2.pdf and
the report on Europe’s 2–4 m telescopes by the European Telescope Strategic
Review Committee222https://www.astronet-
eu.org/sites/default/files/plaquettet2_4m-final-2.pdf was for an intermediate-
resolution ($R\sim 5000$), wide-field ($\gtrsim 1^{\circ}$ diameter) multi-
object spectrograph with a multiplex of $>500$ (and preferably $>1000$) to
follow up the Gaia mission and to allow for precision-cosmology experiments;
an extension to even higher resolution ($R\gtrsim 20\,000$) with the same
instrument was deemed desirable, as this extension would allow for chemo-
dynamical labelling of structures in the Milky Way.

Roughly simultaneously, in 2010 January, the Isaac Newton Group of telescopes
(ING) held a workshop on ‘Science with the William Herschel Telescope
2010–2020’333http://www.ing.iac.es/conferences/wht201020/, where ideas for
wide-field spectroscopy with the William Herschel
Telescope444http://www.ing.iac.es/astronomy/telescopes/wht/ (WHT) were
discussed. Previous, on-going, and then-planned surveys (e.g. SDSS: York et
al. 2000; SEGUE-1: Yanny et al. 2009; SEGUE-2: Eisenstein et al. 2011;
APOGEE-1: Blanton et al. 2017; APOGEE-2: Eisenstein et al. 2011; RAVE:
Steinmetz et al. 2006; LAMOST: Cui et al. 2012; Luo et al. 2015; Gaia-
ESOGilmore et al. 2012; Randich et al. 2022; Gilmore et al. 2022; GALAH: De
Silva et al. 2015) have demonstrated the immense power of wide-field
spectroscopy for answering important questions about the formation and
evolution of galaxies and their stars and the structure of the Universe,
leading to the conclusion – fully supported by the ASTRONET recommendations –
that such a survey instrument was vital for the ING’s long-term vision. During
follow-up discussions, a small team developed a set of desirable requirements
for an instrument capable of both providing the missing aspects of the (then)
upcoming survey instruments Gaia, LOFAR, and Apertif (Verheijen et al., 2008;
van Cappellen et al., 2022), and science goals extending beyond those three
instruments: these are listed in Table 1. These requirements and a well-
developed design for such an instrument were presented to the community in
2015, while members of the community presented their own aspirations for
multi-object spectroscopy for the coming decade at the same conference
(Skillen et al., 2016), resulting in a much broader science case covering
cosmology, galaxy evolution, and stellar evolution on scales from the local
Solar neighbourhood to redshifts in excess of 4.

Table 1: Desirable requirements for follow-up of the Gaia mission (Gaia Collaboration et al., 2016), and LOFAR (van Haarlem et al., 2013) and Apertif (Verheijen et al., 2008) surveys. Instrument | Desirable requirements
---|---
Gaia555https://www.esa.int/Science_Exploration/Space_Science/Gaia,666https://www.cosmos.esa.int/web/gaia/data-release-3 |  $R=5000$ for radial velocities at $17\leq V\leq 20\,\mathrm{mag}$
|  $R=20\,000$ for stellar abundances at $12\leq V\leq 17\,\mathrm{mag}$
|  $10^{7}$ stars over $10^{4}$ contiguous square degrees
LOFAR777https://science.astron.nl/telescopes/lofar/,888https://lofar-surveys.org/dr2_release.html | Continuous wavelength coverage over $370$–$980$ nm
|  $V\leq 21.5\,\mathrm{mag}$ at $S/N=5$ in continuum for redshifts
|  $10^{7}$ galaxies over $10^{4}$ square degrees
Apertif999https://science.astron.nl/telescopes/wsrt-apertif/,101010https://tinyurl.com/2p8d64av | Large IFU and mini-IFUs for spatially resolved spectra of
| gas-rich galaxies
|  $10^{4}$ galaxies over $10^{4}$ continuous square degrees

The WHT Enhanced Area Velocity Explorer
(WEAVE111111https://ingconfluence.ing.iac.es/confluence//display/WEAV, Dalton
et al., 2012, 2016a) is the result of these discussions: a next-generation
wide-field spectroscopic survey facility for the $4.2\,\mathrm{m}$ William
Herschel Telescope at the Observatorio del Roque de los Muchachos on La Palma,
Spain. It provides the spectroscopic follow-up required for full scientific
exploitation of the Gaia, LOFAR and Apertif surveys (for references, see Table
1) in the Northern Hemisphere and, furthermore, to study advanced phases of
stellar evolution, galaxy evolution in different environments over the last
5–8 Gyr, and the changing scale of the Universe and therefore its most basic
parameters since near its beginning. The facility comprises a new $2^{\circ}$
field-of-view prime-focus corrector system with a 1000-multiplex fibre
positioner, 20 individually deployable ‘mini’ integral field units (mIFUs),
and a single ‘large’ integral field unit (LIFU). The fibres feed a dual-beam
spectrograph that will provide full coverage of most of the visible spectrum
in a single exposure at a spectral resolution of $\sim 5000$, or a more
limited wavelength coverage in both arms at a resolution of $\sim 20\,000$.
The timely arrival of WEAVE at the precise moment when Gaia and LOFAR are
producing their groundbreaking results guarantees its utility to the community
in the coming half-decade and beyond.

The design of WEAVE was driven by the fact that neither Gaia nor LOFAR provide
all of the information required to fully exploit the data they produce. Gaia
produces radial velocities mostly for sources with magnitude $G<14.5$ at
present (Gaia DR3, Katz et al., 2022a) and $G\sim 16-17$121212This $G$
magnitude range corresponds to the $G_{\mathrm{RVS}}\leq 16$ limit expected
(Sartoretti et al., 2022). by the end of the Gaia mission. In both cases,
these limiting magnitudes are where the Gaia completeness is reasonably high,
while measurements are still available in a tail of fainter stars. These
limits are also far brighter than the astrometric magnitude limit of $G<20-21$
in both cases. Similarly, although the exquisite low-frequency continuum
sensitivity of the LOFAR Surveys Key Science Project (Röttgering et al., 2011)
observations in the LOFAR two-metre sky survey (LoTSS; e.g. Shimwell et al.,
2017, 2019, 2022; Tasse et al., 2021) enables the detection of extremely faint
radio sources, the redshift (i.e. distance) information that is necessary to
distinguish between their apparent and intrinsic properties must be obtained
via other means. Extensive optical spectroscopy over a large contiguous
wavelength range is the ideal way of achieving this.

Furthermore, other pathfinder telescopes for the Square Kilometer
Array131313http:///www.skatelescope.org, such as the Apertif focal-plane array
system (Verheijen et al., 2008; van Cappellen et al., 2022) on the Westerbork
Synthesis Radio Telescope, will reveal the neutral hydrogen sky through wide-
field surveys not previously possible. These surveys are also incomplete, as
connecting the H i sky – inherently three-dimensional (positions plus radial
velocity are simultaneously observed) – to the optical sky requires three-
dimensional optical data. Obtaining such data is now possible through large-
field and/or multiple integral field units (e.g. Kelz et al., 2006; Drory et
al., 2015). An instrument with both kinds of IFU capabilities, capable of
targeting large and multiple small targets, is necessary for the full
exploitation of upcoming H i surveys.

The following eight independent surveys will be carried out with WEAVE over a
period of (at least) five years, producing more than 30 million spectra of
nearly 10 million objects:

  1. 1.

a survey of the Milky Way galaxy, providing radial velocities and stellar
abundances for stars too faint for these quantities to be measured by Gaia
(WEAVE Galactic Archaeology; Section 4.1);

  2. 2.

a survey characterising young and massive stellar populations and the
interstellar medium, and thus probing star formation and evolution, across the
Galactic Plane (Stellar, Circumstellar, and Interstellar Physics; Section
4.2);

  3. 3.

a survey studying the death of stars and constraining the local star-formation
history of the Galaxy through its white dwarf population (White Dwarfs;
Section 4.3);

  4. 4.

a survey of the stellar and gaseous kinematics and physical properties of gas-
rich galaxies, providing a necessary optical complement to Apertif’s neutral
hydrogen surveys of the local Universe (WEAVE-Apertif; Section 4.4);

  5. 5.

a survey probing the evolution of galaxies as a function of environment, from
the cores of rich clusters to their lively environs, going from their smallest
members out to the field at cosmological distances (WEAVE Galaxy Clusters;
Section 4.5);

  6. 6.

a survey providing the first detailed view of the stellar population
properties of galaxies at $z=0.3$–$0.7$ as a function of galaxy mass and
environment, yielding star formation histories, stellar ages, stellar and gas
metallicities, dust attenuation, gas kinematics and stellar velocity
dispersions (Stellar Population at intermediate redshifts Survey; Section
4.6);

  7. 7.

a survey probing galaxy evolution over cosmic time, providing much-desired
redshifts and galaxy properties of LOFAR’s radio sources (WEAVE-LOFAR; Section
4.7);

  8. 8.

a survey of large-scale structure using quasar absorption lines as a cosmic
ruler to probe the expansion of the Universe, which also extends the study of
gaseous environments to larger scales and earlier epochs (WEAVE-QSO; Section
4.8).

The WEAVE Survey will provide data that will help answer the questions: How
did our Galaxy form and the stars within it evolve? How were other galaxies
assembled? What are dark matter and dark energy? On these topics, WEAVE will
be complementary to the surveys carried out by 4MOST (de Jong et al., 2019),
which has a similar design, including spectral range and resolution, and will
operate in the Southern Hemisphere on a similarly sized telescope, DESI (DESI
Collaboration, 2016; DESI Collaboration et al., 2016), which will focus
largely on baryonic acoustic oscillations of galaxies, SDSS-V (Kollmeier et
al., 2017), which will have complementary, lower resolution optical spectra
and, additionally, infrared spectroscopy, and PFS (Tamura et al., 2018), which
will cover a smaller field on a larger telescope that is not fully dedicated
to survey operations.

This paper first presents the design of the WEAVE instrument – in terms of
both hardware and software – as relevant to WEAVE Survey preparation and
execution (Section 2), followed by the structure of the WEAVE work and
workforce including an overview of the key cross-team working groups involved
in and responsible for the preparation and execution of the Survey (Section
3). We then present overviews of each of the eight component surveys that
comprise the WEAVE Survey (Section 4), with each subsection summarising the
particular survey’s science case and survey plan, as well as providing the
relevant introduction and background to the field. This is followed by a
description of the WEAVE Simulator (Section 5) and a summary of the simulated
implementation of the Survey through ‘Operational Rehearsals’ (Section 6).
Section 7 concludes the paper.

##  2 The WEAVE Facility

The WHT is an alt-azimuth telescope with a Cassegrain focus and two Nasmyth
foci (for a description of the original design features of the WHT, see
Boksenberg, 1985). WEAVE’s predecessor on the WHT was the prime-focus
instrument Autofib-2 (AF2, Parry et al., 1994), which had 150 fibres
deployable over a 2-degree-diameter field of view. WEAVE has nearly 1000
fibres deployable over a field of view four times that of AF2 and, with four
times the number of resolution elements of AF2 and three to four times its
throughput, dramatically increases the resolution and multiplex power of the
telescope.

Full details of the WEAVE design and performance will be presented in a
separate paper (Dalton et al., in preparation). Here, we summarise only the
key parameters of the WEAVE facility as they relate to the detailed design of
the component surveys making up the total WEAVE Survey (cf. Dalton et al.,
2016a).

###  2.1 A new top-end for the WHT

The WEAVE prime-focus corrector system delivers a 2-degree-diameter field of
view, with a flat focal plane at $f/2.78$ with full correction for atmospheric
dispersion from 370–1000$\,\mathrm{nm}$ to $65^{\circ}$ zenith distance (Agócs
et al., 2014). The telecentricity of the input beam will deviate by no more
than $4^{\circ}$ at the edge of the field. For a point-source image in the
absence of seeing, the corrector delivers 80 per cent encircled energy within
0.71 arcsec, corresponding to a full width at half maximum (FWHM) of 0.41
arcsec. The effect of differential image distortion with changing zenith
distance is 0.13 arcsec across the field between $0^{\circ}$ and $65^{\circ}$
zenith distance.

