# Charting the Galactic acceleration field
I. A search for stellar streams with Gaia DR2 and EDR3
with follow-up from ESPaDOnS and UVES

Rodrigo Ibata Université de Strasbourg, CNRS, Observatoire astronomique de
Strasbourg, UMR 7550, F-67000 Strasbourg, France  Khyati Malhan The Oskar
Klein Centre, Department of Physics, Stockholm University, AlbaNova, SE-10691
Stockholm, Sweden  Nicolas Martin Université de Strasbourg, CNRS, Observatoire
astronomique de Strasbourg, UMR 7550, F-67000 Strasbourg, France  Max-Planck-
Institut für Astronomie, Königstuhl 17, D-69117, Heidelberg, Germany
Dominique Aubert  Université de Strasbourg, CNRS, Observatoire astronomique de
Strasbourg, UMR 7550, F-67000 Strasbourg, France  Benoit Famaey Université de
Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, F-67000
Strasbourg, France  Paolo Bianchini Université de Strasbourg, CNRS,
Observatoire astronomique de Strasbourg, UMR 7550, F-67000 Strasbourg, France
Giacomo Monari Université de Strasbourg, CNRS, Observatoire astronomique de
Strasbourg, UMR 7550, F-67000 Strasbourg, France  Arnaud Siebert Université de
Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, F-67000
Strasbourg, France  Guillaume F. Thomas Instituto de Astrofísica de Canarias,
E-38205 La Laguna, Tenerife, Spain  Universidad de La Laguna, Dpto.
Astrofísica, E-38206 La Laguna, Tenerife, Spain  Michele Bellazzini INAF -
Osservatorio di Astrofisica e Scienza dello Spazio, via Gobetti 93/3, I-40129
Bologna, Italy  Piercarlo Bonifacio  GEPI, Observatoire de Paris, Université
PSL, CNRS, 5 Place Jules Janssen, 92190 Meudon, France  Elisabetta Caffau
GEPI, Observatoire de Paris, Université PSL, CNRS, 5 Place Jules Janssen,
92190 Meudon, France  Florent Renaud Department of Astronomy and Theoretical
Physics, Lund Observatory, Box 43, 221 00 Lund, Sweden

###### Abstract

We present maps of the stellar streams detected in the Gaia Data Release 2
(DR2) and Early Data Release 3 (EDR3) catalogs using the STREAMFINDER
algorithm. We also report the spectroscopic follow-up of the brighter DR2
stream members obtained with the high-resolution CFHT/ESPaDOnS and VLT/UVES
spectrographs as well as with the medium-resolution NTT/EFOSC2 spectrograph.
Three new stellar streams that do not have a clear progenitor are detected in
DR2 (named Hríd, Víd and Gunnthrá), and seven are detected in EDR3 (named
Gaia-6 to Gaia-12). Several candidate streams are also identified. The
software finds very long tidal tails associated with the globular clusters NGC
288, NGC 1261, NGC 1851, NGC 3201, M 68, Omega Cen, NGC 5466, Palomar 5, M 5,
M 92 and NGC 6397. These stellar streams will be used in subsequent
contributions in this series to chart the properties of the Galactic
acceleration field on $\sim 100{\rm\,pc}$ to $\sim 100{\rm\,kpc}$ scales.

Galaxy: halo — Galaxy: stellar content — surveys — galaxies: formation —
Galaxy: structure

††journal: ApJ

##  1 Introduction

The distribution of matter in our Galaxy and the fundamental behavior of the
gravitational force are encoded in the three-dimensional acceleration field of
the Milky Way. If we were able to measure the acceleration field accurately,
we could assess whether the dark matter distribution is consistent with
predictions from standard Lambda Cold Dark Matter cosmology (Springel et al.,
2008), or whether alternative dark matter models (warm, fuzzy, self-
interacting, etc) explain the acceleration field more naturally. Given the
absence of a direct detection of a dark matter particle, despite decades of
searches (e.g., Schumann, 2019), it is also conceivable that the gravitational
force does not behave as predicted by General Relativity at galactic scales
(e.g., Skordis & Złosnik, 2020). Such a possibility may also be explored with
an accurate map of the acceleration field throughout our Galaxy.

It was with these goals in mind that our team embarked on a search for stellar
streams in the Milky Way. Dynamically cold stellar streams offer an
opportunity to probe the acceleration field locally over the extent of the
stream structure (Ibata et al., 2002; Johnston et al., 2002; Carlberg, 2012),
and also globally in the host galaxy throughout the spatial volume that the
progenitor satellite travelled through (Johnston et al., 1999). Such streams
are formed from stars lost to dissolving globular clusters (and perhaps very
low mass dwarf satellite galaxies) as they orbit around their host galaxy.
Internal or external processes (or a combination of both) can drive the
dissolution. Among the internal processes that dissolve clusters are
evaporation via two-body scatterings and collectively amplified fluctuations,
or ejection from three-body interactions with binary stars (e.g., Binney &
Tremaine, 2008). External processes which shorten the clusters lifetime
include tidal disruption, disk shocking and interactions with other
substructures, such as giant molecular clouds, spiral arms, the bar, dark
matter substructures, etc (Amorisco et al., 2016; Hattori et al., 2016).

In situations where the progenitors are of low mass and dissolve slowly, and
where the ejected stars are lost with low relative energy, the resulting tidal
streams follow closely the progenitor’s orbit. When the progenitors have
larger mass, the ejected stars have to overcome the internal potential well in
their journey out from the satellite, and as a consequence they can emerge
with a significantly different velocity to the progenitor (Küpper et al.,
2008). While they thus no longer follow the same orbit, the new path (and
hence the locus of the observed tidal stream structure) can be readily
calculated with N-body simulations (or with the Lagrange-point or “streak-
line” approximation, Varghese et al. 2011; Küpper et al. 2012). This procedure
can be inverted: N-body simulations can be fit to an observational
configuration to deduce the orbit of the progenitor. The orbit, of course,
gives direct access to the underlying acceleration field. This is the approach
we intend to implement in this series of papers.

The observational foundation of this study is a survey of stream-like
structures detected in the Gaia Data Release 2 (DR2) (Lindegren et al., 2018;
Gaia Collaboration et al., 2018b) and Early Data Release 3 (EDR3) (Gaia
Collaboration et al., 2020; Lindegren, Lennart et al., 2020; Riello, Marco et
al., 2020; Gaia Collaboration et al., 2018a) catalogs using the STREAMFINDER
algorithm (Malhan & Ibata, 2018). Although a number of these streams were
known before Gaia, 13 were detected with the STREAMFINDER in Gaia (Malhan et
al., 2018; Ibata et al., 2019b), and we will report several more new
detections and candidate detections below. Over the last two and a half years
the brightest stars in these streams were targeted for spectroscopic follow-up
with the high-resolution CFHT/ESPaDOnS and VLT/UVES instruments, with some
preliminary spectra obtained at medium resolution with NTT/EFOSC. The present
work is enabled by the combination of the excellent Gaia astrometry and the
excellent radial velocity measurements from ESPaDOnS and UVES.

Our aim in the present study is first to update the Gaia DR2 stream maps shown
previously in Ibata et al. (2019b, hereafter Paper I) (Section 2), and from
which the targets of the spectroscopic campaign were drawn. Deeper maps based
on the recently published Gaia EDR3 catalog are presented in Section 3. We
summarize the ESPaDOnS and UVES observational campaigns in Section 4. The
detections made with DR2 data (and followed-up with spectroscopy) are
presented in Section 5, while the new findings with the EDR3 catalog are
discussed in Section 6. Finally, in Section 7 we discuss the findings and draw
the conclusions of this study.

