# BAT AGN Spectroscopic Survey XXI: The Data Release 2 Overview

Michael J. Koss Eureka Scientific, 2452 Delmer Street, Suite 100, Oakland, CA
94602-3017, USA  Space Science Institute, 4750 Walnut Street, Suite 205,
Boulder, CO 80301, USA  Benny Trakhtenbrot School of Physics and Astronomy,
Tel Aviv University, Tel Aviv 69978, Israel  Claudio Ricci Núcleo de
Astronomía de la Facultad de Ingeniería, Universidad Diego Portales, Av.
Ejército Libertador 441, Santiago 22, Chile  Kavli Institute for Astronomy and
Astrophysics, Peking University, Beijing 100871, People’s Republic of China
Franz E. Bauer Instituto de Astrofísica and Centro de Astroingeniería,
Facultad de Física, Pontificia Universidad Católica de Chile, Casilla 306,
Santiago 22, Chile  Millennium Institute of Astrophysics (MAS), Nuncio
Monse$\sim$nor Sótero Sanz 100, Providencia, Santiago, Chile  Space Science
Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado 80301, USA
Ezequiel Treister Instituto de Astrofísica, Facultad de Física, Pontificia
Universidad Católica de Chile, Casilla 306, Santiago 22, Chile  Richard
Mushotzky Department of Astronomy, University of Maryland, College Park, MD
20742, USA  Joint Space-Science Institute, University of Maryland, College
Park, MD 20742, USA  C. Megan Urry Yale Center for Astronomy & Astrophysics
and Department of Physics, Yale University, P.O. Box 208120, New Haven, CT
06520-8120, USA  Tonima T. Ananna Department of Physics and Astronomy,
Dartmouth College, 6127 Wilder Laboratory, Hanover, NH 03755, USA  Mislav
Baloković Yale Center for Astronomy & Astrophysics, 52 Hillhouse Avenue, New
Haven, CT 06511, USA  Department of Physics, Yale University, P.O. Box
2018120, New Haven, CT 06520, USA  Jakob S. den Brok Institute for Particle
Physics and Astrophysics, ETH Zürich, Wolfgang-Pauli-Strasse 27, CH-8093
Zürich, Switzerland  Argelander Institute for Astronomy, Auf dem Hügel 71,
53231, Bonn, Germany  S. Bradley Cenko Astrophysics Science Division, NASA
Goddard Space Flight Center, Mail Code 661, Greenbelt, MD 20771, USA  Joint
Space-Science Institute, University of Maryland, College Park, MD 20742, USA
Fiona Harrison  Cahill Center for Astronomy and Astrophysics, California
Institute of Technology, Pasadena, CA 91125, USA  Kohei Ichikawa Frontier
Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai
980-8578, Japan  Astronomical Institute, Tohoku University, Aramaki, Aoba-ku,
Sendai, Miyagi 980-8578, Japan  Max-Planck-Institut für extraterrestrische
Physik (MPE), Giessenbachstrasse 1, D-85748 Garching bei Müunchen, Germany
Isabella Lamperti Centro de Astrobiología (CAB), CSIC–INTA, Cra. de Ajalvir
Km. 4, 28850 Torrejón de Ardoz, Madrid, Spain  Amy Lein Astrophysics Science
Division, NASA Goddard Space Flight Center, Mail Code 661, Greenbelt, MD
20771, USA  Julian E. Mejía-Restrepo European Southern Observatory, Casilla
19001, Santiago 19, Chile  Kyuseok Oh Korea Astronomy & Space Science
institute, 776, Daedeokdae-ro, Yuseong-gu, Daejeon 34055, Republic of Korea
Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku,
Kyoto 606-8502, Japan  JSPS Fellow  Fabio Pacucci Center for Astrophysics $|$
Harvard & Smithsonian, Cambridge, MA 02138, USA  Black Hole Initiative,
Harvard University, Cambridge, MA 02138, USA  Ryan W. Pfeifle Department of
Physics and Astronomy, George Mason University, 4400 University Drive, MSN
3F3, Fairfax, VA 22030, USA  Meredith C. Powell Kavli Institute of Particle
Astrophysics and Cosmology, Stanford University, 452 Lomita Mall, Stanford, CA
94305, USA  George C. Privon National Radio Astronomy Observatory, 520
Edgemont Road, Charlottesville, VA 22903, USA  Department of Astronomy,
University of Florida, P.O. Box 112055, Gainesville, FL 32611, USA  Federica
Ricci Instituto de Astrofísica, Facultad de Física, Pontificia Universidad
Católica de Chile, Casilla 306, Santiago 22, Chile  Dipartimento di Fisica e
Astronomia, Università di Bologna, via Gobetti 93/2, 40129 Bologna, Italy
Mara Salvato Max-Planck-Institut für extraterrestrische Physik (MPE),
Giessenbachstrasse 1, D-85748 Garching bei Müunchen, Germany  Kevin Schawinski
Modulos AG, Technoparkstrasse 1, CH-8005 Zurich, Switzerland  Taro Shimizu
Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse 1,
D-85748 Garching bei Müunchen, Germany  Krista L. Smith Department of Physics,
Southern Methodist University, 3215 Daniel Ave., Dallas, TX 75205, USA  Daniel
Stern Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak
Grove Drive, MS 169-224, Pasadena, CA 91109, USA