###  2.2 The WEAVE Fibres and Fibre Positioner

Figure 1: A computer-aided diagram (CAD) representation of the WEAVE top-end assembly, incorporating the prime-focus (field) corrector system, instrument rotator and fibre positioner. The four mounting units on the outer ring of the telescope structure provide for focus and tilt correction of the whole system. Figure 2: Configured field for OB #3182 from OpR3b (see Section 6) as an example of a representative WEAVE-Survey MOS field. The black, dashed, outermost circle indicates WEAVE’s 2-degree-diameter field of view. MOS fibres and targets are colour-coded as follows – black: science (821); cyan: calibration stars (19); blue: blank sky (100); green: guide stars (8). The two faint inner circles denote the maximum extent of the outer two tiers of MOS fibres. As this particular field is a configuration on plate B, the ‘park’ locations of the 10 pairs of mIFU bundles can also be seen along the periphery. Note that the mIFUs cannot be used concurrently with the MOS fibres. Table 2: Key parameters of the four focal-plane modes of WEAVE. MOS-B and mIFU modes both use plate B, but cannot be employed simultaneously. Focal-plane mode | MOS fibres (MOS-A) | MOS fibres (MOS-B) | mini-IFUs (mIFU) | Large IFU (LIFU)
---|---|---|---|---
Tumbler position | 0° | 180° | 180° | 90°
Diameter of individual fibres | 1.3 arcsec (85 µm) | 1.3 arcsec (85 µm) | 1.3 arcsec (85 µm) | 2.6 arcsec (170 µm)
Multiplex | 960 fibres | 940 fibres | 20 IFUs | 1 IFU
Diameter of field over which | 2° | 2° | 2° | on axis
deployable |  |  |  |
Minimum separation on sky |  $\sim$60 arcsec |  $\sim$60 arcsec |  $\sim$60 arcsec | –
Fibres per IFU | – | – | 37 | 547
IFU field of view | – | – | 11 $\times$ 12 arcsec2 | 90 $\times$ 78 arcsec2
IFU filling factor | – | – | 0.50 | 0.55
Fibres for auto-guiding | 8 $\times$ 3 arcsec2 coherent bundles | 8 $\times$ 3 arcsec2 coherent bundles | 8 $\times$ 3 arcsec2 coherent bundles | Separate camera
|  |  |  | (4 $\times$ 3.7 arcmin2 field of view)
Fibres for sky subtraction |  $\sim$5–10$\%$ of science fibres |  $\sim$5–10$\%$ of science fibres | one of the mIFUs | 8 peripheral bundles
|  |  |  | of 7 fibres each
Configuration time |  $\sim 55$ minutes |  $\sim 55$ minutes | <20 minutes |  $\sim 1$ minute

Behind the prime-focus corrector system, fibres are deployed in the focal
plane using a buffered positioner system developed from the 2dF concept (Lewis
et al., 2002): the positioner can deploy up to 1008 fibre buttons on each of
two field plates using a pair of Cartesian robots sharing a common $x$-axis.
Each science-fibre button is $5\,\mathrm{mm}\times 2\,\mathrm{mm}\times
7\,\mathrm{mm}$ in size and terminates in a $1.5\,\mathrm{mm}\times
1.5\,\mathrm{mm}$ prism with a concave upper surface to expand the telescope
beam to $f/3.2$. Each multi-object spectrograph (MOS) science fibre carries a
$85\,\mbox{\textmu m}$ core fibre that subtends 1.3 arcsec on the sky. Eight
buttons on each field hold coherent fibre imaging bundles to be used for
acquisition and guiding. On the first plate (‘plate A’), each of the eight
guide-fibre bundles is packaged so that each bundle replaces six potential
science fibres, leaving 960 science fibres available. On the second plate
(‘plate B’), 10 groups of six science fibres are replaced by pairs of
deployable mini integral field units (mIFUs). On plate B, the guide-fibre
bundles each replace only a single science fibre to leave 940 science fibres
available. The two MOS plates are at either end of the positioner, such that
when one plate is being used for the current observation, the other can be set
up for the upcoming observation. A MOS field can be configured in under one
hour, thus setting the length of a typical WEAVE observation. Once the current
observation has finished, the positioner tumbles to reverse the roles of the
two plates (Lewis et al., 2014). This results in a marked improvement in
observing efficiency as compared with AF2, as there will, in principle, be no
loss in observing time due to field configurations when switching between the
two MOS plates for observations with the MOS or mIFU modes with WEAVE. Table 2
summarises the key parameters of the four focal-plane modes of WEAVE.

The 20 mIFUs are packaged similarly to the MOS fibres, but with slightly
larger buttons ($7\,\mathrm{mm}\times 5\,\mathrm{mm}\times 7\,\mathrm{mm}$ in
size), each holding a hexagonal close-packed array of 37 fibres with the same
size and aperture as the MOS fibres. The outer diameter of the fibre buffer
layer is $120\,\mbox{\textmu m}$, so each mIFU covers roughly 11 $\times$ 12
arcsec2 on the sky with a filling factor of $\approx 0.50$. The buffered
arrangement of the two field plates also affords an intermediate location
(i.e. at 90∘ to the MOS plates), which accommodates a single, large integral
field unit (LIFU) containing a hexagonal array of 547 fibres with
$170\,\mbox{\textmu m}$ cores 2.6 arcsec, subtending 90 $\times$ 78 arcsec2 on
the sky with a filling factor of $\approx 0.54$, along with eight peripheral
bundles of seven fibres, arranged in a ring of radius 150 arcsec from the
centre of the array, to be used for sky subtraction. The LIFU package includes
a discrete guiding camera mounted within the same physical structure as the
LIFU head.

Each MOS observation is prepared in advance using a purpose-built software
tool (configure: Terrett et al., 2014, see Figure 2), which builds on the
simulated annealing approach of Miszalski et al. (2006) with some elements of
the original ‘Oxford’ algorithm (Colless et al., 2001) to facilitate an
optimal, on-the-fly assignment of fibres for sky-background determination
without impacting the results of the annealing process. The output of
configure is an XML file containing all of the information required to specify
an observation, including observing constraints, spectrograph configuration
and calibration lamp sequences. Observations for the mIFU and LIFU modes
follow a similar but slightly simpler approach. Using configure, the mIFUs in
a given field can be manually allocated to targets such that the central fibre
of each mIFU is located at a desired target position. Final sky coordinates
for the remaining 36 fibres are output by configure, based on the sky position
angle of the mIFU field resulting from this allocation. Each mIFU-field setup
requires at least one of the mIFUs to be dedicated for calibration purposes.
The central seven fibres are identified with the primary calibration target
(i.e. a white dwarf), and the remaining 30 fibres are labelled as sky
positions. LIFU observations are prepared using a similar tool, with the
output XML file providing information on guide-star positions and the precise
on-sky position for each fibre within the LIFU.

###  2.3 The WEAVE Spectrograph

Figure 3: A CAD representation of the WEAVE spectrograph installed in the
GHRIL instrument room on the WHT Nasmyth platform. The spectrograph cameras
are pictured in a configuration unused in practice, with the blue camera at
the LR position and the red camera at the HR position. As a scale reference,
the optical bench is approximately 3 m long.

All four sets of science fibres pass through a cable-wrap around the prime-
focus instrument rotator, along the vanes of the prime-focus support spider
and down the telescope structure, to where they pass over the elevation axis
and into the GHRIL instrument
room141414https://www.ing.iac.es//PR/inst.php?tel=wht&inst=GHRIL on the
Nasmyth platform that houses the WEAVE spectrograph (see Figure 3).

The spectrograph (Rogers et al., 2014) provides full wavelength coverage over
the range 366–959$\,\mathrm{nm}$ at nominal resolving power of $R\sim 5000$
(the low-resolution, ‘LR’, mode), or a pair of restricted wavelength ranges at
$R\sim 20\,000$ (the high-resolution, ‘HR’, mode; note that these resolving
powers are halved when using the LIFU with its two-times-larger fibres). This
is achieved by a dual-beam design with an off-axis catadioptric151515A
catadioptric optical system combines both reflection and refraction to reduce
aberration effects. $f/3.1$ collimator, which delivers a 190 mm collimated
beam. The dichroic is located before the collimator correcting lenses so that
these can be optimised for transmission in each arm. Each arm feeds an $f/1.8$
transmissive camera consisting of 8 lenses (one asphere on the first lens),
designed such that the lens prescriptions are common to both cameras, and only
the coatings and lens spacings differ between the two. The useful focal plane
of each camera is 8192 (spectral) $\times$ 6144 (spatial) pixels, populated by
a pair of e2V CCD 231-C6 CCDs, with the physical size of the pixels being 15
µm. This oversize of the detector avoids use of a custom format, and also
allows the gap in the spectral direction to be chosen in each camera to fall
in a region of low interest. At the same time, placing this gap close to the
centre of the image plane makes the control of Littrow ghosts easy by using
VPH gratings with slightly slanted fringes (Burgh et al., 2007; Bianco et al.,
2018), such that the image of the ghost is formed in the gap. Each arm of the
spectrograph contains a rolling shutter mechanism and a grating exchange
mechanism to allow the choice between two resolution modes, with the
associated change of beam angles accommodated by articulation of the two
cameras. The available modes and associated nominal spectral coverage are
listed in Table 3.

Table 3: Available resolution modes (LR = low resolution, HR = high resolution) and nominal useful spectral coverage for the WEAVE spectrograph in the MOS/mIFU modes, including the wavelength gap between the chips. a‘Blue’ HR grating. b‘Green’ HR grating. Note that the two HR Blue modes cannot be used simultaneously. cDue to the larger fibre cores of the LIFU, the resolution in this mode is halved with respect to the MOS/mIFU cases. Spectrograph | $R^{c}$ |  $\lambda_{\mathrm{low}}$(Å) |  $\lambda_{\mathrm{high}}$ (Å) | gap in $\lambda$
---|---|---|---|---
mode |  |  |  | coverage (Å)
LR Blue | 5 000 | 3660 | 6060 | 5491–5539
LR Red | 5 000 | 5790 | 9590 | 7590–7669
HR Blue-1a | 20 000 | 4040 | 4650 | 4525–4536
HR Blue-2b | 20 000 | 4730 | 5450 | 5302–5315
HR Red | 20 000 | 5950 | 6850 | 6412–6431

Direct illumination of each camera’s focal plane is achieved by a series of
LEDs that are mounted on the spectrograph’s shutter mechanisms. These are used
to provide a direct measure of the pixel-to-pixel response variations of the
detector, and a measure of the non-linearity present in the detector read-out
electronics.

###  2.4 Calibration Unit

The WHT has a set of projector lamps mounted at one of the broken-Cassegrain
foci. These can be used to direct light to the primary focus via the tertiary
mirror. Investigations during the development of WEAVE have shown that the
illumination is adequate – but not perfect – out to the edge of the WEAVE
field of view (Domínguez et al., 2016). Observations of a quartz-halogen lamp
are used to determine the traces of the individual fibre spectra on the
detector mosaic for each camera, and to provide a rough indication of the
illumination variations for each fibre. Observations of a ThArCr lamp are used
to determine a wavelength solution for each fibre. With this lamp, we find
that it is possible to obtain a sufficient density of lines in both low- and
high-resolution modes.

Mapping of the wings of the fibre profiles on the detector is achieved by
configuring three fields for each plate, each sampling every third fibre along
the slit and observing high signal-to-noise arc and flat-field lamp exposures.
As the fibres in these configurations can all be placed on a circle at around
a 20-arcmin radius, these observations can also be used to determine fibre-to-
fibre intrinsic transmission variations.

Flux calibration is achieved on-sky using calibrator stars within each WEAVE
field (Section 4.3.1).

###  2.5 Observatory Control System (OCS)

WEAVE introduces a new top-level Observatory Control System (OCS) to the WHT
that permits queue-based observations (Picó et al., 2018). The XML files
generated by configure are converted to Observation Blocks (OBs), which are
stored in a database and can be scheduled in a queue for the operator, based
on the prevailing conditions and target priorities. At the heart of the OCS is
a sequencer task that coordinates the actions of the telescope, spectrograph,
positioner and calibration system using a CORBA-
based161616https://www.corba.org architecture and a centralised noticeboard.
In this context, the positioner has two functions: one for the observation of
the current field and performing any actions required of the focal-plane
imager, and one for the positioning robots concerning the parallel setting-up
of the next observation.

The ‘WEAVE observation queue scheduler’ (scheduler: Fariña et al., 2018) is
bespoke software written for use by the WEAVE operator to make informed
decisions about which OBs in the database are most suitable for observation,
at least one hour ahead of time, given the typical observation length (which
in turn is driven by the configuration time of a MOS field). Requirements set
by the OBs on instrument configuration (i.e. observing mode) and observing
conditions (e.g. sky brightness, seeing, etc.) are combined with current and
predicted weather conditions and current instrument configuration to allocate
scores to OBs in the OB database. For example, poor seeing conditions might
trigger a LIFU observation, during which no configuration is possible on
either field plate. If this occurs during the night, the most efficient
subsequent observation, once the seeing conditions start improving, is a mIFU
field using plate B, as configuring a mIFU field takes less time (under 20
minutes) compared with a MOS field (just under one hour), during which plate A
can be set up for a MOS observation. If such a LIFU observation takes place at
the start of the night, both field plates will already have been set up with
suitable fields before the beginning of the night. Note that each MOS field
configuration is valid only for a specific length of time, depending on its
elevation, due to the distortion introduced by the effects of differential
refraction.

###  2.6 WEAVE Data Processing and Archiving

From the outset, it was envisaged that WEAVE would require a dedicated data-
processing pipeline to ensure the best outcomes from the facility. Given the
scale and complexity of this task, the activities of this Science Processing
and Analysis (SPA) system have been divided into three distinct sub-systems:
the Core Processing System (CPS) for basic data reduction and calibration, the
Advanced Processing System (APS) for producing derived data products, and the
WEAVE Archive System (WAS) for data archiving.

All WEAVE data frames, including data for individual open-time programmes, are
retrieved from the observatory in near real-time by CPS (Walton et al., 2014)
and processed to remove instrumental signatures, calibrate wavelength and flux
scales (see Section 4.3.1) and subtract sky background. A subset of the CPS
pipeline will run at the telescope to provide a quick-look analysis tool
(Peralta de Arriba et al., 2019) for the observers for the purposes of quality
control during observations. For each OB, the output from the CPS consists of
a multi-spectrum image for each science frame in each arm (with sky and
variance components) and a stacked multi-spectrum image for each arm. These
data are passed to the WAS, together with the raw frames and processed
calibration data.

The APS picks up the processed science frames and produces joined spectra in
both the low and high-resolution modes, before providing an analysis of each
spectrum using a variety of methods (e.g. ferre,
redrock171717https://github.com/desihub/redrock, ppxf, and gandalf: Allende
Prieto et al., 2006; Bolton & Schlegel, 2010; Cappellari, 2012; Sarzi et al.,
2017, the last two of which are wrapped in gist: Bittner et al. 2019a, b). For
extragalactic sources, the analysis yields redshift, galaxy classification,
velocity dispersion, spectral indices and equivalent widths for a list of
standard emission lines. For each stellar source, the analysis produces
velocity, temperature, surface gravity and abundance measures for a variety of
elements. For IFU spectra, the APS produces maps of velocity, velocity
dispersion and abundances. The APS also allows for additional analysis
software (‘Contributed Software’) to be developed by the science teams and
incorporated into the processing chain. All data products produced by the APS
are stored in FITS tables and sent to the WEAVE Archive System.