Throughout the remainder of this study, when referring to the proper motion
components in some spherical coordinate system (say $\alpha,\delta$):
$\mu^{*}_{\alpha}(\equiv\mu_{\alpha}\cos(\delta))$, $\mu_{\delta}$, we will
for convenience drop the asterix superscript from $\mu^{*}_{\alpha}$. All the
extinction-corrected magnitudes listed in this work were obtained by assuming
that the interstellar extinction is in the foreground, and interpolating the
reddening value using the Schlegel et al. (1998) maps, as re-calibrated by
Schlafly & Finkbeiner (2011) with $R_{V}=3.1$.

##  2 Gaia DR2 STREAMFINDER maps

The STREAMFINDER algorithm was designed to hunt for stream-like over-densities
in photometric and astrometric catalogs. This would be a relatively trivial
task if the stellar distances and radial velocities were measured in addition
to sky position and proper motion. One would simply calculate the adiabatic-
invariant actions and group together stars that are close to each other in
action space. Although Gaia provides superb proper motion measurements even
for faint stars, the parallax measurements are relatively poor for distant
populations, and radial velocities are only available for the very brightest
sources (at this time, to $G\sim 12.5$ mag). The missing distance and velocity
information make it challenging to calculate the action variables.

Gaia DR2 detections, $[1,30]{\rm\,kpc}$

Figure 1: Proper motion distribution in the Galactic longitude direction (top)
and the Galactic latitude direction (bottom) of the 5960 stars in the sample
derived from the Gaia DR2 catalog. These sources have a $>7\sigma$ likelihood
of being stream members according to the STREAMFINDER algorithm, and were used
as the basis of the spectroscopic follow-up campaign. The stellar streams are
identified by name, and include three new features labelled Hríd, Víd and
Gunnthrá. A further 11 candidate streams (marked C-1 to C-9) were selected for
observation, but additional spectroscopic information is still required for
their confirmation. The six globular clusters NGC 3201, M 68 (not labelled,
but at $\ell=-60^{\circ},b=36^{\circ}$), Omega Cen, Palomar 5, M 92 and NGC
6397 create tidal tails that are detected in these maps.

A full description of the STREAMFINDER method is given in Malhan & Ibata
(2018) and Paper I. Briefly, the algorithm circumvents the missing distance
data by testing a series of stellar populations templates, and it circumvents
the missing velocity data by scanning through heliocentric radial velocity.
The STREAMFINDER is essentially a friend-finding algorithm that examines, in
turn, every star in the input dataset. The star’s trial orbit is calculated
given the measured proper motion, the measured position, the estimated
distance (consistent with the measured photometry and the trial isochrone
model), for each radial velocity value sampled. (The measured radial velocity
is used for the small subset of stars for which the radial velocity is
actually known.)

Gaia DR2 detections, $[1,30]{\rm\,kpc}$

Figure 2: As Figure 1 (i.e. for Gaia DR2), but showing the heliocentric
distance solutions (top) found by the STREAMFINDER software, and the
corresponding metallicity of the stellar populations template (bottom).

We construct a model of the probability density function of the stream
$\mathcal{P}_{\rm stream}$, which is simply a smeared out version of the trial
orbit of length $L$ (centered on the star), with Gaussian dispersion in sky
position, distance, proper motion and radial velocity. As in Paper I, we
adopted $L=20^{\circ}$, the stream model width was chosen to be
$w_{s}=100{\rm\,pc}$ (we adopt the convention of using $w_{x}$ to denote model
dispersion and $\sigma_{x}$ to denote measurement uncertainty in a variable
$x$), and we took a stream velocity dispersion of $w_{v}=3{\rm\,km\,s^{-1}}$
in radial velocity, together with an identical dispersion in proper motion
$w_{\mu}$ (given the estimated distance $d$). We consider this stream model to
be present in addition to a “contamination” $\mathcal{P}_{\rm cont}$ due to
the normal smooth population of the Galaxy. The logarithm of the likelihood of
this stream model is thus:

${\ln}\mathcal{L}=\rm\sum_{\rm{data}}{\ln}\,[\eta\mathcal{P}_{\rm
stream}(\theta)+(1-\eta)\,\mathcal{P}_{\rm cont}]\,,$ (1)

where $\theta$ represents the stream parameters and $\eta$ is the fraction of
stars in the structure. For each trial distance (obtained from the SSP model)
the algorithm samples over the missing velocity information to find the
highest value of $\mathcal{L}$ and the corresponding value of $\eta$. This
allows us to answer the question: if there were a stream passing through the
(phase-space) location of the star under consideration, how likely is the most
likely stream model, and what fraction of stars are part of the structure?

Gaia DR2 detections, $[1,30]{\rm\,kpc}$

Figure 3: As Figure 1, but showing the sky distribution of the 685 radial
velocity measurements of the DR2 sample (top). All of the streams show
kinematic coherence, but with varying levels of contamination, dependent on
stream distance and the local contrast over the “normal” populations of the
Milky Way. In the bottom panel the different streams have been color-coded to
allow for easier visual discrimination in sky regions where they overlap.

The precise form of $\mathcal{P}_{\rm stream}$ is explained in Paper I, but
briefly it is:

$\mathcal{P}_{\rm stream}(\theta)=\mathcal{P}_{\rm
length}\times\mathcal{P}_{\rm width}\times\mathcal{P}_{\rm
LF}\times\mathcal{P}_{\rm
color}\times\mathcal{P}_{\mu}\times\mathcal{P}_{\rm\varpi}$ (2)

where $\mathcal{P}_{\rm length}=1/L$ is the uniform probability in position
along the stream, while $\mathcal{P}_{\rm width}=\mathcal{N}(\Delta s,w_{s})$
is the Gaussian probability of being at a perpendicular distance $\Delta s$
away from the stream track. $\mathcal{P}_{\rm LF}$ is the probability that the
star is drawn from the luminosity function of the adopted SSP model. $P_{\rm
color}=\mathcal{N}(\Delta(G_{\rm Bp}-G_{\rm Rp}),\sigma_{G_{\rm Bp}-G_{\rm
Rp}})$ is the Gaussian probability of the measured color offset $\Delta(G_{\rm
Bp}-G_{\rm Rp})$ from the adopted isochrone given the measured color
uncertainty $\sigma_{G_{\rm Bp}-G_{\rm Rp}}$. The proper motion probability
term $\mathcal{P}_{\mu}$ is a two-dimensional Gaussian PDF incorporating the
proper motion uncertainties in right ascension and declination and their
cross-terms (as explained in Paper I).

In contrast to the stream PDF model in Paper I, we now include the term
$\mathcal{P}_{\rm\varpi}$ in Equation 2, which accounts for the Gaussian
probability of the (inverse) trial distance to the star given the observed
parallax $\varpi$ and its uncertainty. The small $0.029$ mas parallax bias of
Gaia DR2 (Lindegren et al., 2018) is added to the stream model at this stage.

Figure 4: Improvement in astrometric parameters between DR2 and EDR3. The
proper motion distribution (a) of the sample of streams detected in DR2 is
shown with the DR2 values (red) and EDR3 values (blue). There is a very clear
reduction the proper motion scatter, which implied both that EDR3 has better
proper motion measurements, and that the stream detection are real. The
parallax values (b) also display a clearly improved scatter. (The solid
straight line shows the expected one-to-one relation).