(Accepted May 4, 2022)

###### Abstract

The BAT AGN Spectroscopic Survey (BASS) is designed to provide a highly
complete census of the key physical parameters of the supermassive black holes
(SMBHs) that power local active galactic nuclei (AGN) ($z\lesssim 0.3$),
including their bolometric luminosity ($L_{\rm bol}$), black hole mass
($M_{\rm BH}$), accretion rates ($L_{\rm bol}/L_{\rm Edd}$), line-of-sight gas
obscuration ($N_{\rm H}$), and the distinctive properties of their host
galaxies (e.g., star formation rates, masses, and gas fractions). We present
an overview of the BASS data release 2 (DR2), an unprecedented spectroscopic
AGN survey in spectral range, resolution, and sensitivity, including 1449
optical ($\sim$3200 Å-1 $\mu$m) and 233 NIR (1-2.5 $\mu$m) spectra for the
brightest 858 ultra-hard X-ray (14-195 keV) selected AGN across the entire sky
and essentially all levels of obscuration. This release provides a highly
complete set of key measurements (emission line measurements and central
velocity dispersions), with 99.9% measured redshifts and 98% black hole masses
estimated (for unbeamed AGN outside the Galactic plane). The BASS DR2 AGN
sample represents a unique census of nearby powerful AGN, spanning over 5
orders of magnitude in AGN bolometric luminosity ($L_{\rm bol}\sim
10^{40}-10^{47}\,{\rm erg\,s}^{-1}$), black hole mass ($M_{\rm BH}\sim
10^{5}-10^{10}\,M_{\odot}$), Eddington ratio ($L_{\rm bol}/L_{\rm Edd}\gtrsim
10^{-5}$), and obscuration ($N_{\rm H}\sim 10^{20}-10^{25}\,{\rm cm}^{-2}$).
The public BASS DR2 sample and measurements can thus be used to answer
fundamental questions about SMBH growth and its links to host galaxy evolution
and feedback in the local universe, as well as open questions concerning SMBH
physics. Here we provide a brief overview of the survey strategy, the key BASS
DR2 measurements, data sets and catalogs, and scientific highlights from a
series of DR2-based works pursued by the BASS team.

††journal: ApJS††facilities: Du Pont (Boller & Chivens spectrograph), Keck:I
(LRIS), Magellan:Clay, Hale (DBSP), NuSTAR, Swift (XRT and BAT), VLT:Kueyen
(X-Shooter), VLT:Antu (FORS2), SOAR (Goodman)††software: Astropy
(Collaboration et al., 2013), ESO Reflex (Freudling et al., 2013), IRAF
(National Optical Astronomy Observatories, 1999), Matplotlib (Hunter, 2007),
Numpy (van der Walt et al., 2011), Pandas
(https://doi.org/10.5281/zenodo.3630805)