The WEAVE Archive System (WAS) is the official archive for WEAVE data products
(Guerra et al., 2016). It is based on Apache
Solr181818http://lucene.apache.org/solr/ and provides interactive front-end
access to WEAVE data and data products for WEAVE data users. The WAS not only
stores the results from WEAVE’s two main pipelines, namely the CPS and the
APS, but also stores Contributed Data Products (CDPs), which are specific data
products produced by WEAVE Science Team members in addition to the APS
products. To preserve and ensure the high quality of data available to WAS
users, CDPs are agreed in advance by the Science Team with the Science
Executive (see Section 3). As well as search and download options, the WAS
user interface also offers plotting tools for the visualisation of spectra and
other data products. The WAS manages both internal and public data releases.
Internal data releases are expected on a regular basis, at least once a
semester, while public data releases will be yearly following the first public
data release two years after the start of Survey operations. Data from the
public data releases will also be published to the Virtual
Observatory191919https://ivoa.net/ using the Table Access
Protocol202020https://www.ivoa.net/documents/TAP/20190927/REC-TAP-1.1.html.

##  3 WEAVE’s Organisational Structure

The work and workforce within WEAVE are divided into two entities: the WEAVE
Instrument Consortium and the WEAVE Survey Consortium. The former, also
commonly referred to as ‘the WEAVE Project’, is responsible for the delivery
of the WEAVE facility to the ING. The latter, also commonly referred to as
‘the WEAVE Science Team’, is responsible for preparing and executing a five-
year survey. A simplified view of the organisational structure is shown in
Figure 4.

Figure 4: A simplified version of the WEAVE Project Management Team structure
(the ‘WEAVE organogram’). Arrows point in the direction of reporting. PM:
Project Manager; PI: Principal Investigator; PS: Project Scientist; CPS: Core
Processing System; APS: Advanced Processing System; WAS: WEAVE Archive System;
SWG: Survey Working Group; QAG: Quality Assurance Group.

The WEAVE Project is responsible for the delivery of the complete instrument
along with data-processing software and associated hardware for the data-
processing pipelines, a data archive system (along with a user interface), and
other useful tools such as a quick-look GUI, an operational repository, OB
templates and a file-submission platform (see Sections 2 and 6). The WEAVE
Project is overseen by a Project Board with representatives from the funding
parties, and managed through the Project
Executive212121https://ingconfluence.ing.iac.es/confluence/display/WEAV/Project+Executive.
The latter comprises the Project Manager and Principal Investigators (PIs)
from each of the communities in WEAVE-funding countries. These PIs also
currently compose the Science Executive, which oversees the Survey Consortium.

The WEAVE Survey Consortium – also known as the WEAVE Science Team – is
composed of eight individual science teams, preparing to execute the eight
respective surveys. Each team is led by a Science Team Lead (STL) and each
survey has its own science case and survey plan. The WEAVE Survey Consortium
currently consists of approximately 500
astronomers222222https://ingconfluence.ing.iac.es/confluence/display/WEAV/Science+Team
spread over 11 countries, where membership is contingent on satisfying
eligibility requirements, e.g. any astronomer working in an ING country may
request to join the Survey Consortium. An individual joins the Survey
Consortium by becoming a member of one or more of the eight science teams.
WEAVE membership entitles its members to proprietary WEAVE-data access during
Survey operations, while for certain science teams, this also includes
ancillary data provided through a Memorandum of Understanding (MoU) with
another survey. The component surveys comprising the global WEAVE Survey are
described further in Section 4.

###  3.1 Survey Working Group (SWG)

During Survey operations, the main responsibility of the Survey Working Group
(SWG) is to provide catalogues of targets and configured fields in the form of
XML files using configure (see Section 2.2) on a trimesterly basis, i.e. every
three months. The SWG is composed of representatives from the WEAVE Science
Team, each of whom reports to the relevant Science Team Lead, and is currently
led by the Project Scientist and Deputy Project Scientist. The SWG has been
fundamental in helping to define the structure and details of key data-flow
processes, testing many steps of the Survey preparation process from the
creation of input target catalogues and testing of the configure software for
creating configured fields to trialling the trimesterly XML submission process
via the WEAVE Automated Submission Platform (WASP) in the Operational
Rehearsals (see Section 6).

###  3.2 On-Island Survey Management Team (OISMT)

The On-Island Survey Management Team (OISMT) at the ING is responsible for
many aspects of the running of the WEAVE facility, ranging from the
commissioning of the WEAVE instrument to ensuring the day-to-day running of
WEAVE in the contexts of both the WEAVE Survey and PI programmes232323WEAVE
will be available for community PI programmes in the same manner as for
instruments on other ING telescopes, via the usual ING call for proposals.
Further details can be found at
https://www.ing.iac.es/astronomy/observing/INGinfo_home.html once the
instrument is on sky. With support from the OISMT, a dedicated WEAVE
instrument specialist will be responsible for managing the execution of
nightly OBs, aided by the observation queue created by the scheduler based on
current and predicted weather conditions and the observing requirements of OBs
in the OB database, held at Observatory Control System. Regular communication
between the OISMT and the Survey Working Group is key for ensuring the smooth
and optimised scheduling of OBs from the WEAVE Survey and PI programmes
combined, and this interface has been tested extensively during the most
recent Operational Rehearsal (see Section 6).

###  3.3 Quality Assurance Group (QAG)

Although various checks are performed on the spectra being processed by the
Core and Advanced Processing Systems before reaching the WEAVE Archive System,
some unexpected results requiring closer inspection are anticipated,
especially at the start of Survey operations. To allow such spectra to be
caught and investigated quickly, the Quality Assurance Group (QAG) of each
science team will perform regular checks on data taken for their survey to
ensure that the spectra are of sufficient quality.

The work of the QAG will also be instrumental in decisions to be made on
timescales of several months to a year. Although each of the eight science
teams comprising the global WEAVE Survey has an internally and externally
reviewed five-year survey plan based on its individual science case prior to
start of Survey operations, the detailed planning of which fields/targets are
submitted for observations (and when) is a real-time decision-making operation
that will continue to take place on a trimesterly cycle. The QAG will report
its findings of their quality-assessment work to the STL and SWG member(s) of
their team on a regular basis. This, in turn, will form the basis of decisions
made by the STL and SWG on any necessary strategic planning for upcoming
submissions on how to proceed with their survey. Reports from the QAG will
also be combined with individual survey completion rates and other
considerations such as right-ascension pressure for top-level Survey-planning
decisions to be made by the Science Executive.

The QAG workflow and its tasks have been tested intensively during Operational
Rehearsals involving the WEAVE Science Team (see Section 6).

###  3.4 IFU Working Group (IWG)

The IFU Working Group (IWG) deals with planning, observational, and software
aspects of WEAVE’s integral-field modes. The IWG consists of members of the
software systems, SWG, QAG, and ad-hoc members of the science teams as
required, and discusses issues ranging from observation preparation to
dithering strategies, data reduction and analysis techniques. Given the
complexity of designing IFU observations within the context of the WEAVE
observation preparation system, the IWG has been instrumental in the
development and testing of the ‘IFU workflow’ software for preparation of IFU
observations, which in turn has also significantly contributed to the parallel
development of the ‘MOS workflow’ software for preparation of the multi-
object-spectroscopy observations. During Survey operations, the work carried
out by the IWG will serve as additional input to the SWG and the QAG of
science teams that include IFU observations.

##  4 The WEAVE Survey

Table 4: WEAVE survey parameters, listed in order of survey appearance in the text. Numbers provided are best estimates at the time of writing for a seven-year WEAVE Survey. a‘Survey fraction’ denotes the fraction of total WEAVE-Survey fibres hours currently planned for use for a given WEAVE survey. bMSTO: Main Sequence Turnoff; RGB: Red Giant Branch; BHB: Blue Horizontal Branch. cWhite dwarfs used purely for IFU-mode calibrations. dArea coverage calculated using the field of view of the LIFU, not the field of view of WEAVE. eApplies to WQ-Wide (when J-PAS target selection is available). WQ-HighDens will target $r<23.5$ for $z>2.2$ over $\sim$420 deg2.  WEAVE survey | main targets | number of | area | Survey | observing | resolution | redshift
---|---|---|---|---|---|---|---
(short-hand) |  | objects | (deg2) | fractiona | mode(s) | modes(s) |
Galactic Archaeology | MSTOb stars, RGBb stars, | $\sim 1.6\times 10^{6}$ | 8750 | 0.168 | MOS | LR | 0
(GA-LRhighlat) |  BHBb stars, RR Lyrae |  |  |  |  |  |
Galactic Archaeology | Red Clump stars, | $\sim 1.1\times 10^{6}$ | 1370 | 0.110 | MOS | LR | 0
(GA-LRdisc) |  RGB stars |  |  |  |  |  |
Galactic Archaeology | Main sequence & | $\sim 1.6\times 10^{6}$ | 5650 | 0.309 | MOS | HR | 0
(GA-HR) |  RGB stars |  |  |  |  |  |
Galactic Archaeology | Stars in open clusters | $\sim 1\times 10^{5}$ | 375 | 0.029 | MOS | HR | 0
(GA-OC) |  and star forming regions |  |  |  |  |  |
Stellar, Circumstellar and | OBA stars, ionised nebulae, | $\sim 4\times 10^{5}$ | 1230 | 0.069 | MOS, LIFU | LR, HR | 0
Interstellar Physics (SCIP) |  young stars, compact objects |  |  |  |  |  |
White Dwarfs | white dwarfs | $\gtrsim 5\times 10^{4}$ | $\gtrsim 10\,000$ | $\sim 0.012$ | MOS, mIFUc, | LR, HR | 0
|  |  |  |  |  LIFUc |  |
WEAVE-Apertif |  H i-detected, mostly late-type | 400 (LR), | 0.2d | 0.061 | LIFU | LR, HR | $<0.04$
|  galaxies | 100 (HR) |  |  |  |  |
WEAVE Galaxy Clusters | galaxies in dense environments | $\sim 2\times 10^{5}$ | 1350 | 0.064 | MOS, mIFU, LIFU | LR | $<0.5$
Stellar Populations at intermediate | field galaxies | $\sim 2.5\times 10^{4}$ | 25 | 0.026 | MOS | LR | 0.3–0.7
redshifts Survey (StePS) |  |  |  |  |  |  |
WEAVE-LOFAR | 150 MHz sources | $\sim 7\times 10^{5}$ | 8950 | 0.109 | MOS, mIFU, LIFU | LR | $<6.9$
WEAVE-QSO | bright, $r<21.5$: $z>2.2$; | $\sim 4\times 10^{5}$ | 8950 | 0.056 | MOS | LR, HR | $>2.2$
|  $21.5<r<23.5$: $2.5<z<3$e |  |  |  |  |  |

The WEAVE Survey is composed of eight individual surveys, each with its own
dedicated science team, whose science goals span a wide range of topics
covering various fields of stellar, Galactic and extragalactic astronomy. The
surveys will together use approximately 1150 nights over the course of five
years of WHT time. Their science goals play crucial roles in complementing
major space- and ground-based programmes in the current and coming decade,
including Gaia, LOFAR and Apertif, by providing a dedicated wide-field optical
spectroscopic instrument in the Northern Hemisphere. WEAVE will also be
accessible to the wider astronomical community through open calls outside of
the allocated survey time.

In this section, an overview of each of the planned surveys of the global
WEAVE Survey is provided, starting from surveys focusing on the nearby
Universe and extending towards higher redshifts (see also Table 4).

###  4.1 The WEAVE Galactic Archaeology (GA) Survey

How did our Galaxy and its components assemble and evolve over time? This
question is key to the general understanding of galaxy formation over cosmic
times, as the Milky Way is the system for which we can hope to constrain in
most detail the physical processes that play a role in the formation and
evolution of galaxies. These processes leave behind specific signatures that
are encoded in the location, kinematics and chemistry of stars (e.g. Freeman &
Bland-Hawthorn, 2002). The ultimate goal of Galactic Archaeology as a field of
study is to obtain a comprehensive census of the positions, orbits, ages and
chemical compositions of stars in all major stellar structures of our own
Galaxy to enable a complete reconstruction of its formation and subsequent
evolution.

Many questions remain open in understanding how the complex structure of the
Milky Way was assembled, with its stellar populations occupying a bulge, a
thin and thick disc and a halo. Whether the different stellar populations have
truly different origins, what the relative importance is of internal (in-situ
star formation, secular evolution of the disc, etc.) and external (accretion
events, gas accretion through filaments, etc.) processes in forming and
shaping the Galaxy, and how universal these processes are in the evolution of
galaxies in general, are questions to which Galactic Archaeology as a field of
astronomical study tries to find answers. The most outstanding open questions
currently include: What is the assembly history of the Milky Way mass? What is
the role of the structure currently known as the Milky Way thick disc at the
earliest times, and what is its relationship with the Milky Way bulge and
halo? What are the relative fractions and properties of the Galactic stellar
halo, formed within the Milky Way (in-situ) and accreted from ‘building
blocks’ of satellite systems? What is the general shape of the potential of
the Milky Way’s halo out to large distances, and how lumpy is this potential?
Is the Galactic disc duality (that refers to the existence of a thin and thick
disc) real, and what is the cause of this duality if so? What causes the
[$\alpha$/Fe] bimodality in the Galactic disc? What is the Galactic disc
potential in detail, and how important are deviations from axisymmetry and
stationarity? What are the origins of the chemical elements in the Galaxy?

The European Space Agency mission Gaia (Gaia Collaboration et al., 2016) was
launched in 2013 December. During its nominal five-year mission (and up to 10
years pending approvals of mission extensions), Gaia is already producing –
and will continue to produce – the most accurate astrometric data (positions,
parallaxes and hence geometrical distances, and proper motions) ever produced
in the optical domain for more than a billion stars in the Milky Way. In
addition, Gaia is providing very accurate photometry in three bandpasses
(Riello et al., 2021a), complemented since its DR3 by shallower photometry in
a fourth bandpass ($G_{\mathrm{RVS}}$, Sartoretti et al., 2022), as well as
spectrophotometric information from the Gaia BP/RP spectrophotometer (De
Angeli et al., 2022).