This new inclusion of parallax in Equation 2 means that we cannot simply reuse
the empirical contamination model $\mathcal{P}_{\rm cont}$ of Paper I. We re-
create it in essentially the same way as we did in that contribution, but with
an additional parallax dimension. Briefly, we generate 1000 realizations of
the Gaia catalog in which the stellar coordinates are each shifted with a
$2^{\circ}$ Gaussian random deviate. We combine these smoothed catalogs into
sky maps (with pixels of $1.4^{\circ}\times 1.4^{\circ}$ in zenithal equal
area projection) as a function of color and magnitude (using intervals of
$0.05\,{\rm mag}$ and $0.10\,{\rm mag}$, respectively). We refer to this as
the fine spatial map $P_{\rm fine}(\alpha,\delta,G,G_{\rm BP}-G_{\rm RP})$,
which is identical to that of Paper I. In a similar way to that contribution,
we use the Armadillo software package (Sanderson & Curtin, 2016) to create
Gaussian Mixture Model (GMM) decompositions in larger $5.6^{\circ}\times
5.6^{\circ}$ spatial bins to the color-magnitude, the proper motion, and (now
new) the parallax information. We fit this five-dimensional model $P_{\rm
GMM}(\mu_{\alpha},\mu_{\delta},G,G_{\rm BP}-G_{\rm RP},\varpi|\alpha,\delta)$
using 100 GMM components, including cross-terms (the same number as used
previously in Paper I). Cutting through this model at the observed $G,G_{\rm
BP}-G_{\rm RP}$ value of each star yields the conditional probability $P_{\rm
GMM}(\mu_{\alpha},\mu_{\delta},\varpi|\alpha,\delta,G,G_{\rm BP}-G_{\rm RP})$.
The contamination model can now be calculated as:

$\displaystyle\begin{split}\mathcal{P}_{\rm
cont}(\alpha,\delta,\mu_{\alpha},\mu_{\delta},G,G_{\rm BP}-G_{\rm
RP},\varpi)=&\\\ P_{\rm fine}(\alpha,\delta,G,G_{\rm BP}-G_{\rm RP})\times&\\\
P_{\rm GMM}(\mu_{\alpha},\mu_{\delta},\varpi|\alpha,\delta,G,G_{\rm BP}-G_{\rm
RP})\,\,.\\\ \end{split}$ (3)

In our initial development of the software we realised that it was undesirable
to retain dense sources such as globular clusters in the input catalog.
Otherwise, if their proper motions are poorly constrained, we find disk-like
regions of fake high likelihood of size $\sim L$ (the stream search length
parameter). On the other hand if the cluster’s proper motions are well
measured, the central cluster regions dominate the stream signal out to a
distance $\sim L$. To avoid these undesirable effects we therefore removed all
stars in the Gaia catalogs (DR2 and EDR3) within two tidal radii of the
globular clusters listed in Harris (2010), within 7 half-light radii of the
Galactic satellites listed in McConnachie (2012), and within a $3^{\circ}$
radius of M31 and $1^{\circ}$ radius of M33. We also excised sources near the
(low-latitude) open clusters NGC 188, Berkeley 8, NGC 2204, NGC 2243, NGC
2266, Melotte 66, NGC 2420, NGC 2682 and NGC 6939 (using a suitable radius,
typically $\sim 0.5^{\circ}$).

Gaia EDR3 detections, $[3,12]{\rm\,kpc}$

Figure 5: New Gaia EDR3 map of the proper motion distribution in $\mu_{\ell}$
(top) and in $\mu_{b}$ (bottom) for the heliocentric distance range
$[3,12]{\rm\,kpc}$. The sources are filtered so that only stars that have
$>10\sigma$ likelihood of being stream members are shown. The globular
clusters NGC 3201, M 68, $\omega$Cen, M 5 and M 92 are also shown, as their
streams are visible on this map. The Anticenter Stream (marked ACS, e.g. Ramos
et al. 2020) appears in this map of thin streams, although note that the
algorithm is finding a denser enhancement in that wide structure. Figure 6: As
Figure 5, but zoomed-in to the Phlegethon stream, and using a lower $7\sigma$
threshold. This region just south of the Galactic bulge is particularly rich
in stream-like sub-structure, but it exemplifies the fact that many stream
candidates become visible as we lower the detection threshold.

Gaia EDR3 detections, $[10,30]{\rm\,kpc}$

Figure 7: New EDR3 map of the proper motion distribution in $\mu_{\ell}$ (top)
and in $\mu_{b}$ (bottom) for the heliocentric distance range
$[10,30]{\rm\,kpc}$. The sources are filtered to have $>10\sigma$ likelihood
of being stream members, and to have metallicities ${\rm[Fe/H]<-1.2}$. Figure
8: Properties of the Hríd stream. From top to bottom, the panels show proper
motion $\mu_{\alpha}$ and $\mu_{\delta}$, the parallax $\varpi$, the
heliocentric line of sight velocity $v_{h}$ and Galactic latitude position of
the stream stars, as a function of Galactic longitude $\ell$. The red points
are the 156 members of the Gaia DR2 catalog with five-component astrometric
solutions that are identified by the STREAMFINDER software as stream stars
using a $7\sigma$ threshold. The ($1\sigma$) uncertainties on these points are
shown with error bars. The points in the fourth panel mark those stream
members with measured radial velocity. As explained in the text, we fitted an
orbit model to the astrometric and radial velocity data. The corresponding
best-fit model is shown with a dashed line, and can be seen to give a
reasonable representation of the parameter profiles. Additionally, the bottom
panel also shows the $E(B-V)$ extinction map, extracted from Schlegel et al.
(1998), from which the reader may verify that the detected stream does not
follow any obvious structure in the interstellar extinction. Figure 9: As
Figure 8, but for the Gunnthrá stream. A total of 61 stars are identified as
candidate members of this structure at $>7\sigma$ confidence.

The additional slight differences with respect to the analysis of Paper I are
that here we consider distance solutions in the range $[1,30]{\rm\,kpc}$
(rather than just $[1,10]{\rm\,kpc}$), we consider SSP metallicity templates
with ${\rm[Fe/H]=-2.2,-1.9,-1.5,-1.3,-1.1,-0.7}$ (rather than
${\rm[Fe/H]=-2.0,-1.6,-1.4}$), and we process the full sky (rather than just
$|b|>20^{\circ}$). Figure 1 shows the sky distribution of the 5960 stars
selected for spectroscopic follow-up with a $7\sigma$ stream detection
threshold (an $8\sigma$ threshold was used in Paper I). As in paper I, we
adopt an extinction-corrected limiting magnitude of $G_{0}=19.5$ for the
search in DR2.

The sample shown in Figure 1 was selected to have
$\sqrt{\mu_{\ell}^{2}+\mu_{b}^{2}}>2{\rm\,mas/yr}$, so as to avoid the
contamination from low proper motion objects discussed at length in Paper I.
However, in order to avoid gaps that could be introduced by this proper motion
selection, we inspected each stream and added in any of its members with
$\sqrt{\mu_{\ell}^{2}+\mu_{b}^{2}}<2{\rm\,mas/yr}$. Sagittarius stream stars
(identified as discussed in Ibata et al. 2020a) were removed from this sample.
Furthermore, at latitudes below $30^{\circ}$, we selected features manually to
optimize the spectroscopic follow-up campaign. However, all stars (above the
chosen $7\sigma$ stream threshold) of a given stream or stream candidate were
included. Above $|b|\lower
2.15277pt\hbox{$\;\buildrel>\over{\sim}\;$}30^{\circ}$ the STREAMFINDER
detections are unambiguous, but closer to the Galactic plane the algorithm
finds high-significance stream-like correlated behavior in millions of Gaia
stars. Since we do not have the resources to follow-up those potential
detections, we took a pragmatic approach of selecting low-latitude structures
by hand that were clearly well-separated from the bulk of the Galactic
populations in the parameter space of observables (proper motion, parallax,
color and magnitude). This approach allowed us to identify the low-latitude
Hríd stream discussed below, as well as enabling us to follow previously-known
structures in dense regions close to the Galactic plane.