##  1 Introduction

Although active galactic nuclei (AGNs) and the supermassive black holes
(SMBHs) that power them have been studied for decades, there still many key
unresolved questions concerning the nature of these systems and how their
evolution may be related to the galaxies that host them. While there are some
clear SMBH-host correlations, such as the one between SMBH mass and bulge
properties (Kormendy & Ho, 2013) or the similar redshift evolution of star
formation and SMBH growth (e.g., Heckman & Best, 2014; Yang et al., 2019), it
is not yet clear whether SMBH growth and AGN output (“feedback”, e.g.,
radiation, winds, jets) affects the host galaxy interstellar medium, star
formation, and molecular gas (e.g., Fabian, 2012). On smaller scales, it is
not yet entirely clear what is the structure of the obscuring torus, what is
its connection to its surroundings (e.g., Netzer, 2015) and what is the role
of obscuration by galaxy-scale gas and nuclear starbursts. Moreover, the
stochasticity of both SMBH fueling and corresponding AGN emission (e.g.,
Hickox et al., 2014; Schawinski et al., 2015), combined with inherent biases
in AGN survey techniques when identifying obscured AGN (e.g., Hickox &
Alexander, 2018). Therefore, the only way to address the many open questions
concerning SMBHs and AGNs, even in the local universe, and to construct
statistically significant global trends and/or correlations, necessitates
large samples (e.g. $N>100$) surveyed with multiwavelength observations.

Extragalactic hard (${>}2$ keV) X-ray surveys provide one of the most complete
ways to study growing black holes in an unbiased way (see, e.g., Brandt &
Alexander, 2015; Brandt & Yang, 2021). A large fraction, and indeed the
majority, of the AGN population is obscured and therefore the construction of
a complete AGN census requires the identification of both obscured and
unobscured sources (see review by, e.g., Hickox & Alexander, 2018). At even
higher energies, the ultra-hard X-rays ($>10$ keV) provide a more complete
tracer of the radiation for obscured AGN ($N_{\rm H}>10^{22}\,{\rm cm}^{-2}$)
and even some Compton-thick (CT) AGN ($N_{\rm H}>10^{24}\,{\rm cm}^{-2}$;
e.g., Ricci et al., 2015; Koss et al., 2016). An all-sky survey in the ultra-
hard X-ray band (${>}$10 keV) thus provides an important way to answer the
fundamental questions of SMBH growth and its links to host galaxy evolution,
as well as many open questions concerning AGN physics, for a complete,
unbiased sample of AGN.

Over the past 20 years, great progress has been made in surveying the ultra-
hard X-ray sky to increasing depths with the Burst Alert Telescope (BAT;
Barthelmy et al., 2005) at 14-195 keV on board the Neil Gehrels Swift
Observatory (Gehrels et al., 2004) and the IBIS instrument (Ubertini et al.,
2003) at 17-60 keV onboard the INTEGRAL observatory (Winkler et al., 2003).
Thanks to its wide field of view (FOV; 1.4 sr half coded), BAT monitors
roughly 80% of the sky every day, providing regularly sampled average emission
properties of objects. INTEGRAL/IBIS, with a roughly 13 times smaller FOV
(0.11 sr half coded) but better angular resolution (12′ vs. 19.5′ for BAT),
has focused on targeting the Galactic plane and particularly the Galactic
center region. Thus, the Swift BAT survey provides a uniform all-sky census of
the average ultra-hard X-ray emission of AGN.