However, for a number of objectives, Gaia will not be sufficient by itself,
and ground-based spectroscopic information provided by large-scale surveys
such as WEAVE are mandatory. Gaia’s on-board Radial Velocity Spectrograph
(RVS), with a resolution of $\sim 11\,500$ over the wavelength range 845–872
nm (Cropper et al., 2018; Katz et al., 2022b), will not reach the same depth
as the rest of the Gaia instruments242424Gaia will deliver astrometry down to
$G\sim 21$, while Gaia RVS will deliver radial velocities only down to $G\sim
16$–$17$. leaving the vast majority of the survey without the third (radial)
velocity dimension. Furthermore, the RVS’s limited wavelength coverage and
resolution will not allow for detailed studies of a large variety of elemental
abundances, with reliable detailed elemental abundances only for sources
brighter than $G\approx 12$252525This limiting magnitude was obtained by
extracting from the DR3 Astrophysical Parameters catalogue in the Gaia archive
all stars with at least a [Ca/Fe] abundance measurement with a corresponding
flag of 0, which shows a strong drop in number count for $G>12.5$. (Recio-
Blanco et al., 2022).

The WEAVE Galactic Archaeology survey is tailored to complement Gaia in two
primary ways:

  * •

by providing measurements based on low-resolution spectra of radial velocities
(expected precision $\pm 1$–$2\,\mathrm{km\,s^{-1}}$), effective temperature,
surface gravity and metallicity (expected precision $\pm 0.2\,\mathrm{dex}$)
for $\sim 1.8$–2.6 million targets in the faint part of the Gaia catalogues
with magnitudes in the range $16<G<20.7$, which are too faint for the RVS (see
Figure 5);

  * •

by yielding accurate measurements based on high-resolution spectra of
abundance ratios covering the main nucleosynthetic channels (light, $\alpha$-,
Fe-peak, and $s$\- and $r$-process neutron-capture elements, to better than
$\pm 0.05$–$0.2\,\mathrm{dex}$), depending on the element and type of star,
and radial velocities (to better than $\sim\pm 0.5\,\mathrm{km\,s^{-1}}$) for
$\sim 1.1$–1.6 million stars in the magnitude range $12<G<16$, for which the
RVS will again not be able to provide the relevant data.

Figure 5: Expected radial velocity accuracies for WEAVE in its $R=5000$ mode
(thick solid coloured lines) for different stellar effective temperature
ranges, based on the WEAVE ‘Operational Rehearsal’ (OpR) simulations
(specifically OpR2.5 – see also Section 6; final improvements to the APS
pipeline based on OpR3 data have confirmed the results presented here, albeit
on a smaller sample), together with the post-launch predictions for Gaia at
the end of the nominal five-year mission (thin solid coloured lines) for
different stellar types. Dashed coloured lines represent Gaia end-of-mission
photometric limits for astrometry of different spectral types.

Studying the origin and evolution of the main different stellar components of
the Milky Way (halo, thin and thick discs) calls for massive spectroscopic
surveys to provide line-of-sight velocities, distances and chemical abundances
of several million stars, and WEAVE is a cornerstone for this endeavour, as
one of the first – and for its high-resolution capability, the only – high-
multiplex wide-field optical multi-object spectrograph in the Northern
Hemisphere. These spectroscopic surveys complement Gaia’s superb astrometry
while benefiting from Gaia for optimal target selection. A WEAVE survey
starting in the coming months will be particularly timely, benefiting from
Gaia Data Release 3 in 2022 June (DR3: Gaia Collaboration et al., 2022a), and
producing its final science in the same time-frame as the Gaia end-of-mission
catalogues (the Gaia legacy release, planned for 2030). Gaia’s third data
release (DR3, released as the EDR3 on 2020 December 4 and DR3 on 2022 June 13:
Gaia Collaboration et al., 2022a) are being used for WEAVE target selection,
providing a homogeneous source for astrometry (positions, parallaxes, proper
motions – Lindegren et al., 2021) and broad-band photometry (Riello et al.,
2021b), to enhance the capabilities of WEAVE to build large source catalogues
tailored to studying specific aspects of the Milky Way’s stellar populations,
with well-controlled and reproducible target selection criteria.

Figure 6: Footprint of the WEAVE GA surveys on the sky in Galactic coordinates
$(\ell,b)$ in an Aitoff projection overlaid on the Gaia DR3 (Gaia
Collaboration et al., 2022a) density contours of stars with parallax measures,
$G<16$, and $-10^{\circ}<\mathrm{\delta}<80^{\circ}$ (in stars deg-2). HR
chemo-dynamical survey: black dots; Open Clusters survey: red stars; Disc
dynamics LR survey: dark green dots; High-latitude LR survey (shared with the
WEAVE-LOFAR and WEAVE-QSO surveys) yellow outline, where the hashed yellow
outline region shows the provisional area to be surveyed at $\sim 100$ per
cent coverage factor, while the remaining area for this survey will be
surveyed with a $\sim 30$ per cent coverage factor. Known stellar streams and
dwarf spheroidal galaxies covered within the pointed part of the High-latitude
LR survey: light green stars. The available declination range arises from the
impact of differential atmospheric refraction on targets near the edge of the
field on the typical 1-hour WEAVE-Survey OB. Near-polar targets like NGC 188
require short exposures outside of the normal WEAVE-Survey OBs. Figure 7:
Expected coverage of the WEAVE GA sub-surveys in the $(X,Z)$ plane (Cartesian
coordinates centred on the Sun’s position in the Galaxy) with the Galactic
Centre located at $(-8.5,0)$ kpc. This is based on the target-selection-scheme
footprints of each of the sub-surveys, applied to simulations performed with
GUMS (Gaia Universe Model Snapshot, Robin et al., 2012) and Galaxia (Sharma et
al., 2011), taking into account the expected uncertainties on the Gaia
astrometric data used in the target selection, estimated using the PyGaia
toolkit (for Gaia performances, see:
https://www.cosmos.esa.int/web/gaia/science-performance).

The WEAVE Galactic Archaeology surveys fall into four independent categories
listed below, according to the spectral resolution of the observations (LR or
HR) and the stellar populations or Galactic locations targeted, as illustrated
in Figures 6 and 7. These complementary surveys, together with the SCIP and
White Dwarfs surveys (see Sections 4.2 and 4.3, respectively), nearly
completely sample the Milky Way’s stellar populations accessible from the
Northern Hemisphere.

  * •

The high-latitude LR survey (‘LR-highlat’): This low-resolution wide-area
survey of 6000–8500 deg2 at high Galactic latitude will observe $1.10\times
10^{6}$ (five-year survey) to $1.55\times 10^{6}$ (seven-year survey) stars in
the magnitude range $15<r_{\mathrm{SDSS}}<20$–$21$262626Magnitudes with
subscript ‘SDSS’ refer to Sloan Digital Sky Survey magnitudes (see Albareti et
al., 2017). to complement Gaia with line-of-sight velocities and metallicities
for stars too faint for the RVS, and thereby yield the best-yet constraints on
the halo potential’s lumpiness and total mass to large distances ($\sim$100
kpc, e.g. Cautun et al., 2020), address the formation of the Galactic halo,
and characterise its progenitors (including substructures such as streams and
dwarf galaxies). The search for traces of accretion events has recently been
exemplified by Gaia’s finding of a significant merger, known as Gaia Enceladus
or the Gaia Sausage, that has been shown to make up a significant fraction of
the inner Galactic halo (e.g. Belokurov et al., 2018; Helmi et al., 2018;
Haywood et al., 2018; Myeong et al., 2019; Gallart et al., 2019; Naidu et al.,
2020; Malhan et al., 2022). A number of new cold streams have also been
discovered thanks to Gaia astrometry (e.g. Malhan et al., 2018; Ibata et al.,
2021), which anticipates the wealth of discoveries awaiting us with the
combination of Gaia and WEAVE. While the main target of this survey is the
Galactic halo, the survey will also probe the thick disc of the Milky Way,
complementing the HR survey (see section below). To achieve these goals, the
LR-highlat wide-area survey will target the following: colour- and magnitude-
selected main-sequence turnoff (MSTO) stars; red-giant-branch (RGB) stars
selected by colour, magnitude and Gaia astrometric data (parallaxes and proper
motions, to reject local red main-sequence stars from the selection);
extremely metal-poor star candidates selected from narrow-band photometry
(e.g. Pristine: Starkenburg et al., 2017), broad-band photometry (e.g. SDSSS,
Abolfathi et al., 2018) or Gaia’s on-board spectrophotometry (Gaia Data
Release 3 and onwards; see Sartoretti et al., 2022); blue-horizontal-branch
(BHB) stars and blue stragglers (using, for example, a combination of colours
including the $u$ band, e.g. Deason et al., 2012); RR Lyrae stars (e.g. Sesar
et al., 2017; Holl et al., 2018; Iorio & Belokurov, 2021; Clementini et al.,
2022); hyper-velocity stars (e.g. Brown et al., 2018; Contigiani et al., 2019;
Evans et al., 2022; Marchetti et al., 2022). This survey is foreseen to share
fibres on the sky with the WEAVE-LOFAR and WEAVE-QSO surveys (see Sections 4.7
and 4.8, respectively), and hence share the same footprint. In addition to
this wide-area component, the LR-highlat survey will drill a few deeper (down
to at least $r_{\mathrm{SDSS}}=21$) four-hour pointings (and even up to seven
hours in some specific cases) around specific objects of interest, namely
dwarf spheroidal galaxies and known stellar streams, where the multi-epoch
data will allow for the detection of binary stars to refine the line-of-sight
velocity dispersion profiles of these objects and to understand better the
dark-matter distribution (e.g. Posti & Helmi, 2019; Cautun et al., 2020). This
pointed survey is expected to cover a total of $\sim 200\,\mathrm{deg}^{2}$ on
the sky.

  * •

The disc-dynamics LR survey (‘LR-disc’): This low-resolution, high radial-
velocity accuracy survey will observe $\sim 500$ lines of sight through the
Galactic disc at very low latitudes to probe fundamental Galactic disc
dynamics processes in a volume inaccessible to Gaia alone. The survey will
observe $7\,\times 10^{5}$ (five-year survey) to $1.1\times 10^{6}$ (seven-
year survey) stars selected mainly to be red-clump giants with Pan-STARRS1
(Chambers et al., 2016) $r$-band magnitudes in the range
$15<r_{\mathrm{PanSTARRS}}<19$272727Magnitudes with subscript ‘PanSTARRS’
refer to PanSTARRS PS1 magnitudes (Tonry et al., 2012)., spanning evenly the
distance range $\sim$2–8 kpc from the Sun, or from $\sim 2$–15 kpc in
Galactocentric radius. Towards the outer disc, all RGB stars will be allowed
into the selection function. The fundamental goal of this survey is to
understand the phase-space distribution of stars of the Galactic disc, in
particular those effects associated with mergers and/or resonances of non-
axisymmetric perturbations (bar and spiral arms), both in the inner and outer
disc (see also Famaey et al., 2016). This survey is likely to provide unique
insights into the effects of various internal and external perturbers on the
secular evolution of the Galactic disc at large scales, in a similar fashion
to the local non-stationarity found locally in Gaia DR2 (e.g. Gaia
Collaboration et al., 2018b; Antoja et al., 2018; Bennett & Bovy, 2019; Bland-
Hawthorn & Tepper-García, 2021; Bovy et al., 2019; Ruiz-Lara et al., 2020).
Thanks to the Northern location of WEAVE, this survey will also provide a
unique view of the Galactic Anticentre (cf. Gaia Collaboration et al., 2021).

  * •

The HR chemo-dynamical survey (‘HR’): This high-resolution survey, covering
$\sim 4000$–5600 deg2 at intermediate and high Galactic latitudes, will target
stars with $12<G<16$ to complement Gaia’s phase-space information with full
chemical information, thereby opening the full 15+ dimensional space of the
chemo-dynamical picture of the three major Galactic populations: the thin and
thick discs and the halo. The thick discs will be sampled to large distances,
allowing us to study its connection to the thin disc and the inner halo,
together with its age-density and age-kinematics relations through mono-
abundance and mono-age groups (e.g. Mackereth et al., 2017; Mackereth & Bovy,
2020). Ancient accretions that have now dissolved in the halo and discs will
be detected chemo-dynamically through the wealth of abundance information for
different nucleosynthetic channels that will be provided by the HR spectra
(eg. Mackereth et al., 2019; Di Matteo et al., 2019). In the volume within
which Gaia parallaxes are most accurate (i.e. accurate to better than 10 per
cent), high-quality stellar parameters (effective temperature, gravity,
metallicity and [$\alpha$/Fe]) for $>7\times 10^{5}$ (five-year survey) to
$1.1\times 10^{6}$ (seven-year survey) MSTO and sub-giant stars will also
allow us to constrain stellar ages to exquisite precision (Bland-Hawthorn et
al., 2019; Kordopatis et al., 2022) within a sphere of radius $\sim 2$ kpc.
The target selection function, based on Gaia DR3 (or from later releases)
absolute magnitudes and their associated uncertainties, will include all stars
with absolute magnitudes $M_{G}<4.5$, thus also selecting intrinsically bright
targets such as RGB stars that will probe the Milky Way’s thin and thick discs
and halo to distances of $\sim 10$ kpc and beyond. Targets of special interest
will be given higher priority to ensure proper sampling. These include very
and extremely metal-poor stars with prior information on metallicity (e.g.
Pristine: Starkenburg et al., 2017), RR Lyrae (see e.g. Clementini et al.,
2022) and Cepheid (both in this HR survey, as well as in the LRdisc survey
above; see e.g. Ripepi et al., 2022) variable stars, known exoplanet hosts,
bright members of known streams (e.g. Martin et al., 2022), and stars with
high tangential velocities to increase the halo fraction in the sample (from
Gaia DR3; see e.g. Gaia Collaboration et al., 2018a; Koppelman & Helmi, 2021).