We expect the present stream sample to have uniform completeness for
$|b|>30^{\circ}$, but towards lower latitude the sample completeness
diminishes, as we were progressively forced to select only the most
significant structures. For $|b|<20^{\circ}$, the sample is highly incomplete,
and the selection is not remotely objective. However, the proof that the
STREAMFINDER is providing useful detections is that all of the stream
structures followed up spectroscopically to date have coherent line of sight
kinematics.

Having processed the Gaia catalog with 6 different SSP model metallicity
values, we select the solution that yields the highest likelihood, thus
providing a distance and metallicity estimate for each star. Note that the
distance is constrained by both the Gaia parallax and photometry (given the
trial SSP model). The corresponding distance and metallicity solutions are
shown in Figure 2.

##  3 New Gaia EDR3 STREAMFINDER maps

The recent publication on December 3rd 2020 of the Gaia Early Data Release 3
catalog has allowed us to re-run the STREAMFINDER algorithm on this refined
dataset. The procedure was essentially identical to that described in Section
2, with a few exceptions. In EDR3 the parallax bias appears to be non-trivial
to correct (Lindegren et al., 2020), but the offset is tiny as far as the
present work is concerned. So instead of the value of $0.029$ mas (Lindegren
et al., 2018) used in our DR2 search, we assumed a null parallax bias. As
before, we used a template of age $12.5{\rm\,Gyr}$ and the metallicity values
${\rm[Fe/H]=-2.2,-1.9,-1.5,-1.3,-1.1,-0.7}$ were probed. To speed up the
computation we undertook separate searches in two overlapping distance
intervals: $[3,12]{\rm\,kpc}$ and $[10,30]{\rm\,kpc}$. The algorithm becomes
computationally very expensive to run close to the Galactic plane, because of
the large numbers of stars that need to be examined along each orbit. Because
of this, we also decided to limit the present search to $|b|>20^{\circ}$.
Given the increased astrometric and photometric precision of the EDR3 dataset,
we also increased the limiting magnitude of the search from $G_{0}=19.5$ mag
to $G_{0}=20$ mag. We expect later contributions in this series will provide
updated maps that reach closer to the Galactic plane, and also perhaps to
deeper limiting magnitudes.

However, we begin by showing the improvement that EDR3 brings to the problem
of detecting stellar streams. In Figure 4 we have cross-identified our DR2
stream catalog with EDR3 to obtain the new proper motion values. In panel (a),
the new measurements (blue) are clearly less scattered than the old DR2 values
(red). The increased accuracy of EDR3 should thus increase the contrast of the
streams in proper motion space. Furthermore, the fact that the streams are
tighter given better data provides further confirmation that the stream
detections are real. We also show the improvement in the parallax of the
sample in panel (b), plotted against the reciprocal of the distance estimated
by the STREAMFINDER (from the DR2 data). Again there is a striking reduction
in the parallax scatter, and it can also be seen that the algorithm produces
distance estimates that are in broad agreement with the new parallax
measurements.

The new sky map derived from EDR3 for the $[3,12]{\rm\,kpc}$ distance range is
shown in Figure 5. As for the DR2 case above, we again select stars with
proper motion $\sqrt{\mu_{\ell}^{2}+\mu_{b}^{2}}>2{\rm\,mas/yr}$ and that are
not Sagittarius stream members. Stars with $10\sigma$ likelihood of being
stream members are selected. No other filtering has been applied (i.e. there
has been no manual selection as in Figure 5). The stream features previously
seen in Figure 1 are also seen here, and several new structures are also
apparent. In Figure 6, we show the effect of decreasing the detection
threshold to $7\sigma$ in a particularly busy region that skims the southern
bulge. A large number of kinematically coherent groups can be seen to be
criss-crossing this field. The full sky view of the ($10\sigma$) streams in
the $[10,30]{\rm\,kpc}$ distance window is shown in Figure 7. Again, many new
structures are revealed.

##  4 Spectroscopic Observations

To measure the radial velocities and metallicities of the DR2 sample presented
above we secured 75 hours of service-mode observation with the CFHT/ESPaDOnS
instrument (Donati, 2003) over five semesters between 2018 and 2020. We used
ESPaDOnS in the “object + sky” non-polarimetric configuration, where it
operates as a standard high-resolution spectrograph of resolution $R=68,000$
covering the wavelength range $370$ to $1,050$$\;\buildrel\circ\over{\rm A}$.
Our observing strategy was to aim for signal to noise of $S/N\sim 7$ per pixel
for the majority of the fainter targets (up to $G_{0}=17.6$), while obtaining
high signal to noise ($S/N>30$) for the brighter stars if it cost less than 20
min to do so. All the data were reduced with the Libre-ESpRIT pipeline (Donati
et al., 1997) through to extracted and wavelength-calibrated one-dimensional
spectra. A total of 163 stars were observed with ESPaDOnS up to the end of
June 2020.

A further 6 nights of observation were obtained with the UVES spectrograph
(D’Odorico et al., 2000), 3 in June 2019 and 3 in January 2020111Runs
0103.B-0568(B) and 0104.B-0406(B).. Our aim with UVES was to probe fainter
targets than can be efficiently observed with ESPaDOnS, and to reach regions
of the southern sky that are inaccessible to the CFHT. UVES was used with the
DIC2 dichroic beamsplitter in the “437+760” setting, covering
3730–4990$\;\buildrel\circ\over{\rm A}$ and
5650–9460$\;\buildrel\circ\over{\rm A}$. For increased efficiency, we used
$2\times 2$ pixel binning when reading out the CCDs, and a
$1\mbox{\mbox{${}^{\prime\prime}$}\hskip-3.00003pt.}$0 slit, which results in
a resolution of $R\sim 40,000$. As with the ESPaDOnS observations, we
calibrated the exposures to obtain sufficient signal to achieve
$1$–$2{\rm\,km\,s^{-1}}$ radial velocity accuracy for the fainter stars, while
aiming for high S/N in the brightest targets, thus enabling chemical abundance
studies. All the spectra were extracted and wavelength-calibrated with the
“esoreflex” pipeline. Despite poor weather during the June 2019 run, we were
able to measure the radial velocities of 174 stars with the UVES spectrograph
over the two runs. (A final UVES run scheduled for April 2020 has been
postponed to 2021 because of the Covid-19 pandemic).

As the UVES observations aimed to probe deeper than what we had managed to
reach with ESPaDOnS, we considered it prudent to first check the velocities of
the survey stars with a low resolution spectrograph before devoting precious
VLT time to the follow-up. To this end we secured two 6-night runs with
NTT/EFOSC2222Runs 0103.B-0568(A) and 0104.B-0406(A). immediately before each
UVES run. We used a narrow
$0\mbox{\mbox{${}^{\prime\prime}$}\hskip-3.00003pt.}5$ slit together with
grism #19 to cover the spectral region 4400–5100$\;\buildrel\circ\over{\rm
A}$, yielding spectra with a resolution of $R\sim 3000$. The EFOSC2
observations were entirely reduced with the IRAF
software333http://ast.noao.edu/data/software. Most exposures were 20 min long,
resulting in typical radial velocity uncertainties of $\sim
10{\rm\,km\,s^{-1}}$. However, we quickly realised that the stream sample did
not suffer from substantial contamination, and adapted our observation
strategy accordingly. A total of 60 stars of the stream sample have an EFOSC2
velocity measurement but no UVES velocity measurement (either due to
insufficient time to complete the follow-up or because the EFOSC2 measurement
indicated that the star was unlikely to be a stream member).