Early analysis (e.g., Markwardt et al., 2005; Tueller et al., 2008) of the
(ongoing) all-sky survey with BAT found that the brightest ultra-hard-X-ray-
selected AGN in the sky can probe nearby ($z<0.1$) AGN, including highly
obscured systems; low-redshift, high-luminosity AGN and quasars ($0.05\lesssim
z\lesssim 0.3$), and much more distant, beamed AGN (reaching out to $z\gtrsim
3$). Specifically, the unbeamed BAT-detected AGN span the moderate-to-high-
luminosity end of the X-ray luminosity function (XLF; e.g., Maccacaro et al.,
1991, Comastri et al., 1995, Gilli et al., 2007, Ueda et al., 2014, Aird et
al., 2015, Buchner et al., 2015, Ananna et al., 2019) at all accessible
redshifts, thus providing critical low-redshift templates for high-$z$ sources
detected in much deeper, pencil-beamed surveys (e.g., CDF-S, Luo et al., 2017;
COSMOS, Civano et al., 2016). The low-redshift regime of the BAT AGN survey
thus strongly complements high-redshift AGN surveys, providing a legacy
dataset with both high spatial resolution111The angular resolution at the
median redshift $z{=}0.04$ is 10${\times}$ sharper than at $z{=}1$, which is
typical for deeper and narrower surveys. and very high signal-to-noise ratio
data obtained through relatively short observations. Finally, some AGN are
uniquely identified as AGN in the hard X-rays (e.g., Smith et al., 2014).
Other all-sky selection methods, such as WISE mid-IR (MIR) colors, are only
able to uniformly classify the most luminous AGN ($L^{\rm obs}_{\mathrm{2-10\
keV}}$ ${>}10^{44}\,{\rm erg\,s}^{-1}$; e.g., Ichikawa et al., 2017) owing to
contamination from star formation in the host galaxies. Moreover, methods that
use strong emission-line ratio diagnostics to identify AGN (e.g., “BPT”
Baldwin et al., 1981; Veilleux & Osterbrock, 1987; Kewley et al., 2006) only
select about half of the ultra-hard X-ray-selected BAT AGN (Koss et al.,
2017). This is likely due to the contribution of star forming regions in the
AGN host galaxies (e.g., Koss et al., 2011b), combined with dust obscuration
of the central, AGN-dominated line-emitting regions (e.g., Koss et al., 2010).

The ultimate goal of the BAT AGN Spectroscopic Survey (BASS) is to provide the
largest available spectroscopic sample of Swift BAT ultra-hard X-ray (14–195
keV) detected AGN. BASS DR1 (Koss et al., 2017) used mostly archival optical
spectra for 641 AGN from the 70-month BAT catalog (Baumgartner et al., 2013).
This was complemented by detailed 0.5–200 keV spectral measurements using
Swift, Chandra, and XMM-Newton for 838 AGN (Ricci et al., 2017a). A further
102 near-IR (NIR; 1–2.5 $\mu{\rm m}$) spectra of BAT AGN were reported on by
Lamperti et al. (2017). Other data include high spatial resolution NIR
adaptive optics (AO) imaging (Koss et al., 2018); extensive continuum modeling
of the MIR and far-IR (FIR) emission from WISE, IRAS, Akari, and Herschel
(Ichikawa et al., 2017, 2019); radio emission in several resolution regimes
(Baek et al., 2019; Smith et al., 2020); and host-scale molecular gas
measurements (Koss et al., 2021). From these data, numerous links and
correlations were investigated, such as between X-ray emission and high-
ionization optical lines (Berney et al., 2015), and/or ionized gas outflows
(Rojas et al., 2020). A major result was the realization that the Eddington
ratio is a key parameter in some of these links and scaling relations (e.g.,
Ricci et al., 2017b; Oh et al., 2017; Ricci et al., 2018). Other BASS DR1
studies focused on AGN clustering (Powell et al., 2018), the surprisingly weak
(or indeed insignificant) correlation between the X-ray photon index,
$\Gamma$, and Eddington ratio (Trakhtenbrot et al., 2017), the most luminous
obscured AGN (Bar et al., 2017), optically unobscured AGN with massive X-ray
absorbing columns (Shimizu et al., 2018), BAT blazars (Paliya et al., 2019),
and a search for BH binaries (Liu et al., 2020).

Since making BASS DR1 public, the BASS team has pursued additional optical and
NIR spectroscopy towards the second data release of BASS (DR2), to obtain
higher spectral resolution ($R\gtrsim 1500$) over the broadest possible
spectral range (i.e., 3200–10000 Å), for the most complete sample of AGN drawn
from the 70-month BAT catalog. Below we present an overview of the BASS DR2,
including a summary of our survey design and goals, datasets, key scientific
results, and a comparison to other large AGN surveys. A more detailed,
technical description of the DR2 observations and data is provided in Koss et
al. (2022a).