  * •

The Open Clusters survey (‘OC’): This is a survey (mostly in high resolution)
of a sizeable sample ($\sim 120$) of old and young open clusters (OCs) and
tiling three cluster-formation regions, aimed at obtaining homogeneous
information on radial velocities and chemical information. This will
complement Gaia’s superb distances and proper motions of individual stars in
known and newly discovered OCs (e.g. Castro-Ginard et al., 2020, 2022, and
references therein) up to 5 kpc from the Sun, with a precision in distance of
1 per cent up to a distance of 1.5 kpc and 10 per cent for more distant
targets, leading to an accurate and reliable definition of membership even for
the most distant objects. This survey will constrain our understanding of the
formation and disruption processes of OCs (Carrera et al., 2019b; Cantat-
Gaudin, 2022, and references therein), the chemical evolution of the Milky Way
disc using OCs as probes (Carrera et al., 2019a; Cantat-Gaudin et al., 2020;
Spina et al., 2022), and provide insights into star formation and early
stellar evolution (Valentini et al., 2019; Magrini et al., 2021).

A careful validation and accurate calibration of the derived atmospheric
parameters, measured abundance ratios and radial velocities will ensure that
the data from the aforementioned four GA sub-surveys will be on the same
scale. The WEAVE GA sub-surveys will therefore include dedicated observations
for this purpose, including the observation of globular clusters, well-studied
open clusters, field giant stars with stellar parameters derived from
asteroseismic data from the CoRoT (e.g. Anders et al., 2017) or Kepler (e.g.
Borucki et al., 2010; Kallinger et al., 2010; Yu et al., 2018) missions, and
field stars (Gaia benchmark stars, Heiter et al., 2015; Hawkins et al., 2016),
as well as stars in previous high-resolution spectroscopy studies (e.g.
Soubiran et al., 2016). This will allow us to combine the results of the GA
sub-surveys both among themselves and with several other stellar Galactic
surveys including Gaia-ESO (Gilmore et al., 2012), GALAH (Buder et al., 2021),
APOGEE (Majewski et al., 2017), 4MOST (de Jong et al., 2019) and others; see
e.g. Soubiran et al. (2022) for a comparison of metallicity determinations for
FGK stars across public data releases from several such spectroscopic surveys.

In addition to dedicated calibration fields, it is highly desirable that a
significant overlap is built between the high and low spectral resolution (HR
and LR, respectively) Galactic Archaeology surveys in order to ensure
homogeneity of the stellar parameters and individual chemical abundances that
are derived between these two instrumental modes. While calibration stars
(cluster and reference field stars observed specifically with both modes)
provide the absolute anchor to these quantities through comparisons with
independent estimates, it is desirable to build an overlap of stars surveyed
in both LR and HR that would enable us to better map the systematics between
stellar parameters and abundances derived from these two types of WEAVE
spectra. Furthermore, it is expected that it will not be possible to measure
the abundances of some elements in the restricted wavelength range of the HR
mode; among these are, for example, Nitrogen (and Carbon in the green+red HR
mode). A programme to utilise sub-optimal and/or relatively under-subscribed
observing conditions has thus been devised to observe a fraction of the GA-HR
survey observing blocks in LR mode, to be executed in weather conditions when
the main surveys cannot be executed. Thanks to the limited depth of the GA-HR
survey (12$\leq$G$\leq$16), this programme is expected to be executed in
bright time and modest seeing conditions (seeing$<$2 arcsec), thus
complementing the suite of programmes with observations suitable for the
relatively under-subscribed bright time. This GA programme will allow us: (i)
to map systematics between the LR and HR analysis of WEAVE spectra over a
large part of the Hertzsprung-Russell diagram, and reflecting the target
selection of the GA surveys, with sufficiently large statistics to allow this
mapping to be worked out with machine-learning methods (e.g. Nandakumar et
al., 2022); and (ii) to potentially build a significant sample in which the HR
suite of elemental abundances is complemented by the few elements that are
only accessible (or better measured) in LR.

###  4.2 The WEAVE Stellar, Circumstellar and Interstellar Physics (SCIP)
Survey

Figure 8: The SCIP survey footprint, mapped in Galactic coordinates, starting
from $\ell\simeq 25^{\circ}$ (top right) and ending finally at $\ell\simeq
210^{\circ}$ (bottom left), with an island of pointings that encloses the
Rosette Nebula, NGC 2264 and the Monoceros supernova remnant. The pattern of
LR pointings, covering $\sim$1200 deg2, is traced by the orange circles
representing the WEAVE field of view. In the second Galactic quadrant
($95^{\circ}<\ell<170^{\circ}$), the footprint is shifted towards positive $b$
to better track the warp of the Galactic disc. The regions to be studied
intensively using the blue and red HR gratings are outlined in blue: these are
in Cygnus (top panel, seven tailored pointings) and the Galactic Anticentre
(using the same field centres as the LR survey). Figure 9: The distribution of
potential WEAVE diffuse-ISM targets for the Rosette/NGC 2264/Monoceros
supernova remnant region, derived from IGAPS/IPHAS H$\alpha$+[NII] narrow-band
imagery (see www.igapsimages.org and Greimel et al., 2021). The planned SCIP
LR fields are superimposed as blue circles. The candidate fibre positions,
before final downsizing for configuration, are plotted on a colour scale
according to narrow-band surface brightness (in
$\mathrm{erg}\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}\,\mathrm{arcsec}^{-2}$). The
brightest diffuse structure here is the Rosette Nebula, at the lower left. In
the upper right is the NGC 2264 region, with the $\sim$220 arcmin diameter
Monoceros supernova remnant in between. Figure 10: The discovery space for the
high-resolution Cygnus component of the SCIP survey, expressed in Gaia BP
magnitudes. Known OB stars are shown in orange, while those stars available
for confirmation with WEAVE are shown in cyan after scaling down by a factor
of 0.3. These fall within the seven fields planned for the survey in the
Galactic longitude range $75^{\circ}<\ell<81^{\circ}$ (see Figure 8).

Advancements in characterising the stellar and interstellar processes
currently at work in the Milky Way are helping us to better understand the
physics that shape the appearances, content and evolutionary histories of
galaxies in the wider and younger Universe. Only locally can we easily access
individual examples of the least well-described (and often short-lived) phases
in stellar evolution down to sub-solar luminosity, and study the relationship
between stars and the interstellar medium (ISM) at maximum angular resolution.
Topics of current significance include: environmental factors in the formation
of stars (Vincke & Pfalzner, 2016; Guarcello et al., 2021); mapping the ISM in
terms of its extinction and its ionisation (Maíz Apellániz & Barbá, 2018;
Dharmawardena et al., 2022; Rathjen et al., 2021); filling in the Galactic
massive-star Hertzsprung–Russell diagram (Berlanas et al., 2020; Holgado et
al., 2020); how young stellar objects disperse into the field (Kounkel &
Covey, 2019; Wright, 2020); the approach to stellar end states via cooling
white dwarfs, X-ray and other interacting binaries (Pala et al., 2020; Inight
et al., 2021).

SCIP will use the products of the IPHAS (Drew et al., 2005; Barentsen et al.,
2014; Monguió et al., 2020) and UVEX (Groot et al., 2009; Monguió et al.,
2020) surveys executed on the Isaac Newton Telescope and (to a lesser extent)
VPHAS$+$ (Drew et al., 2014) at the VLT Survey Telescope, for much of its
target selection. Between them, these surveys map the whole Galactic plane at
$\sim$1 arcsec resolution within $|b|<5\degr$ and in five optical bands. These
photometric surveys have captured more than half a billion point sources in
the Milky Way’s main mass component. Well-tuned samples of a range of object
classes are being extracted from them for WEAVE, mostly within the magnitude
ranges $11<i<18$ and $13<B<20$. SCIP will pursue a number of scientifically
distinct goals in Galactic astronomy, permitting full use of the available
fibres in every pointing. The SCIP survey footprint is shown in Figure 8.

The SCIP survey targets the high-mass and young/old extremes of stellar
evolution along with the ISM on a large scale, capturing both the emissive,
ionised nebulous component, and the ISM seen in absorption against background
starlight. Since over half of the planned targets are relatively luminous OBA
stars, the total space volume accessed will be very large, creating the
opportunity to map Galactic disc structure out to heliocentric distances of
$\sim 10\,\mathrm{kpc}$. The $\sim 200\,000$ BA stars to be surveyed, falling
within the SCIP footprint, will provide a comprehensive account of young
stellar kinematics in the Galactic disc on combining them with Gaia proper
motions. The feasibility of this via fibre spectroscopy has been tested by
Harris et al. (2018), where it was shown that stellar parameters of sufficient
quality can be derived from the far-red calcium-triplet region of the spectrum
alone. WEAVE in its $R\sim 5000$ mode captures this region well, and is
accessible for Galactic plane extinctions up to at least $A_{V}\sim 6$. The
$\sim 20\,000$ O and B stars to be targeted will be most abundant in the first
Galactic quadrant ($\ell<90^{\circ}$). These spectra will build a more
complete picture of the Galactic massive-star Hertzsprung–Russell diagram, and
inform data-starved stellar evolution models. Parameter determination will
proceed via the methods presented by Simón-Díaz (2010), Holgado et al. (2018),
Carneiro et al. (2019) and others.

Pan-optical multi-object spectroscopy of stellar sight-lines at $R\sim 5000$
provides many types of information, including: radial velocities to precisions
of a few $\mathrm{km\,s^{-1}}$; reliable stellar effective temperatures and
surface gravities; overall metallicity and first evidence of chemical
peculiarity; detection of the more obvious spectroscopic binaries;
interstellar absorption features; a suite of nebular/circumstellar emission
line diagnostics; mass transfer/loss signatures; and markers for magnetic
activity. In WEAVE’s HR mode at $R=20\,000$, observations in the Cygnus region
and the Galactic Anticentre will push even further, from the measurement of
projected stellar rotation speeds down to lower limits, to the determination
of individual heavy-element abundances. Greater precision of radial-velocity
measurements will better expose the velocity dispersions of the local
environments. Spectroscopic binaries will be detected via careful programmes
of repeat observations.

The SCIP programme will be split between a LR survey, taking roughly two-
thirds of the SCIP survey time, and two HR components that will equally split
the remainder in bright time.

  * •

The LR survey: Around 1200 deg2 of the northern Galactic Plane will be
observed. The largest target group is comprised of the higher-mass stars: this
includes OBA stars and their evolved counterparts (including red supergiants),
emission-line objects and Cepheid variables. The aim is to densely sample most
Galactic longitudes in the thin disc accessible from La Palma. A novel feature
of SCIP’s target selection is the use of H$\alpha$ imagery from IPHAS to
identify, in addition, a web of H$\alpha$-bright diffuse-ISM positions
spanning H ii regions, supernova remnants and other nebulae (Greimel et al.,
2021, and Figure 9). These will complement the O- and B-star targets, with
spectroscopy of the associated ionised ISM. The targeted range is from surface
brightnesses of $6\times
10^{-16}\,\mathrm{erg\,cm\,^{-2}\,s^{-1}\,arcsec^{-2}}$ up to $\sim
10^{-13}\,\mathrm{erg\,cm\,^{-2}\,s^{-1}\,arcsec^{-2}}$, with full abundance
analysis achievable above 3–$5\times
10^{-15}\,\mathrm{erg\,cm\,^{-2}\,s^{-1}\,arcsec^{-2}}$. Radial velocities
will be obtained for all targets. A third focus is on the unbiased exploration
of the incidence of young stellar objects in order to understand how they
disperse across the Galactic field. The target selection draws significantly
from infrared and optical photometric surveys and Gaia astrometry, yielding
candidates of young stellar objects outside of the traditionally explored
cores of OB associations and open clusters. The majority of targets will be
selected using the method of Wilson et al. (MNRAS, submitted); for HAeBe
stars, seeVioque et al. (2020). WEAVE spectroscopy will enable definitive
classifications, as well as measures of radial velocities and accretion rates.
A minority of fibres per pointing will also be allocated to compact binaries
and white dwarfs (see also Section 4.3).

  * •

The HR surveys: These also place high priority on O, B and A stars. In the
Cygnus region, the aim is to obtain spectroscopy of prominent OB-associations
(for a recent review, see Wright et al., 2022) to build a full picture of
kinematics and elemental abundances across the region, extending the studies
first initiated by Berlanas et al. (2018). As shown in Figure 10, the
discovery potential is very large. In the Anticentre fields, the emphasis
shifts to observations of late-B and A stars, where the science aims concern
both the stars themselves and describing Galactic disc kinematics as imprinted
on this young, dynamically cold population. Measuring constraints on binarity
via repeat exposures is an important aspect in both HR programme components.
Most stars in the Universe form in binaries or higher-order multiples (Sana,
2022). In their review, Duchêne & Kraus (2013) cite studies that indicate that
$\sim$50 per cent of B and A stars have binary companions (see also Moe & Di
Stefano, 2017; Banyard et al., 2022). In the case of O stars, it has been
estimated that over half will experience binary mass exchange in their
lifetimes (Sana et al., 2013, 2014).