The radial velocities of all the stars in the sample observed with ESPaDOnS,
UVES or EFOSC2 were measured using the fxcor algorithm in IRAF, by cross-
correlation against the spectrum of the radial velocity standard star HD
182572.

We complemented these velocities with measurements taken from public
spectroscopic surveys. We cross-matched the stream sample against APOGEE-2
(Majewski et al., 2017), finding 16 in common; we find 2 stars in common with
the GALAH DR3 survey (Buder et al., 2018); 25 in common with the Gaia Radial
Velocity Spectrometer (RVS) catalog; 80 in common with the LAMOST DR5 survey
(Cui et al., 2012); 4 in common with the Radial Velocity Experiment (RAVE DR5)
(Kunder et al., 2017); and 150 in common with SDSS/Segue survey (Yanny et al.,
2009). A further 7 stars are present in the kinematic survey of Palomar 5 by
Odenkirchen et al. (2009), and 22 stars in the survey of Ibata et al. (2017).

In those cases with multiple velocity measurements, we adopt the velocity
value with the lowest uncertainty. In this way, a total of 685 stars from our
sample of 5960 stream candidate stars (i.e. 11.5%) end up having velocity
measurements. Of these, 264 have velocity uncertainties
$\sigma_{v}<1{\rm\,km\,s^{-1}}$, 355 have $\sigma_{v}<2{\rm\,km\,s^{-1}}$, 393
$\sigma_{v}<3{\rm\,km\,s^{-1}}$, 491 have $\sigma_{v}<5{\rm\,km\,s^{-1}}$, and
602 have $\sigma_{v}<10{\rm\,km\,s^{-1}}$. The stars with measured velocities
are displayed in color in Figure 3a. Each marked stream shows up as grouping
of stars of similar radial velocity, or as a feature with a position-dependent
velocity gradient. In Figure 3b we have assigned different colors to the
various streams to allow the reader to distinguish the individual structures
more easily.

The STREAMFINDER catalog of 5960 sources derived from Gaia DR2, but with
updated Gaia EDR3 information is provided in Table 1. The radial velocity
information is also listed. The catalog of new detections in EDR3 will be
published when the full search is completed (the low-latitude disk and bulge
region take millions of CPU hours of computation).

Table 1: The first 10 rows of the STREAMFINDER catalog of 5960 stars detected in Gaia DR2, with updated EDR3 astrometry. EDR3 ID | $\alpha$ | $\delta$ | $\varpi$ | $\mu_{\alpha}$ | $\mu_{\delta}$ | $G_{0}$ | $(G_{BP}-G_{RP})_{0}$ | $d_{SF}$ | $v_{h}$ | $\delta v_{h}$ | s | S
---|---|---|---|---|---|---|---|---|---|---|---|---
| ∘ | ∘ | ${\rm\,mas}$ | ${\rm\,mas/yr}$ | ${\rm\,mas/yr}$ | mag | mag | ${\rm\,kpc}$ | ${\rm\,km\,s^{-1}}$ | ${\rm\,km\,s^{-1}}$ |  |
4976492500371673600 | 0.555095 | -50.846349 | -0.155 | 8.388 | -4.010 | 19.452 | 0.668 | 9.333 |  |  |  | 1
4973150156822139264 | 1.515135 | -51.976175 | 0.174 | 6.889 | -3.500 | 18.507 | 0.697 | 11.940 |  |  |  | 1
4973592611470354688 | 1.773110 | -50.834348 | 0.140 | 7.972 | -3.240 | 19.180 | 0.582 | 10.044 |  |  |  | 1
2429494258672369152 | 1.875748 | -8.429741 | -0.075 | 0.264 | 0.520 | 19.235 | 0.794 | 7.041 |  |  |  | 2
4973559626121475968 | 2.854625 | -50.733300 | 0.430 | 7.136 | -3.877 | 19.429 | 0.574 | 10.274 |  |  |  | 1
4973378962616486912 | 3.111449 | -50.615773 | 0.121 | 7.975 | -3.357 | 17.038 | 0.897 | 11.033 | 27.900 | 0.430 | 13 | 1
2424367510829683456 | 3.216603 | -12.313105 | -0.002 | 0.174 | -0.165 | 18.903 | 0.659 | 7.205 |  |  |  | 2
2424399499746804608 | 3.824391 | -12.055727 | -0.147 | -0.136 | 0.055 | 18.853 | 0.706 | 6.833 |  |  |  | 2
2424146577712223616 | 3.963230 | -12.643203 | 0.041 | -0.078 | 0.012 | 17.946 | 0.622 | 6.922 |  |  |  | 2
2417215325130569216 | 4.717180 | -13.883024 | -0.178 | -0.763 | -0.479 | 19.269 | 0.659 | 6.833 |  |  |  | 2
Figure 10: As Figure 8, but for the Víd stream stream. A total of 46 stars are
identified as candidate members of this structure at $>7\sigma$ confidence.
Figure 11: As Figure 8, but for the NGC 6397 stream. A total of 276 stars are
identified as candidate members of this structure at $>7\sigma$ confidence.
The globular cluster itself is represented with a star symbol, and the dotted
line corresponds to the orbit of this object, as obtained by integrating using
Gaia DR2 proper motions and position, the radial velocity and the heliocentric
distance listed in Vasiliev (2019). The Galactic potential model #1 of Dehnen
& Binney (1998) was used here. Figure 12: As Figure 11, but for the NGC 3201
stream. A total of 388 stars are identified as candidate members of this
structure at $>7\sigma$ confidence.  Figure 13: As Figure 12, but showing a
larger area of sky to encompass the Gjöll stream. The orbit shown is not a
fit, but is rather simply the extrapolation from the measured phase-space
values of NGC 3201. The excellent correspondence in position, distance, proper
motion and radial velocity indicates that Gjöll is the trailing stream of NGC
3201.

##  5 Gaia DR2 Detections

We will first discuss the 32 streams or stream candidates detected in Gaia
DR2, and presented in Figures 1–3 and listed in Table 1. Some of these streams
were previously known, or were announced over the course of our spectroscopic
follow up campaign: GD-1 Grillmair & Dionatos (2006), Orphan (Grillmair,
2006), Palomar 5 (Odenkirchen et al., 2001), Jhelum (Shipp et al., 2018), C-1
(which turns out to be the M 5 stream Grillmair 2019), and M 92 (Thomas et
al., 2020; Sollima, 2020). The candidate streams C-1 to C-9 are structures
that appeared real given extant astrometry and photometry, but which we could
not confirm with complete confidence due to inexistent or insufficient follow-
up observations. However, as we shall see below, EDR3 data has clarified the
nature of some of them. The new stream structures presented in this section
are the features named Hríd, Gunnthrá and Víd (following our previous
nomenclature of adopting the names from Norse mythology of the streams of the
gaping abyss that existed before the beginning of the world). We will also
discuss the streams emanating from the globular clusters NGC 6397 and NGC
3201.