Throughout this work we adopt $\Omega_{\rm M}{=}0.3$,
$\Omega_{\Lambda}{=}0.7$, and $H_{0}{=}70\,{\rm km\,s}^{-1}\,{\rm Mpc}^{-1}$.
To determine the extinction due to Milky Way foreground dust, we use the maps
of Schlegel et al. (1998) and the extinction law derived by Cardelli et al.
(1989).

##  2 Overview of DR2 Special Issue

BASS DR2 provides optical spectra and associated redshifts, broad and narrow
emission line measurements, velocity dispersions, and derived quantities —
particularly BH mass and Eddington ratio estimates — for a nearly complete
(e.g. $>$95%) sample of the 858 AGN from the Swift BAT 70-month survey.

###  2.1 Major Science Goals and Key measurements

The measurements and derived quantities provided through the BASS project are
critical for the major science goals of the survey:

  1. 1.

Provide a (nearly) complete census of the brightest local AGN from the
unobscured to highly obscured.
Swift BAT, with its ultra hard X-ray sensitivity, serves as a primary
discovery and survey tool for these local AGN as the energy range is
uncontaminated by star formation and (nearly) unaffected by obscuration, and
hence naturally provides a reliable tracer of AGN emission and SMBH growth
within a volume-limited, highly complete survey.

  2. 2.

Connect BH growth to their host galaxy properties to understand fueling and/or
feedback.
It is critical to understand how the large range of SMBH-related properties —
such as the bolometric luminosities, BH masses ($M_{\rm BH}$), accretion rates
($L_{\rm bol}/L_{\rm Edd}$), kinetic power of AGN-powered outflows and jets,
and circumnuclear obscuration — relate to key host galaxy properties as traced
by multiwavelength data, such as star formation rates (SFRs), stellar and
molecular gas content, morphologies, and merger activity.

  3. 3.

Provide critical nearby templates for luminous high-redshift AGN.
The BASS AGN are local analogs of the powerful AGN that form the bulk of the
X-ray-detected population in deep pencil-beam surveys where high spatial
resolution (e.g., hundreds of parsecs) and sensitivity cannot be achieved.

  4. 4.

Provide critical diagnostics for rare and/or “abnormal” AGN.
The unprecedentedly rich collection of multiwavelength data collected for the
BASS AGN and its unique selection in the ultra-hard X-rays allow one to
identify, calibrate, and/or test selection criteria for highly obscured AGN,
highly accreting AGN (i.e., $L_{\rm bol}/L_{\rm Edd}\gtrsim 1$), and other
challenging subclasses, to be used with future facilities, surveys, and
models.

###  2.2 Revised 70-month AGN Catalog

We briefly review the AGN catalog changes compared to BASS DR1, with further
details provided in Koss et al. (2022a). The published 70-month BAT catalog
(Baumgartner et al., 2013) is composed of 1210 ultra-hard X-ray sources,
including 822 classified as AGN or associated with a galaxy and likely an AGN,
287 Galactic sources (e.g., high-mass X-ray binary, low-mass X-ray binary,
cataclysmic variable, pulsar), 19 clusters, and 82 unknown sources. In the
BASS DR1 (Ricci et al., 2017a, see Appendix A) new AGN candidates were
identified among BAT detections based on WISE and soft X-ray data to increase
the number of 70-month AGN to 838.

However, even after the DR1, 44 unknown BAT sources, typically near the
Galactic plane ($\lvert b\rvert{<}10^{\circ}$), had not been associated with
counterparts. Of these sources, 22, were found to be AGN. Fifteen of these
unknown sources were subsequently identified as Galactic with NuSTAR and/or
Chandra follow-up surveys (e.g., Yukita et al., 2017; Kennedy et al., 2020;
Halpern et al., 2018) and/or based on optical spectroscopy obtained in the
BASS/DR2 (see Koss et al., 2022a, for details). Another two sources that were
classified as AGN in the DR1 based solely on their hard X-ray spectra were
found to be Galactic.