The normal SCIP survey planning for MOS observations assumes an upper limit to
the seeing of 1.2 arcsec, appropriate for point sources observed using fibres
of diameter 1.3 arcsec. This constraint is unnecessary for extended sources,
such as Galactic planetary nebulae (PNe), where abundance patterns as a
function of position are an important goal (García-Rojas et al., 2022). In
poor seeing conditions ($\sim$2 arcsec or more) that are unlikely to be usable
for primary science objectives of the WEAVE surveys, SCIP will exploit the
WEAVE (m/L)IFU capability to provide complete spatial chemo-dynamical maps of
a significant sample of PNe. The key science aim is to probe the physical
processes governing the late-stage evolution of intermediate-mass stars, and
to determine how the abundances and dynamics of larger PNe are governed by
their central stars (single or binary). PNe targets have been picked out that
suit the 1.3–1.5 arcmin field of view of the LIFU sampled by 2.6 arcsec
fibres. These are large, bright nebulae where, in some, the central stars are
known to be close binaries; such objects have markedly different chemical
properties to other PNe (Wesson et al., 2018). Our list includes well-known
objects that have yet to be mapped comprehensively (e.g. the Owl and Cat’s-Eye
nebulae). There will also be instances of several smaller nebulae in the field
of view, for which the mIFU mode could also be usefully utilised.

###  4.3 The WEAVE White Dwarfs Survey

Figure 11: Examples showing a variety of white dwarf spectra in the range
3850-5550 Å. White dwarfs have large surface gravities ($\log g\simeq 8$),
which result in the chemical stratification of their constituents. Most white
dwarfs have atmospheres dominated by hydrogen or helium, and their spectra
contain only Balmer (DA) or He (DB) lines. However, $\simeq 20$ per cent of
white dwarfs exhibit spectroscopic peculiarities. Accretion of disrupted
planetesimals results in photospheric contamination by metals (DAZ, DBZ, DZ),
providing a unique window into the bulk abundances of exoplanetary bodies.
Cool white dwarfs with deep convective envelopes may dredge up carbon and
oxygen from their cores (cDQ), allowing sensitive tests of stellar evolution.
A small number of hotter white dwarfs with carbon-dominated atmospheres (hDQ)
are thought to be products of white dwarf mergers, possibly descending from R
Corona Borealis stars. Finally, up to 10 per cent show magnetic fields, across
all atmospheric compositions (e.g. DAH, DBH), and serve as laboratories for
atomic physics under extreme conditions. The WEAVE Survey will target
$\simeq$100 000 white dwarfs selected from Gaia DR3 (Figure 12).

Figure 12: Hertzsprung-Russell diagram (left) and magnitude distribution
(right) of the 104 198 high-confidence white dwarf candidate targets (red,
$\mathrm{Dec}>-10^{\circ}$ and $G\leq 20$), selected from Gaia DR2 (Gentile
Fusillo et al., 2019). Synthetic photometry was computed for main-sequence
stars, sub-giants and giants (the blue lines labelled V, IV, and III,
respectively) using the spectral library of Pickles (1998) and for white
dwarfs with masses of 0.2, 0.6, and 1.0 M⊙ (black lines, top to bottom) using
the cooling sequences of Holberg & Bergeron (2006) and model spectra from
Koester (2010).

All stars born with masses $\lesssim 8$–$10\,\mbox{$\mathrm{M}_{\odot}$}$
eventually become white dwarfs (Ritossa et al., 1999; Doherty et al., 2017):
Earth-sized electron-degenerate stellar embers. The short main-sequence
lifetime of stars with $M\gtrsim 1.5\,\mbox{$\mathrm{M}_{\odot}$}$ implies
that the majority of A/F-type stars formed throughout the history of the
Galaxy are now white dwarfs. As such, white dwarfs play a central role across
a variety of areas in astrophysics. Homogeneous samples of white dwarfs with
accurate physical parameters are essential for constraining and calibrating
stellar evolution theory, including mass loss on the asymptotic giant branch
(intimately linked to the initial-to-final mass relation, e.g. Williams et al.
2009; Cummings et al. 2018), internal rotation profiles and loss of angular
momentum (Hermes et al., 2017), and fundamental nuclear reaction rates (Kunz
et al., 2002), with important implications for stellar population synthesis
and galaxy evolution theory (Maraston, 1998; Kalirai et al., 2014). Because of
their well-constrained cooling ages, white dwarfs provide an insight into the
age of the Galactic disc (Winget et al., 1987; Oswalt et al., 1996), open
clusters (García-Berro et al., 2010) and globular clusters (Hansen et al.,
2007), and can even trace variations in the Galactic star-formation rate
(Tremblay et al., 2014).

White dwarfs will be observed as flux calibrators in WEAVE Survey
observations, and the resulting spectra will provide a rich database for
white-dwarf-enabled science.

####  4.3.1 White dwarfs as flux calibrators

White dwarfs are routinely used as spectro-photometric flux standards (Bohlin
et al., 2001, 2014; Moehler et al., 2014), as their spectra are extremely
simple (i.e. pure hydrogen atmospheres in most cases) and can easily be
modelled to high precision (1–2 per cent across all wavelengths) with only two
free parameters: effective temperature and surface gravity. Moreover, in a
magnitude-limited sample, the majority of white dwarfs have effective
temperatures $\gtrsim 8000$ K, i.e. they are blue objects with significant
amounts of flux at the shortest wavelengths.

The surface density and magnitude distribution of the Gaia white-dwarf
population Pancino et al. (2012) is well-matched to the field of view of WEAVE
and the aperture of the WHT. In the MOS mode, 10–15 white dwarfs are expected
to be observed per WEAVE pointing for calibration purposes in the low-
resolution (LR) configuration. The number of suitable white dwarfs will likely
be around 3–5 for the high-resolution (HR) MOS configuration due to the
magnitude limit for this mode, dictated by the observational constraints set
by the science targets in the field, being brighter than for a LR observation
(see Section 4.3). In the mIFU and LIFU modes, constraints set by the number
and placement of the IFU(s) in the field mean that no more than one white
dwarf is expected to be observed per pointing; bootstrapping from other
exposures or other information (such as known magnitudes) are required for
these observations.

####  4.3.2 White dwarf science with WEAVE

A significant fraction of local white dwarfs are members of binaries (Toonen
et al., 2017), providing a benchmark population for investigating complex
interactions (e.g. Zorotovic et al., 2010), including the progenitors of type
Ia supernovae. Degenerate stars with precise cooling ages in wide binaries can
also be used to calibrate main-sequence ages (Rebassa-Mansergas et al., 2016).
The host stars of virtually all planetary systems – including the Sun – will
evolve into white dwarfs, and a sizable fraction of white dwarfs host the
remnants of planetary systems (Farihi et al., 2009; Koester et al., 2014).
White dwarfs that accrete tidally disrupted planetesimals display photospheric
trace metals, which provides a unique opportunity to measure the bulk
composition of extra-solar planets (Zuckerman et al., 2007; Gänsicke et al.,
2012; Xu et al., 2017). Finally, white dwarfs are laboratories of extreme
physics that are unachievable on Earth, including atomic and molecular physics
in the presence of strong magnetic fields (Guan, 2006) and high-density
plasmas (Kowalski, 2006).

The extremely large surface gravities of white dwarfs, $\log g\simeq 8$,
result in chemical stratification and atmospheric compositions dominated by
hydrogen and/or helium (e.g. Eisenstein et al. 2006; Giammichele et al. 2012).
About 20 per cent of white dwarfs display spectroscopic peculiarities,
including metal pollution from accreted planetary debris or from dredge-up of
core material, and Zeeman-splitting in magnetic fields of up to
$10^{9}\,\mathrm{MG}$, see Figure 11 for examples. Spectroscopy spanning the
full optical range is therefore critically important for the study of these
degenerate stellar remnants.

Because of their small size, white dwarfs are intrinsically faint, and
difficult to distinguish from more distant main-sequence stars with similar
colours. Consequently, most known white dwarfs that were identified as
ultraviolet-excess objects (Green et al., 1986; Liebert et al., 2005; Kleinman
et al., 2013) are young ($\lesssim 1$ Gyr) and hot
($\mbox{$T_{\mathrm{eff}}$}\gtrsim 10,000$ K), and are utterly
unrepresentative of the Galactic white dwarf population as a whole. Gaia DR2
finally provided the information necessary to break the degeneracy between
nearby white dwarfs and background main-sequence stars: accurate parallaxes.
Gentile Fusillo et al. (2019) assembled the first unbiased all-sky magnitude-
limited ($G\simeq 20$) sample of 260 000 white dwarf candidates using Gaia
DR2, followed by an updated catalogue using Gaia EDR3 data in Gentile Fusillo
et al. (2021). While the Gaia data are sufficient for identifying white dwarfs
with high confidence, follow-up spectroscopy is required (Figure 11) to
determine their physical properties (effective temperature, surface gravity,
atmospheric composition, magnetic field strength, multiplicity) and derive
fundamental properties (mass, cooling age, progenitor mass) that are necessary
to address the science areas outlined above.

WEAVE will target $\simeq$100 000 white dwarfs with $G\leq 20$ (Figure 12) for
LR spectroscopy, and will roughly triple the number of white dwarfs with high-
quality spectroscopy in the Northern hemisphere (Kleinman et al., 2013;
Gentile Fusillo et al., 2015). The WEAVE White Dwarfs survey will establish
the first large and homogeneous spectroscopic sample that is not subject to
complex selection effects, and is therefore ideally suited for detailed
statistical analyses of white dwarfs in the context of galactic, stellar, and
planetary structure and evolution. The combination of accurate Gaia parallaxes
and photometry with spectroscopic mass determinations will result in an
extremely stringent test of the mass-radius relation of white dwarfs (see e.g.
Parsons et al., 2017; Tremblay et al., 2017; Tremblay et al., 2019; Genest-
Beaulieu & Bergeron, 2019). The blue coverage (down to 3700 Å) of WEAVE is
essential for covering the higher Balmer lines, which are important
diagnostics for the surface gravity (Kepler et al., 2006), and to probe for
atmospheric pollution by planetary debris via Ca H/K absorption. Our large
sample size will result in the identification of rare white dwarf species,
tracing the extremes of parameters space of short-lived phases in their
evolution, such as stellar remnants from stars near the core-collapse boundary
(Gänsicke et al., 2010), products of thermonuclear supernovae (Vennes et al.,
2017; Raddi et al., 2018b, a; Shen et al., 2018), of as-yet not fully
understood binary interactions (Dufour et al., 2007; Kepler et al., 2016), and
possibly examples of entirely new evolutionary channels (Marsh et al., 2016).
WEAVE spectroscopy will also provide radial velocity measurements that are an
important complement to the Gaia proper motions – most white dwarfs have no
features in the Gaia RVS spectra, or are too faint for Gaia radial velocities
altogether. This first large sample of white dwarfs with full 3D kinematics
will allow us to distinguish them into their thin and thick disc, and halo
populations (Pauli et al., 2003, 2006; Anguiano et al., 2017), with an
expected $\simeq$1 per cent of halo white dwarfs. Furthermore, the kinematics
will provide constraints on the age-velocity dispersion relation, and insight
into the mass distribution of white dwarfs that formed via binary mergers
(Wegg & Phinney, 2012).

Given their low luminosity, the surface density of white dwarfs varies only
mildly, with on average $\simeq 5$ stars per square degree, or $\simeq 15$ per
WEAVE pointing. The WEAVE White Dwarfs survey is therefore multiplexed into
the entire WEAVE Survey footprint and the white dwarfs will also be used for
the spectro-photometric flux calibration of the entire WEAVE Survey (Section
4.3.1).

###  4.4 The WEAVE-Apertif Survey

Figure 13: Examples of typical galaxies to be observed in the WEAVE-Apertif
survey. Top panels: H i contours plotted over Sloan Digital Sky Survey (SDSS)
colour images. The WEAVE LIFU footprint is indicated in red over the SDSS
image. The galaxy on the left, UGC 9837, is the proto-typical example of a
regular galaxy, whereas the galaxy on the right, NGC 3769, represents a
perturbed case. Bottom panel: example of spectral instrumental setups used for
the survey. The black line shows the wavelength range of our low resolution
(LR) observations ($R\sim 2500$), while the red and green lines indicate the
wavelength coverage of the two high-resolution (HR) arms ($R\sim 10\,000$).
Shaded areas mark the location of some of the main spectral features in our
wavelength range, which can be found in absorption (blue), emission (red) or
both (green).

The detailed analysis of galaxies in the local Universe is crucial to
understanding galaxy evolution. However, recent local IFU surveys of galaxies
(e.g. CALIFA, Sánchez et al. 2012; SAMI, Croom et al. 2012; MANGA, Bundy et
al. 2015) suffer from an optical selection bias that makes it difficult to
address the effects of environment on the evolution of galaxies. The gas
content of galaxies is less strongly bound to the gravitational potential well
than its stellar content. Thus, one possible approach to study the effect of
environment on galaxy evolution is to characterise the morphology of galaxies
seen in the atomic gas, which indicates interactions that have taken place,
and to select galaxies with a range of features. To this end, a wide-area
survey of resolved neutral atomic hydrogen (H i) is needed.

####  4.4.1 Apertif and synergy with WEAVE LIFU observations

In the last decade, the Westerbork Synthesis Radio Telescope (WSRT) was
upgraded with a new receiver: an innovative focal-plane array system named
Apertif (Verheijen et al., 2008; van Cappellen et al., 2022) that allowed for
the execution of wide-field radio surveys, and H i surveys out to cosmological
distances ($z\sim 0.2$). Apertif survey operations were conducted between 2019
July 1 and 2022 February 28, surveying a large portion of the Northern
hemisphere in 1.4 GHz radio continuum, polarization, and the 21 cm spectral
line of neutral atomic hydrogen. The Apertif instrument was the first working
focal-plane array capable of full Westerbork resolution
($15\,\mathrm{arcsec}\times 15\sin\mathrm{(Dec)}\,\mathrm{arcsec}$
beam282828The factor of sin(Dec) is distinctive to WSRT as an East-West
array.) over a field of view of 6.5 deg2 per pointing, after tiling, and with
H i spectral resolution of down to 2.6 km s-1. The 300 MHz bandwidth could be
tuned to operate between roughly 1130–1750 MHz, although observations were
heavily impacted by radio frequency interference below $\sim 1300$ MHz. With
about half of the sensitivity of the original single-pixel WSRT front-end, the
wide field of view of Apertif nonetheless drastically enhanced the survey
speed of WSRT by imaging an area on the sky about 25 times the size of the
full moon in a single pointing.