###  5.1 Hríd

Hríd is a low latitude stream that is very challenging to detect because it
lies superposed on dense regions of the disk. We detect 156 members of this
structure in Gaia DR2, and have 24 radial velocity measurements along it,
including 8 UVES and 14 ESPaDOnS high precision measurements. Its properties
are displayed in Figure 8, where we show, as a function of Galactic longitude,
the proper motion $\mu_{\alpha}$ (panel a), the proper motion $\mu_{\delta}$
(b), the parallax $\varpi$ (c), the heliocentric radial velocity $v_{h}$ (d)
and its trend with Galactic latitude (e). We are able to follow the structure
over $62^{\circ}$ until it becomes lost in the high extinction at $b\sim
10^{\circ}$. The distance estimates provided by the STREAMFINDER algorithm
suggest that it lies at a heliocentric distance of $\sim 3.1{\rm\,kpc}$.

We fit an orbit to the astrometric and radial velocity data in the Dehnen &
Binney (1998) Milky Way model ‘1’ using a simple simplectic leapfrog
integrator (the fitting procedure is identical to that outlined in Ibata et
al. 2018). The best fit orbit (dashed line) can be seen to follow closely the
trends in each of the observed parameters. This orbit is extremely radial in
the Dehnen & Binney (1998) potential: it has a pericenter of only
$0.86{\rm\,kpc}$ and an apocenter of $34.0{\rm\,kpc}$. A dynamical study of
this object could be interesting as it is hard to understand how it could have
survived as a coherent structure until the present day.

The most likely metallicity found by the STREAMFINDER is ${\rm[Fe/H]=-1.1}$,
which agrees well with the spectroscopic value of ${\rm[Fe/H]=-1.05\pm 0.04}$
from one star in LAMOST and ${\rm\langle[Fe/H]\rangle=-1.13\pm 0.04}$ from two
stars in SEGUE. Thus it does not appear to be typical of the halo population.
We will examine the constraints on the abundance distribution of this stream
derived from the high-resolution spectroscopy in a subsequent contribution.

###  5.2 Gunnthrá

At first sight the $20^{\circ}$-long Gunnthrá feature appears to be a
continuation of the prominent retrograde Phlegethon stream (Figure 5), as its
stars have large proper motions that seem to continue the trend discerned in
Phlegethon. However, the radial velocity of the member stars (Figure 9d) are
more than $200{\rm\,km\,s^{-1}}$ lower than the trend observed in Phlegethon
(also listed in Table 1).

All the spectroscopy of this stream is derived from the EFOSC2 spectrograph,
and the spectra were not of sufficient quality to allow a metallicity
measurement to be made. However, the STREAMFINDER finds the highest likelihood
solutions with a metal-rich template of ${\rm[Fe/H]=-0.7}$. With this template
the structure is estimated to be at a distance of $\sim 3{\rm\,kpc}$. The
fitted orbit has a pericenter of $4.8{\rm\,kpc}$, an apocenter of
$7.6{\rm\,kpc}$ and a maximum disk height of $4.0{\rm\,kpc}$, so it appears
thick disk-like, but retrograde. This orbit is also quite diffefrent to that
of Phlegethon (pericenter of $19.8{\rm\,kpc}$, and apocenter of
$4.9{\rm\,kpc}$). Thus the alignment on the sky and in proper motion is a
coincidence.

###  5.3 Víd

The Víd stream (Figure 10) is located close to the southern Galactic pole
where we follow it over an $18^{\circ}$-long arc (Figure 10). Excellent
ESPaDOnS (5 stars) and UVES (5 stars) radial velocities show a large
$69{\rm\,km\,s^{-1}}$ gradient from one side of the structure to the other.
The highest likelihood solution suggests a metallicity of ${\rm[Fe/H]=-1.9}$
and a corresponding distance of $\sim 24{\rm\,kpc}$. Víd lies superimposed on
the Sagittarius stream, but has a perpendicular track to Sagittarius. The best
fit orbit has a pericenter of $15.1{\rm\,kpc}$ and an apocenter of
$50.1{\rm\,kpc}$.

###  5.4 NGC 6397

One of the highlights of the search is the detection of the stellar stream of
the nearest globular cluster NGC 6397 (Figure 11). The software finds stars
from this cluster spread along an $18^{\circ}$-long arc in that extremely low
latitude field. Our spectroscopic follow-up of 8 stars with UVES confirmed the
velocity membership, while of the 5 other stars found in public spectroscopic
surveys only 1 is a velocity non-member.

As far as we know, this is the first detection of an extended tidal stream
associated to this nearby ($d_{\odot}=2.3{\rm\,kpc}$, Harris 2010), old
(age$=12.6\pm 0.7{\rm\,Gyr}$, Correnti et al. 2018) and metal-poor
(${\rm[Fe/H]=-2.1}$, Husser et al. 2016) globular cluster. The cluster is not
included in the list by Piatti & Carballo-Bello (2020). Leon et al. (2000)
reported the detection of extra-tidal stars within $150\arcmin$ from the
center of the cluster, with an hint of a tail reaching $\pm 100\arcmin$ from
the center, approximately oriented along the E-W direction, in broad agreement
with our findings. More recently Kundu et al. (2020) used Gaia DR2 data to
identify a handful of extra tidal stars within $5.0^{\circ}$ of the cluster
center. On the other hand, the stream detected here, albeit irregular, extends
$18^{\circ}$ on the sky.

###  5.5 NGC 3201 and Gjöll

NGC 3201 is also a low-latitude globular cluster that is suffering tidal
stripping (Bianchini et al., 2019). The STREAMFINDER is able to identify a
$12^{\circ}$-long feature in the immediate vicinity of the cluster, as we show
in Figure 12. Unfortunately, we obtained only 4 velocity measurements in this
region, 2 data from Gaia RVS and 2 UVES measurements. Of these 4 stars only 2
were confirmed as bona-fide members (panel d), but together with the
astrometric data they constrain the orbit very well. Figure 13 shows the
result of extrapolating this orbit towards lower Galactic longitude $\ell$. We
find an excellent correspondence with the Gjöll stream, previously detected in
Paper I. No additional fitting was performed. Clearly, Gjöll is the tidal
stream of this globular cluster, spanning along a $98^{\circ}$-long track on
the sky. As pointed out already in Paper I, this stream is retrograde.

##  6 Gaia EDR3 Detections

The EDR3 maps (Figures 5–7) extend most of the streams seen in the DR2 maps
(Figures 1–3). Several previously-known structures now come into view which
were not present in Figure 1: the M 5 stream (Grillmair, 2019), the Anticenter
Stream (ACS, Rocha-Pinto et al. 2003), the Indus stream (Shipp et al., 2018)
and the Phoenix stream (Balbinot et al., 2016). Seven new clear streams are
also detected, which we name Gaia-6 to Gaia-12 (having ran out of names for
Norse streams of the underworld). We also identify three candidate stream
structures (C-10 to C-12), which appear plausible but require confirmation
with follow-up observations.

Figure 14: Properties of the new streams Gaia-6 to Gaia-12 found in EDR3. The
columns show, from left to right, the track of the stream on the sky, the
distance profile as a function of Galactic longitude as estimated by the
STREAMFINDER, the proper motion gradient in $\mu_{\alpha}$ (red) and
$\mu_{\delta}$ (cyan), and the color-magnitude distribution of the group.

Before proceeding to discuss the new detections, we note briefly that of the
candidate structures marked in Figure 1, we find that C-1 is a stream
associated to M 5 (Grillmair, 2019), while C-2, C-3 and C-6 are clearly
confirmed thanks to the excellent EDR3 data and are now named Gaia-9, Gaia-8
and Gaia-11, respectively. The position, estimated STREAMFINDER distance,
proper motion and photometric properties of the new stream discoveries are
summarized in Figure 14.