Unfortunately, their were still seven sources that lie very close to the
Galactic plane ($\lvert b\rvert{<}3^{\circ}$) owing to their very high
extinction values ($A_{V}\sim 5-43$ mag) and very high source confusion in the
optical/NIR (i.e., multiple stars per 1′′ sq area), made optical and NIR
spectroscopy of the counterpart impractical. The number of AGN is 858 after
these updates since two sources in the DR1 were discovered to be
Galactic.222838 DR1 AGN - 2 DR1 AGN found to be Galactic + 22 New AGN= 858 DR2
AGN + 7 unknown sources at $\lvert b\rvert{<}3^{\circ}$

For consistency with BASS DR1 and earlier studies, we classify AGN according
to the presence (or absence) of broad emission lines. Specifically, Sy 1 are
AGN with broad H$\beta$ line emission, Sy 1.9 have narrow H$\beta$ and broad
H$\alpha$, while Sy 2 AGN have both narrow H$\beta$ and narrow H$\alpha$
(including small numbers of LINERs and AGN in H2-dominated regions). AGN type
based on optical spectroscopy. For beamed AGN, the types include those with
the presence of broad lines (BZQ), only host galaxy features lacking broad
lines (BZG), or traditional continuum-dominated blazars, with no emission
lines or host galaxy features (BZB).

In addition to unbeamed AGN, the Swift BAT survey includes also beamed and/or
lensed AGN, which are important to separate for most scientific analyses. The
DR2 has 105 beamed AGN as identified by their multiwavelength emission, and
particularly radio emission, and/or Gamma-ray emission detected by Fermi
(e.g., Paliya et al., 2019). This includes both blazars where the boosted
continuum emission completely dominates the (rest-frame) UV/optical regime and
no significant emission lines are seen, and flat-spectrum radio quasars
(FSRQs), which do show broad emission lines (e.g., Paliya et al., 2019). There
are additionally two lensed AGN. One of the beamed AGN (SWIFT J1833.7-2105,
aka PKS 1830-211 at $z{=}2.5$) is also lensed (Lidman et al., 1999)) by a
foreground galaxy. Thus, the total sample of 858 includes 752 unbeamed AGN,
104 beamed AGN, 1 beamed and lensed AGN, and 1 lensed and unbeamed AGN.

An X-ray luminosity and redshift plot of the BASS DR2, with the newly revised
redshifts and AGN classifications, is shown in Figure 1. The figure also shows
a few other deep, and narrow X-ray AGN surveys. We discuss these samples and
comparison in more detail in subsection 5.3; however, it is clearly evident
that BASS provides a natural low-redshift benchmark for distant surveys.

Figure 1: The rest-frame 2–10 keV luminosities of the BASS DR2 AGN and of
higher-redshift X-ray AGN surveys. Although BASS AGN are selected based on
their 14–195 keV emission, we plot the best-fit intrinsic 2–10 keV emission
based on detailed X-ray spectral modeling (Ricci et al., 2017b). Unbeamed BASS
AGN are shown with purple stars, while beamed AGN are shown with purple
triangles. The unbeamed AGN in the BASS sample tend to span the moderate-to-
high-luminosity end of the XLF. We also show samples drawn from deeper X-ray
AGN surveys, including Stripe 82X (red pentagons; LaMassa et al., 2016; Ananna
et al., 2017); CDF-S (brown pentagons; Luo et al., 2017); Chandra COSMOS
Legacy survey (blue circles; Civano et al., 2016; Marchesi et al., 2016); XMM
XXL (green diamonds; Pierre et al., 2016); and the NuSTAR Serendipitous Survey
(yellow squares; Lansbury et al., 2017). We also show contours for eFEDS
containing 99% of the data (Salvato et al., 2021, dashed grey,). We limit our
comparison to X-ray sources with confirmed counterparts with spectroscopic
redshifts. For eFEDS, unlike the other surveys, the soft-band flux (0.2-2.3
keV) was used to estimate the hard X-ray 2-10 keV emission assuming a power-
law spectral model with $\Gamma=1.7$ since only a small number of sources were
detected above 2.3 keV ($<$1% Brunner et al., 2021). For these higher-redshift
X-ray surveys we assumed a power-law spectral model with $\Gamma=1.7$, to
bring each X-ray luminosity to the rest frame by $K$-correcting the apparent
luminosities based on the observed redshifts into the 2-10 keV rest frame. The
deeper, higher-redshift samples tend to sample a luminosity range that is
consistent with that covered by BAT, but at higher redshift.