Apertif conducted a two-tiered H i imaging
survey292929https://www.astron.nl/telescopes/wsrt-apertif that provides a
strong synergy with WEAVE: a medium-deep survey of 150 deg2 and a wide-area
survey of approximately 2200 deg2 between $\sim+27$ to $65$°declination (Hess
et al., in preparation). The wide-area survey targeted areas with multi-
wavelength coverage from Northern sky surveys including LOFAR, SDSS, and the
Hobby-Eberly Telescope Dark Energy Experiment (HETDEX, Hill et al., 2008) in
the Spring sky; and PanSTARRS (Chambers et al., 2016) in both the Fall and
Spring skies. The medium-deep survey targeted areas with substantial ancillary
data in the Herschel-ATLAS (Eales et al., 2010) North Galactic Pole field
(Smith et al., 2017), and a volume of particular environmental interest in the
Perseus-Pisces supercluster. Apertif released its first year of imaging data
to the community in late 2020303030https://www.astron.nl/telescopes/wsrt-
apertif/apertif-dr1-documentation (Adams et al., 2020), and the data are
described in further detail by Adams et al. (2022). A companion paper
describes the radio continuum source catalogue for DR1 (Kutkin et al., 2022).
Processing of the full survey data is currently ongoing. Together, the two
Apertif surveys are expected to yield thousands of H i-detected galaxies, of
which about 10–15 per cent have optical dimensions and a surface brightness
distribution that fit the field of view of WEAVE’s LIFU for our required
signal-to-noise ratio. The Apertif H i surveys will provide redshifts, neutral
gas content, environment densities, morphologies, kinematics and dynamical
masses at the aforementioned resolutions. Furthermore, Apertif will also
detect and measure H i absorption against radio-loud active galactic nuclei,
yielding information on gas accretion and outflows, provide spatially
resolved, extinction-free star-formation maps from the radio continuum
emission, and identify OH megamasers as locations of intense star formation
(Hess et al., 2021).

The natural goal when exploiting the synergy between a spatially resolved H i
survey and the data coming from the LIFU mode of WEAVE is to compare in detail
the cold gas properties of the H i-selected sample with the spatially resolved
properties (stellar and ionised gas) of those galaxies. WEAVE-Apertif will
study the transformation of gas into stars within galaxies by comparing the
optical and ISM properties of galaxies belonging to the so-called main star-
forming sequence, the red sequence and the intermediate region, distinguishing
whether the last represents a transition population triggered by a quenching
episode in the fairly recent past (e.g. Schawinski et al., 2014) or smoothly
evolving on timescales comparable to the age of the Universe (e.g. Casado et
al., 2015).

With this goal in mind, we will select a sample of around 400 galaxies from
the Apertif survey covering the following parameters with at least five
galaxies per bin: H i mass, H i morphology (perturbed or not), stellar mass
and star formation. The unique aspect of this survey (compared to other IFU
surveys) resides in the H i selection of targets, which enables the detailed
analysis of galaxies in different stages of the transformation process. We
will use the LIFU in both low and high spectral resolution modes to study
nearby galaxies. A large-scale integral-field survey with WEAVE creates a
strong synergy with the Apertif imaging surveys, significantly increasing the
power of both instruments as compared to each on its own.

Figure 13 illustrates the kind of regular and perturbed galaxies that we
expect to probe during the WEAVE-Apertif survey and the typical spatial
coverage of the LIFU. In addition, the bottom panel shows examples of the
spectral coverage and instrumental modes that will be used for the different
science cases planned for the survey. While the LR mode will be most useful
for emission-line morphologies/diagnostics and stellar-population studies, the
HR mode will allow us to measure the stellar kinematics of both the stars and
ionised gas with unprecedented accuracy, especially for low-mass systems.

####  4.4.2 Scientific goals of the WEAVE-Apertif Survey

In the optical, galaxies are distributed according to their colour and
luminosity (based on the colour-magnitude diagram (CMD), e.g. Bell et al.
2004) in two main groups – the red sequence and blue cloud, with a
transitioning class named the green valley. The physical processes governing
the movement of a galaxy from one group to another are still unclear, with the
following questions still open: Why is there a bimodality in galaxy properties
today, such that there are both star-forming blue galaxies and red-and-dead
galaxies? Where does the gas fuelling the star formation in these galaxies
come from? And how is star formation in galaxies regulated?

WEAVE-Apertif will analyse the spatially resolved stellar and ionised gas
properties of galaxies, sampling the CMD with respect to their H i properties
(e.g. mass and morphology) and general environment to address the possible
mechanisms that regulate the conversion of gas into stars within galaxies,
including potential rejuvenation or quenching episodes. We will establish what
processes – revealed by the Apertif H i images, such as major mergers,
accretion of small gas-rich satellites, tidal- or ram-pressure gas stripping –
have a direct impact on i) the star-formation history of galaxies, ii) the
spatial distribution of star formation in their discs, iii) the physical state
of the ionised ISM, and iv) the foreseeable evolution of galaxies. To this
aim, we will observe the full sample of $\sim 400$ galaxies in the LR LIFU
mode of WEAVE ($R\sim 2500$) with a signal-to-noise ratio per Å per spatial
bin sufficient to derive stellar population parameters. These data will allow
us to derive spatially resolved mean stellar ages and metallicities, as well
as $[\alpha/\mathrm{Fe}]$ ratios. The data will also make possible the
detection of kinematic twists, warps and decoupled components. The WEAVE-
Apertif survey will also deliver spatially resolved stellar age distributions,
ionised-gas metallicities and star-formation rates that, together with the H i
data, will provide a true legacy survey.

As the effects of the environment on the general properties of galaxies will
have been revealed by the LR dataset, a detailed study of a subsample of
galaxies using the high-resolution capabilities of the LIFU will help to
establish the effects of H i accretion history in the secular (i.e. internal)
evolution of spiral-galaxy discs (see for a review van Gorkom, 2013). The
impact of the H i accretion history on the chemo-dynamical properties of
galaxies can be quantified by analysing the stellar kinematics and stellar
properties of a sample of galaxies with perturbed H i morphologies, and
comparing them to those of galaxies with no signs of perturbed H i
morphologies. Our study will allow us to provide answers to crucial questions
such as the following: What is the process driving radial migration of stars?
What is the level of scattering and radial motions induced by spiral arms and
bars in stars? What is the impact of the accretion of satellites in the chemo-
dynamical properties of galaxies? And how does this all connect with
observations in the Milky Way, as revealed, for example, through the Galactic
Archaeology survey described in Section 4.1?

To accomplish these aims, we will analyse in detail the line-of-sight velocity
distribution (velocity, velocity dispersion and high-order moments) of a
sample of low-inclination nearby objects (including both high- and low-
surface-brightness (H/LSB) galaxies), exploiting WEAVE’s HR LIFU mode ($R\sim
10\,000$). The sample will include 30–50 galaxies with perturbed H i
morphologies and approximately the same number of galaxies with non-perturbed
H i morphologies, all taken from the parent low-resolution sample of 400
galaxies. The success of the project and its legacy impact rely on obtaining
quality data reaching $\mu_{g}=24\,\mathrm{mag\,arcsec^{-2}}$ at $\sim
3\sigma$.

Finally, these datasets will enable us to constrain the disc mass of the full
sample of 400 Apertif galaxies, allowing us to relate dynamical mass to the
environment. The dark-matter content in both HSB and LSB galaxies can be
constrained by combining kinematic data from both H i and optical data and
applying the traditional technique of rotation-curve decomposition as well as
other dynamical modelling techniques. However, the actual total mass residing
within a galaxy’s disc remains a controversial matter. This is an important
calculation to get right, as the mass-to-light ratio of a disc determines the
disc’s contribution to the galaxy’s rotation curve, which in turn constrains
the density and scale-length of the halo. The WEAVE-Apertif survey will be
able to expand on previous efforts on this topic (e.g. DiskMass survey,
Bershady et al., 2010) and constrain the surface mass density of discs using
stellar velocity dispersions, with the HR mode being of particular value to
the challenges presented by low-mass systems. The mass modelling will be
possible for the entire sample of galaxies included in the two projects
mentioned above, using different modelling techniques.

In line with many of the other WEAVE surveys, an extra programme has also been
designed within the WEAVE-Apertif survey to help maximise the use of sub-
optimal weather conditions and/or under-subscribed observing conditions, while
simultaneously extending the scientific output of the survey by relieving some
of our most stringent constraints on sample selection. Within this programme,
there will be no restrictions to stay within the Apertif footprint, and the
removal of this condition will allow for a much more flexible scheduling that
will enable us to venture into the ALFALFA survey (Giovanelli et al., 2007) to
obtain upper limit H imass estimates. In addition, we will allow for the
inclusion of highly inclined galaxies and compact ($\leq$0.2 arcmin) high-
surface-brightness systems. This will enable us to obtain observations for
certain types of galaxies that would otherwise be missed in the main survey,
for example Markarian galaxies, which usually display peculiar H iproperties.

###  4.5 The WEAVE Galaxy Clusters Survey

Figure 14: The WEAVE Galaxy Clusters survey encompasses three layers that
observe in the three instrument modes, as illustrated in this image of cluster
Abell 2142 (background image from PanSTARRS1). Layer 1 focuses on cluster-
dwarf-galaxy evolution and uses the mIFU mode centred on selected dwarf
galaxies (red circles), which will be selected from previous MOS spectra. The
zoomed-in image shows the mIFU arrangement of 37 fibres placed on a single
dwarf galaxy. Layer 2 focuses on the filamentary large scale structures that
feed galaxy clusters, and uses WEAVE’s $\sim$1000-fibre MOS mode (yellow
diamonds). Each cluster will be covered by up to 20 pointings. Layer 3 focuses
on the evolution of central cluster galaxies, and cosmological constraints
that can be drawn from cluster scaling relations. It uses WEAVE’s LIFU mode
(blue squares), highlighted in the zoomed-in image that shows a LIFU pattern
placed on a group falling into Abell 2142.

On the largest scales, matter is distributed as a vast network of filaments
and sheets that connect dense galaxy clusters and groups. This cosmic web
(Bond et al., 1996) is the environment in which galaxies form and evolve.
Relative to the small volume they occupy (much less than one per cent of the
volume of the Universe), clusters and groups contain the largest fraction of
the mass of the Universe (Cautun et al., 2012; Tempel et al., 2014; Ganeshaiah
Veena et al., 2019). Observations of galaxies in clusters are therefore of
great interest, both for a test of galaxy formation in dense environments and
as a measure of environmental influences on galaxy evolution. Consequently, in
order to make meaningful progress in our understanding of galaxy evolution, we
must consider galaxies evolving in relation to their environment, defined both
by their local density and the global large-scale structure.

Observations of cluster galaxies have firmly established the importance of the
environment in addition to galaxy mass for shaping galaxy properties:
morphology, colour, star-formation rate (SFR), stellar age and active galactic
nuclei (AGN) fraction correlate with both local galaxy density and location
inside or outside clusters (e.g. Dressler, 1980; Blanton et al., 2005; Postman
et al., 2005; Smith et al., 2005; Bamford et al., 2009). In clusters, the
fraction of early-type galaxies, defined either morphologically or by the
amount of current star formation, is much higher than in the average, lower-
density Universe, often referred to as the ‘field’. Related studies show that
cluster galaxies possess much less cold gas than field galaxies (e.g. Cayatte
et al., 1990; Serra et al., 2011) and that galaxy luminosity functions are
steeper for cluster galaxies, owing to the change in the relative contribution
of massive quiescent galaxies (e.g. De Propris et al., 2003). These well-
established findings may be summarised as manifestations of the morphology-
density relation (Dressler, 1980) and are related to the result of a series of
processes involving the mass assembly, star formation and morphological
evolution of these galaxies (see e.g. Boselli & Gavazzi, 2006). A variety of
interactions with the hot intracluster medium (e.g. Gunn & Gott, 1972; Bekki
et al., 2001; Kawata & Mulchaey, 2008; Zinger et al., 2018), as well as
interactions and fly-bys of galaxies in this medium (e.g. Gnedin, 2003;
Aguerri & González-García, 2009; Park & Hwang, 2009; Sinha & Holley-
Bockelmann, 2012; Méndez-Abreu et al., 2014) affect the gas – and often the
stellar – content of galaxies. Importantly, the accumulated effect of various
galaxy encounters and the loss of gas, ultimately resulting in the suppression
of star formation, may stimulate the transformation of late-type into early-
type galaxies in dense environments (Moore et al., 1996; Boselli & Gavazzi,
2006; Cappellari, 2013; Kuchner et al., 2017; Joshi et al., 2020).

Despite active research on this topic, it is still a matter of debate as to
which of the proposed physical mechanisms drive the evolution of galaxies in
high-density environments and their relative importance (for a review, see
Boselli & Gavazzi, 2014). The difficulty lies in disentangling subtle and
competing processes that act on different timescales. In addition,
quantification of the observed changes has mainly been focused on the end-
point in the virialized regions of clusters. However, half of a nearby
cluster’s galaxy population originates from the cosmic web (McGee et al.,
2009; Dressler et al., 2013). Clusters grow by accreting galaxies from their
surroundings, and a significant fraction of cluster galaxies have therefore
been environmentally affected long before they fall into the cluster. This
effect is called ‘pre-processing’ and is receiving increasing attention by the
community (e.g. Wetzel et al., 2013).