###  6.1 Gaia-6

Gaia-6 is a $22^{\circ}$-long feature that lies in the northern Galactic sky
and appears strongest in the $[3,12]{\rm\,kpc}$ maps. The orbital solutions
provided by the algorithm imply that it is situated at a distance of
$d_{\odot}\sim 8{\rm\,kpc}$. The very coherent proper motions in both
$\mu_{\alpha}$ and $\mu_{\delta}$, the clearly defined color-magnitude
distribution, and the fact that the object is detected at $15\sigma$ make this
an unambiguous detection.

###  6.2 Gaia-7

Gaia-7 is a short stream that extends $14^{\circ}$ in length. This stream is
considerably closer to us, $d_{\odot}\sim 5{\rm\,kpc}$, and it seems to
possess negligible distance gradient. We find Gaia-7 to be highly coherent in
proper motion, with a large magnitude of $\mu_{\alpha}$ that renders it easily
distinguishable from the contamination. Given that this stream appears of
highest contrast with the template model of ${\rm[Fe/H]=-0.7}$, it may well be
that the progenitor of this stream was not a halo system. The highest
likelihood regions of Gaia-7 are detected at $18\sigma$.

###  6.3 Gaia-8

Gaia-8 is another very interesting northern stream for which STREAMFINDER
finds a large number of stars ($>1000$ sources) over its $37^{\circ}$ length.
In the highest contrast regions it is detected at $14\sigma$ with the
${\rm[Fe/H]=-1.5}$ template. This stream lies at a distance of $d_{\odot}\sim
7{\rm\,kpc}$, and has a high measured proper motion value with an average of
$\sim 12.5{\rm\,mas/yr}$.

###  6.4 Gaia-9

Gaia-9 runs for $40.5^{\circ}$ roughly parallel to the Galactic plane. It
appears as a thick stream in Figure 5, partially due to its proximity,
although it possesses a significant distance gradient ranging from
$d_{\odot}\sim 3$–$7{\rm\,kpc}$. This distance solution is based on a metal
poor template of ${\rm[Fe/H]=-1.9}$. The tight proper motion profiles, as well
as the color-magnitude distribution and $14\sigma$ detection, make this stream
unambiguous.

###  6.5 Gaia-10

Gaia-10 is the farthest system that we have discovered in the present study,
and it ranges from $d_{\odot}\sim 12-18{\rm\,kpc}$ according to the preferred
template of metallicity ${\rm[Fe/H]=-1.9}$. This $17.6^{\circ}$-long system is
oriented almost perpendicular to the “GD-1” stream on the sky. We also detect
several red giant branch stars for this stream (as can be seen from its color-
magnitude distribution). The highest contrast regions of the structure are
detected with $20\sigma$ confidence.

###  6.6 Gaia-11

Gaia-11 is a $19^{\circ}$-long and extremely narrow structure, found at a
distance of $d_{\odot}\sim 11{\rm\,kpc}$ with the ${\rm[Fe/H]=-1.9}$ template.
Given its narrowness, we suspect the progenitor to be a globular cluster. The
color-magnitude distribution of this stream is less well-defined than the
other streams, but it is nevertheless detected at the $12\sigma$ confidence
level. Future spectroscopic information should better inform us about the true
nature of this stream.

###  6.7 Gaia-12

We find Gaia-12 as a $14^{\circ}$-long stream in the southern Galactic sky.
With the preferred ${\rm[Fe/H]=-1.9}$ metal-poor template, the system lies at
$d_{\odot}\sim 11{\rm\,kpc}$, but with a clear distance gradient. Despite the
low number statistics, the detection is unambiguous at $13\sigma$.

In future, we hope to undertake a careful investigation of the kinematics,
stellar population and chemistry of these new “Gaia 6–12” streams, along with
many other candidates that can be conspicuously seen in Figures 5–7.

Figure 15: Globular clusters with long tidal tails detected by the
STREAMFINDER. The panels have the same layout as Figure 14. Figure 16:
Continuation of Figure 15.

###  6.8 Globular clusters

In the Gaia DR2 maps (Figure 1), we found clear streams emanating from NGC
3201, Omega Cen, NGC 6397, M 68, Palomar 5 and M 92. The new processing of
EDR3 maps also show long streams that are very likely associated to NGC 288,
NGC 1261, NGC 1851, NGC 5466 and M 5. In Figures 15 and 16 we display all
these structures (except those that are shown above or that lie at
$|b|<20^{\circ}$) derived from the Gaia EDR3 catalogs and that are identified
as $>7\sigma$ in the range $[3,12]{\rm\,kpc}$ and $10\sigma$ for the range
$[10,30]{\rm\,kpc}$ by the algorithm. These clusters have good matches in
position, distance and proper motion with extended streams or fans that were
found by the STREAMFINDER. In some cases, the stream is highly structured and
further analysis and modelling is required to understand the structure and to
disentangle the stream actually associated with the cluster from other
unrelated structures that may cross that spot of sky just by chance. We defer
the detailed analysis of these complex structures to a later contribution.

For a comparison with the literature, in addition to our own search on each
individual cluster, we took advantage of the recent work by (Piatti &
Carballo-Bello, 2020, P20 hereafter), who scanned the literature on globular
cluster tidal tails and extra-tidal stars, providing a final list of 53
clusters with reliable information on the presence or absence of extra-tidal
stars. These authors classify the listed clusters into three groups: G1 for
clusters with tidal tails, G2 for clusters with extra-tidal features, and G3
for clusters with no reported sign of extra-tidal stars in spite of dedicated
studies. A systematic study on globular cluster tidal tails, based on Gaia DR2
data has been recently published by (Sollima, 2020, S20 hereafter). This is a
useful reference study to compare with, but only in the vicinity of the
cluster, as the S20 analysis is limited to a circular area of radius
$5^{\circ}$ around the considered clusters.

NGC 288: Shipp et al. (2018) present a possible detection for this cluster
based on deep Dark Energy Survey (DES) photometry. The extra-tidal feature
found by these authors extends over $\sim 8^{\circ}$, oriented along the
proper motion vector in the innermost 2–3∘ and in the North-South direction
beyond. In comparison, we trace the stream over $>10^{\circ}$ degrees,
approximately in the direction SE–NW, nicely aligned with the simulated orbit
presented by Shipp et al. (2018). Additional detections within
$2.0^{\circ}$–$5^{\circ}$ from the cluster center have been reported by Leon
et al. (2000), Kaderali et al. (2019), and S20. NGC 288 is classified as G1 by
P20.

N1261: The cluster is classified as G2 by P20, based on the detection of an
extra-tidal envelope out to $r\simeq 0.5^{\circ}$ from the cluster center by
Kuzma et al. (2018). Shipp et al. (2018) report the possible detection of a
$\sim 5^{\circ}$ long tail, with similar orientation to the stream detected
here. A photometric detection on small ($<1^{\circ}$ deg) scale was reported
by Leon et al. (2000).

N1851: Shipp et al. (2018) discussed the possible detection of a $\sim
8^{\circ}$-long tail, approximately oriented as the inner part of the S-shaped
tail detected here (as seen in equatorial coordinates). The detection of an
extra-tidal envelope on the scale of $\lesssim 2^{\circ}$ was presented by
Olszewski et al. (2009) and Kuzma et al. (2018). Possible spectroscopic
detections were reported by Sollima et al. (2012) and Kunder et al. (2014).
The cluster is class G1, according to P20.