###  2.3 Survey Strategy and Observations

The sky distribution of all BASS DR2 optical spectroscopic observations is
presented in Figure 2. Table 2 summarizes all of the BASS DR2 data, with
further details provided in Koss et al. (2022a). The majority of the spectra
used for the catalog measurements presented in DR2 papers come from either the
Double Beam Spectrograph (DBSP) mounted on the Palomar Hale 5 m telescope (402
AGN, mainly in the northern hemisphere) or the X-Shooter spectrograph at the
Very Large Telescope (VLT; 211 sources, mainly southern).

Figure 2: Overview of the BASS DR2 optical spectroscopy as observed on the sky
(shown in equatorial coordinates and a Mollweide projection). The Galactic
plane is indicated by the light grey line.

In terms of DR2 spectroscopic targeting, our goals were to (1) provide the
largest possible sample of black hole mass measurements from either broad
Balmer emission lines or stellar velocity dispersion measurements and (2)
cover broadest possible spectral range (e.g., 3200–10000 Å) for emission-line
measurements, for the entire catalog of 858 AGN. In practice, unless one uses
echelle instruments (e.g., X-Shooter), the latter goal requires either spectra
with broad wavelength coverage and lower resolution or, alternatively,
multiple, higher-resolution gratings and narrower wavelength coverage. The
former goal motivates a spectral resolution of $R\gtrsim 1000$ ($\Delta
v\lesssim 300\,{\rm km\,s}^{-1}$ for broad lines), and yet higher resolution
($>$2000) for stellar velocity dispersion measurements. Our targeting strategy
was further complicated by the fact that for a significant number of targets
we did not know a priori the broad/narrow-line nature of the sources, and thus
whether high-resolution stellar velocity dispersion measurements were
required. Repeated observations were therefore done if either the S/N of the
broad Balmer lines (H$\beta$ or H$\alpha$) or the S/N and/or spectral
resolution of the stellar absorption features were too low. Repeated
observations with higher spectral resolution but limited spectral range were
primarily done for obscured AGN (Sy1.9 and Sy2) to measure velocity
dispersions (and deduce BH masses), as velocity dispersion measurements are
much more difficult and less reliable for AGN-dominated continuum. We did not
reobserve targets with acceptable spectra and measurements from the Sloan
Digital Sky survey (SDSS; York et al., 2000) included in Data Release 16
(DR16; Ahumada et al., 2020).

As the goal of BASS is to provide the best and most complete set of derived
measurements (e.g. for narrow lines, broad lines, velocity dispersions), we
did not require that all studies use the same single best spectra for an AGN
in the DR2 as was done in the DR1. So for instance, a single Sy 1.9 AGN may
have a broad H$\alpha$ measurement from SOAR/Goodman using the lower-
resolution 400 lines mm-1 setting, along with measurements of narrow emission
lines, but have a velocity dispersion measurement from CaT using the 1200 line
mm-1 setting.

We also did not specifically exclude sources with high Galactic extinction if
there was an obvious optical counterpart. This resulted in some spectra with
very high extinctions (between $A_{\mathrm{V}}=5$ and 10 mag), that are only
suitable for basic emission-line identification/classification and for
redshift measurement.

###  2.4 Comparison with BASS DR1

The BASS DR1 was composed mostly of past archival optical spectroscopic data
from a variety of sources. Almost all the observations were from smaller
(1.5-2.5m) telescopes, with only 35 from the Palomar 5m telescope (DBSP) and
29 from the 8.1m Gemini North and South telescopes. Many of the spectra were
taken from various surveys and studies that used various reduction routines,
leading to substantial inhomogeneity in quality and parameter constraints.
Some of the spectra, particularly those from the 6dF Galaxy Survey (6dFGS;
Jones et al., 2009), had no proper flux calibration. Apart from the subset of
DR1 spectra taken from the SDSS, most BASS DR1 spectra had a spectral
resolution that is too low ($R<1000$) to robustly measure stellar velocity
dispersions. Finally, in most cases, the spectral setups did not include
coverage below 4000 Å or beyond 7000 Å.