The morphology-density relation is particularly strong for dwarf galaxies
(Peng et al., 2010; Kovač et al., 2014; Tal et al., 2014; Wetzel et al.,
2015): early-type dwarfs (dwarf elliptical and dwarf spheroidals) dominate the
galaxy population in clusters, but they are very rarely found in the field
(Geha et al., 2012; Grossi et al., 2016). Despite their smooth appearances,
these systems are more complex than they might appear: dwarf ellipticals can
host stellar late-type features such as discs and spiral arms (Jerjen et al.,
2000; Lisker et al., 2006; Lisker et al., 2007; Janz et al., 2012), some with
clear signs of rotation (Pedraz et al., 2002; Toloba et al., 2009) and atomic
gas or dust components (di Serego Alighieri et al., 2007, 2013; Hallenbeck et
al., 2017). Star-forming dwarf galaxies accreted into a cluster are easily
stripped of their gas and dust when interacting with the cluster environment
(Grossi et al., 2015, 2016), but the variety of signatures suggests that more
than one process may determine dwarf-galaxy evolution.

Evidently, stellar mass is a dominant driver for galaxy evolution (e.g.
Kauffmann et al., 2004; Peng et al., 2010). However, intrinsic properties like
galaxy mass are highly dependent on their surrounding large-scale environment
through their assembly process. Thus, a comprehensive theory of galaxy
formation and evolution must account for the influence of the environment on
galaxy evolution while carefully controlling for stellar mass. Within this
context, several, large, multi-wavelength studies of galaxy clusters have
successfully used cluster number counts as a function of mass and redshift to
constrain cosmological models (Rozo et al., 2009; de Haan et al., 2016; Planck
Collaboration et al., 2016a, b, c; Pacaud et al., 2018). The complexity of the
physical processes acting in clusters complicates the obtaining of reliable
mass determinations, forcing us to fall back on having to rely on combining a
number of scaling relations at various wavelengths. This is still a paramount
challenge for performing precision cosmology using clusters for next
generation surveys such as the LSST, and surveys with Euclid and the Nancy
Grace Roman Space Telecope (see e.g. Salvati et al., 2020).

The WEAVE Galaxy Clusters survey was designed as a response to these
challenges. The main goals of the survey are to provide a better understanding
of the impact of different environmentally induced physical mechanisms on
galaxy evolution. Three layers focus on three complementary aspects: WEAVE
Galaxy Clusters aims to (1) gain a detailed understanding of the formation
history of low-mass dwarf galaxies in dense environments; (2) follow mass and
stellar assembly of clusters from the cosmic web through filamentary
accretion; and (3) obtain an accurate calibration of the scaling relations and
perform precision cosmology on a complete sample of Sunyaev-Zeldovich (SZ)
effect-selected clusters (Planck Collaboration et al., 2016c), and infer the
star-formation histories of the galaxies in the cores of these clusters (see
Figure 14, which summarises the three observing modes for WEAVE Galaxy
Clusters).

Figure 15: The WEAVE Nearby Cluster survey aims to obtain high-resolution
spectroscopy in both MOS and mIFU observing modes for a significant population
of dwarf galaxies in every cluster. The example in the middle panel shows
archival images for A2152 from the SDSS with two selected dwarf galaxies with
absolute magnitudes $M_{r}\sim-17$. Top panel: a one-dimensional spectrum of
an example dwarf galaxy (not shown in the image in the middle panel) is
extracted from Toloba et al. (2014). Bottom panel: two-dimensional velocity
and velocity dispersion maps of another example dwarf galaxy from Ryś et al.
(2013).

  * •

Layer 1 – WEAVE Nearby Clusters survey: Low-mass systems dominate the galaxy
population at all redshifts, and are crucial for the hierarchical build-up of
galaxies. However, the processes driving the formation of dwarf galaxies and
how the environment affects their evolution are poorly understood (e.g. Silk &
Mamon, 2012). The transformation from a star-forming to a passive dwarf galaxy
could involve ISM removal by ram-pressure, a series of tidal interactions that
kinematically heat the stellar disc and thereby modify its morphology (Gnedin,
2003; Lisker et al., 2009; Toloba et al., 2015). The main goal of the Nearby
Clusters survey is to study the properties and formation history of dwarf
galaxies in high-density environments.

Specific scientific questions addressed in this layer of the survey include:
Are dwarf galaxies primordial or the end-products of galaxy transformations?
How do dwarf galaxies contribute to ‘archaeological downsizing’ of galaxies
(e.g. Thomas et al., 2005; Nelan et al., 2005)? What processes drive the
transformation of dwarf galaxies in clusters? What are the internal dynamics
of dwarfs, are they dark-matter-dominated systems, and what are their angular
momenta? How do these processes depend on properties of the galaxies
themselves, e.g. stellar populations, metallicities, mass and star formation
history as well as their local environment?

The WEAVE Nearby Clusters survey (WCN) is a low-spectral-resolution programme
combining the MOS and mIFU modes of WEAVE (see Figure 15). WCN will observe an
X-ray flux-limited sample of 47 nearby galaxy clusters in the redshift range
$0.01<z<0.04$, covering a large range in cluster mass
($13.2<\log(M_{\ast}/\mbox{$\mathrm{M}_{\odot}$})<14.5$), corresponding to
X-ray luminosities of $\log(L_{500})\in[42,45]$. This survey will provide
single-fibre spectroscopic information for several thousands of dwarf galaxy
($M_{r}<-16$) members, thus providing the largest spectroscopically confirmed
sample of dwarf-galaxy cluster members observed so far. In addition, we will
obtain spatially resolved spectroscopic information for $\sim 1000$ dwarf
cluster members using the mIFU mode.

The cluster sample is compiled from ROSAT All-Sky Survey data: the ROSAT
Brightest Cluster Sample (Ebeling et al., 1998, BCS) and its extension
(Ebeling et al., 2000, eBCS). The BCS is 90 per cent complete for fluxes
higher than $4.4\times 10^{-12}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$ in the ROSAT
0.1–2.4 keV band. The eBCS extends the BCS down to $2.8\times
10^{-12}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$ with 75 per cent completeness. The
WEAVE field of view allows us to observe galaxies in the range $0\leq
r/r_{200}<1$–$2$ in a single pointing for the selected redshift range, where
$r_{200}$ is the cluster radius within which the mean density is 200 times the
critical energy density of the universe at that redshift. The general survey
strategy of using 1-hour observing blocks alongside dedicated, existing
photometry ensures that selected targets will have a signal-to-noise ratio
($S/N$) $\geq 5$ per Å in the galaxy spectra for galaxies with mean surface
brightness within a 1.3 arcsec aperture of $22\,\mathrm{mag\,arcsec^{-2}}$. We
thus expect to obtain LR spectra with high enough $S/N$ to obtain reliable
radial velocities for all galaxies down to $M_{r}\sim-16.0$ in our selected
clusters313131At the mean redshift of the cluster survey, the limiting
magnitude of the observations will be $M_{r}\sim-15.0$, and for the closest
ones $M_{r}\sim-14.0$.. This is equivalent to tracing the galaxy luminosity
function down to at least $M^{*}+6$ for all of the clusters, where $M^{*}$ is
magnitude of the knee of the luminosity function. The sample is statistically
large enough to study the effects of cluster properties (e.g. mass, galaxy
density, cluster velocity dispersion) on the evolution of dwarf galaxies. We
will therefore be able to explore systematic differences in dwarf galaxy
properties related to the main properties of cluster environments, together
with cluster-to-cluster variations.

Figure 16: The WEAVE Wide-Field Cluster Survey (WWFCS) will observe 20 nearby
clusters with several WEAVE pointings to cover at least $5R_{200}$. The left
side shows the observing strategy of WWFCS based on simulations from The
ThreeHundred simulations tailored to the survey (Cornwell et al., 2022). Each
blue disk represents one WEAVE pointing; the black circles are at 1 and
$5R_{200}$. This particular cluster is covered with 7 pointings, but others
require fewer or more. The right side of the figure shows filaments identified
using the software Disperse and groups in the infall region of the same
cluster simulation (see Kuchner et al. 2020 for details).

  * •

Layer 2 – WEAVE Wide Field Cluster survey: To understand the effect of
environment on galaxy evolution, we must understand processes acting during
the assembly of clusters. This includes the collapse and feeding of clusters
via filaments and groups from the cosmic web. This layer of the WEAVE Galaxy
Clusters survey concentrates on the still-to-be-explored accretion physics of
cluster infall regions. The WEAVE Wide Field Cluster survey (WWFCS) will
observe galaxy clusters out to five times their viral radius to include
galaxies in filaments directly feeding the clusters as well as infalling
groups. WWFCS will help answer important questions such as: What kind of
galaxies feed clusters? Does their fraction vary with the mass and dynamical
state of the cluster? Is pre-processing in the form of gas-specific (e.g. ram-
pressure stripping) or gravitational (e.g. galaxy-galaxy mergers) processes
significant in groups or filaments outside the cluster cores? How and where do
changes in morphology and star formation occur, over what timescales and at
which mass? How is AGN activity related to filaments? The WWFCS will observe
16–20 clusters at $z\sim 0.05$ with
$13.8<\log(M_{*}/\mbox{$\mathrm{M}_{\odot}$})<15.5$ (where $M_{*}$ is the
stellar mass of the cluster) that are drawn from the WINGS sample (Fasano et
al., 2006), making use of the availability of extensive ancillary data in the
inner region. We will obtain a complete census of galaxies in and around
clusters over a broad range of masses
($10^{9}<(M_{*}/\mbox{$\mathrm{M}_{\odot}$})<10^{11.5}$), corresponding to a
magnitude limit of $r=19.75$. To ensure uniform coverage out to at least $\sim
5R_{\mathrm{vir}}$, each cluster will be covered by between 6 and 18 MOS
pointings, each with WEAVE’s $2^{\circ}$ field of view (see left side of
Figure 16). Given the low density contrast of filaments (Martínez et al.,
2015), it is important to target the highest possible fraction of cluster and
infall region members. This will be achieved by selecting galaxies with
magnitudes, colours and, importantly, photometric redshifts. Photometric
redshifts are obtained from the 12-band photometry of the Javalambre
Photometric Local Universe Survey (J-PLUS, Cenarro et al., 2019) fields and
open-time observations using the spectral energy distribution fitting code
LePhare (Arnouts & Ilbert, 2011). Tests have shown that rejecting targets with
$\sim 3$ times the typical photometric redshift error of J-PLUS introduces at
most $\sim 3$ per cent incompleteness while increasing the number of cluster
structure members by a factor of $\sim 2$ in comparison to a pure magnitude
and colour cut. This leads to an unprecedented total of 4000–6000 structure
member galaxies per cluster, which will ensure well-sampled density fields,
crucial for mapping and characterising the filamentary structure around these
clusters (see right side of Figure 16).

The WWFCS programme will use WEAVE’s MOS mode in low spectral resolution,
covering 370–960 nm at $R\sim 5000$. The spectra will yield emission-line
diagnostics, accurate spectral breaks, and absorption line information for all
galaxies. Velocity dispersions and accurate absorption-line information will
be available for the brighter half of the galaxies individually, and
statistically for the fainter half through spectra stacking. This will shed
light on the preferred routes of mass accretion and investigate the effect
that this has on galaxy properties. Furthermore, the spectroscopic redshifts
will be crucial for identifying cluster volumes for accurate filament
extraction in redshift space (Kuchner et al., 2021). Defining filaments is a
non-trivial task due to the multi-scale and diffuse nature of the cosmic web
(Rost et al., 2021). The challenge is to map galaxy environments in sufficient
detail within, and to sufficient distance around, the dominating clusters,
where the filaments converge (see right side of Figure 16). In preparation for
this survey, we have evaluated the performance of filament-finding strategies
for WWFCS and quantified their robustness on hydrodynamical simulations of The
ThreeHundred project (Kuchner et al., 2020, 2021, 2022). The simulations are
tailored to represent WWFCS cluster observations, which will also aid the
interpretation of the survey’s findings (Cornwell et al., 2022).

Figure 17: Top panel: LIFU footprint (with three dithers) superimposed on
PanSTARRS1 imaging of cluster PSZ2-77. Middle panels: sum of simulated blue-
arm (top left) and red-arm (bottom right) data cubes from OpR3b. The circles
highlight the brightest cluster galaxy; green circles label the extraction
apertures in each data cube. A projection of the blue-arm data cube (short
wavelengths at bottom left, long wavelengths at top right) is shown diagonally
across the middle panels. Bottom panel: one-dimensional spectral extraction of
the (simulated) brightest cluster galaxy with spectral features indicated in
the plot. The blue- and red-arm spectral regions are covered by the blue and
red spectra, respectively.

  * •

Layer 3 – WEAVE Cosmological Clusters survey:

The main goal of the Cosmological Clusters survey is to study the evolution of
galaxies in the cores of clusters out to $z\sim 0.5$ and to place constraints
on cosmological parameters and global scaling relations using a complete
sample of SZ-selected clusters. The baryonic component of clusters contains a
wealth of information about the processes associated with galaxy formation,
including the efficiency with which baryons are converted into stars and the
effects of the resulting feedback processes on galaxy formation, which then
allows us to estimate the total baryon ($\Omega_{\mathrm{b}}$: e.g. Voit et
al. 2005; Borgani & Kravtsov 2011) and dark-matter densities. Moreover,
cluster abundance studies as a function of mass and redshift, $N(M,z)$, are
powerful cosmological tools (e.g. Carlstrom et al., 2002) that allow us to set
constraints on cosmological parameters such as the dark-matter and dark-energy
densities, or the equation of state of the dark energy (e.g. Vikhlinin et al.,
2009; Mantz et al., 2010). Furthermore, by selecting galaxy clusters at
$z\lesssim 0.5$, WEAVE can directly observe the evolution of the stellar
populations of galaxies in cluster cores over a cosmic epoch over which galaxy
transformations are already well documented (e.g. Dressler & Gunn, 1982;
Butcher & Oemler, 1984; Dressler et al., 1997) and which can be directly
compared with other WEAVE surveys like StePS (see Section 4.6). Specific