M68: Palau & Miralda-Escudé (2019) associated this cluster with the Fjörm
stream, providing a detection, based on Gaia DR2, fully consistent with that
presented here. The cluster is P20 class G1, and the stream was detected also
by S20.

M5: Grillmair (2019), using Gaia DR2 data, detected an extended tail in
excellent agreement with our finding. P20 class G1.

NGC3201: Very recently Palau & Miralda-Escudé (2020) detected the stream
around the cluster and the associated Gjöll stream, in full agreement with our
results. However, their detection of Gjoll is more extended than ours,
reaching $(\alpha,\delta)=(40,+20)$ instead of $(\alpha,\delta)=(68,0)$. These
authors suggest that the gap between the two branches is due to an high
extinction region intervening along the line of sight. A good chemical match
between the cluster and Gjöll has been found by Hansen et al. (2020), based on
the abundance analysis of four kinematic members of the stream. See also
Bianchini et al. (2019), S20 and Kunder et al. (2014) for previous additional
detections on small scales or based on small samples.

##  7 Discussion and Conclusions

We have embarked on a large program to detect and characterize the stellar
streams of the Milky Way so as to finally employ them as a means to constrain
the properties of dark matter and compare the merits of different theories of
gravity. The first step of this endeavor was the development of an optimized
search tool, the STREAMFINDER (Malhan & Ibata, 2018) to hunt through
astrometric datasets. Initial results of the application of that algorithm to
Gaia DR2 were reported in Malhan et al. (2018) and Ibata et al. (2019b).

In this contribution we first presented the stream sample (Figure 1) that we
constructed as the foundation for a high-resolution spectroscopic follow-up
campaign conducted from 2018–2020 with CFHT/ESPaDOnS in the northern
hemisphere and ESO/UVES in the south. The source catalog of 5960 stars, along
with the radial velocity measurements for 685 of this set, are provided in
Table 1. We find that all of the detected streams have coherent kinematics, as
expected.

We have taken the opportunity of the very recent publication of the EDR3
catalog of the Gaia mission to update further our view of the ancient
accretions onto the Milky Way. EDR3 provides a significant improvement in
astrometric quality over DR2, which causes streams to be tighter and hence
higher-contrast features in the parameter space of observables (Figure 4).

We find ten new streams without an obvious progenitor (3 in our DR2 search,
and 7 additional ones in the EDR3 survey), ranging in distance from $\sim
3{\rm\,kpc}$ to $\sim 20{\rm\,kpc}$. Our choice to filter sources with
$\sqrt{\mu_{\ell}^{2}+\mu_{b}^{2}}<2{\rm\,mas/yr}$ undoubtedly biasses us
against finding very distant structures. The filter was applied to avoid
contamination from the numerous sources that appear close to being at rest. We
have already upgraded the algorithm (Ibata et al., 2020b) to be able to use
ancillary multi-band photometric data (such as from Pan-STARRS), which may
allow us to avoid the proper motion filter and thus search for more distant
streams with our technique. This will be investigated in future work.

Since the pioneering work of Leon et al. (2000), an ever-increasing list of
globular clusters have been found to possess tidal tails (e.g., Odenkirchen et
al. 2001; Ibata et al. 2019a; Palau & Miralda-Escudé 2019; Bianchini et al.
2019; Thomas et al. 2020; Sollima 2020). The present work reveals that at
least 11 clusters have exceedingly long tidal tails with numerous member stars
measured with exquisite astrometric precision in the Gaia DR2 and EDR3
catalogs. These structures are clearly ideal candidates for probing the
Galactic potential, especially since it will be possible to anchor their
absolute distances to excellent accuracy, and the relative distances can also
be measured very well because the cluster provides a perfect stellar
populations template for the stream. They are (in order of increasing right
ascension of the remnant cluster): NGC 288, NGC 1261, NGC 1851, NGC 3201, M
68, Omega Cen, NGC 5466, Palomar 5, M 5, M 92 and NGC 6397 (the closest
globular cluster). Studying how these clusters managed to survive until the
present day in the complex environment of the Milky Way may place interesting
constraints on the dynamical evolution of the clusters and the mass
distribution of their host.

Gaia has opened up a spectacular vista onto the Milky Way’s halo, revealing a
lacework of criss-crossing ancient and ongoing accretions that testify to the
violent formation history of our Galactic home. As we turn from the halo
towards the inner Galaxy, the weave becomes yet more complex, resembling a
ball of wool (Figure 6). It will clearly take some effort to disentangle these
structures, but now, finally with Gaia such an endeavor is becoming possible.
In future contributions in this series, we will attempt to decipher this rich
trove, presenting the detailed chemical abundances of the streams, developing
N-body models of each structure, and performing a conjoint analysis of their
constraints on the large- and small-scale Galactic acceleration field.

RI, NM, DA, BF, GM and AS acknowledge funding from the Agence Nationale de la
Recherche (ANR project ANR-18-CE31-0006, ANR-18-CE31-0017 and
ANR-19-CE31-0017), from CNRS/INSU through the Programme National Galaxies et
Cosmologie, and from the European Research Council (ERC) under the European
Unions Horizon 2020 research and innovation programme (grant agreement No.
834148). FR acknowledges support from the Knut and Alice Wallenberg
Foundation. MB acknowledges the financial support to this research by INAF,
through the Mainstream Grant 1.05.01.86.22 assigned to the project ?Chemo-
dynamics of globular clusters: the Gaia revolution? (P.I. E. Pancino). We
gratefully acknowledge the High Performance Computing centre of the Université
de Strasbourg for a very generous time allocation and for their support over
the development of this project. This work has made use of data from the
European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia),
processed by the Gaia Data Processing and Analysis Consortium (DPAC,
https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has
been provided by national institutions, in particular the institutions
participating in the Gaia Multilateral Agreement. Based on observations
obtained at the Canada-France-Hawaii Telescope (CFHT) which is operated by the
National Research Council of Canada, the Institut National des Sciences de
l’Univers of the Centre National de la Recherche Scientique of France, and the
University of Hawaii. Based on observations collected at the European Southern
Observatory under ESO programmes 103.B-0568(A), 103.B-0568(B), 0104.B-0406(A)
and 0104.B-0406(B). Funding for SDSS-III has been provided by the Alfred P.
Sloan Foundation, the Participating Institutions, the National Science
Foundation, and the U.S. Department of Energy Office of Science. The SDSS-III
web site is http://www.sdss3.org/. SDSS-III is managed by the Astrophysical
Research Consortium for the Participating Institutions of the SDSS-III
Collaboration including the University of Arizona, the Brazilian Participation
Group, Brookhaven National Laboratory, Carnegie Mellon University, University
of Florida, the French Participation Group, the German Participation Group,
Harvard University, the Instituto de Astrofisica de Canarias, the Michigan
State/Notre Dame/JINA Participation Group, Johns Hopkins University, Lawrence
Berkeley National Laboratory, Max Planck Institute for Astrophysics, Max
Planck Institute for Extraterrestrial Physics, New Mexico State University,
New York University, Ohio State University, Pennsylvania State University,
University of Portsmouth, Princeton University, the Spanish Participation
Group, University of Tokyo, University of Utah, Vanderbilt University,
University of Virginia, University of Washington, and Yale University.
Guoshoujing Telescope (the Large Sky Area Multi-Object Fiber Spectroscopic
Telescope LAMOST) is a National Major Scientific Project built by the Chinese
Academy of Sciences. Funding for the project has been provided by the National
Development and Reform Commission. LAMOST is operated and managed by the
National Astronomical Observatories, Chinese Academy of Sciences.

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