The DR2 results, which are based primarily on Palomar/DBSP or VLT/X-Shooter
spectra, are largely a separate release from DR1, even though the AGN samples
overlap. Aside from 142 SDSS spectra commonly used for both samples, the only
other spectral overlap between DR1 and DR2 is for 35 Palomar/DBSP
observations, which were uniformly reprocessed in DR2 using the new molecfit
(Smette et al., 2015) procedure for telluric corrections.

For the BASS DR2 catalogs of broad and narrow emission line measurements, we
allowed some DR1 spectra to be used to have the best and most complete set of
derived quantities (i.e., BH mass, etc.). This was done because the two main
DR2 observing setups (VLT/X-Shooter and Palomar/DBSP) used for the majority of
sources both had the break in the blue and red CCDs at $\sim$5500 Å, making
measurements of the spectral complex around the (redshifted) H$\beta$ line
problematic (e.g. at $z\approx 0.1$). For stellar velocity dispersion
measurements, only DR2 data were used.

###  2.5 NIR DR2

The goal of the BASS NIR spectroscopy is to obtain wide spectral coverage
across the full NIR range (${\sim}1-2.4\,{\mbox{$\mu{\rm m}$}}$) for a large
sample of BAT AGN. The 102 AGN composing the NIR DR1 (Lamperti et al., 2017)
were primarily for nearby AGN ($z<0.075$) observed with the SpeX instrument
(Rayner et al., 2003) at the NASA Infrared Telescope Facility (IRTF) in the
northern hemisphere ($0.8-2.4\,{\mbox{$\mu{\rm m}$}}$) along with archival
Gemini/GNIRS data ($0.8-2.5\,{\mbox{$\mu{\rm m}$}}$).

The NIR DR2 AGN sample includes 233 new NIR spectra of BASS AGN. The new DR2
data were obtained from the southern hemisphere and include 168 new
VLT/X-Shooter (Brok et al., 2022) and 65 Magellan/FIRE spectra (Ricci et al.,
2022), both with substantially higher spectral resolutions ($R\sim
5000-10000$) than the DR1 data ($R\sim 800-1000$). When including the NIR DR1,
the total NIR sample provided as part of DR2 includes NIR spectroscopy of 322
unique AGN333Four DR2 observations overlap with DR1, while nine DR2
Magellan/FIRE observations overlap with VLT/X-Shooter. The NIR catalog
measurements include broad, narrow, and coronal emission line measurements
between $1-2\,{\mbox{$\mu{\rm m}$}}$, but not the $K$-band
($2-2.5\,{\mbox{$\mu{\rm m}$}}$).

The NIR spectroscopic survey of 322 AGN is distinct from the DR2 optical
spectroscopy, as it is an interim release rather than a complete sample (i.e.,
the NIR DR2 is very far from providing NIR spectra for all 858 AGN in the
optical BASS DR2). Moreover, this NIR survey release also includes some
105-month BAT sources (Oh et al., 2018) and the VLT/X-Shooter spectroscopy
only includes observations carried out through 2019 October.

Future releases (i.e., the NIR DR3) will include additional NIR spectroscopy
efforts within BASS that are currently ongoing, including additional
VLT/X-SHOOTER, Magellan/FIRE, and Palomar/TSpec observations, as well as
velocity dispersion measurements and emission-line fitting in the $K$ band
(${\sim}2-2.4\,{\mbox{$\mu{\rm m}$}}$). As of 2021 July, an additional 267
BASS AGN have NIR spectroscopy that are not part of the DR2 release.

###  2.6 Survey Uniformity and Completeness

BASS DR2 is nearly spectroscopically complete with respect to the 70-month BAT
all-sky survey, with $>99$% of the AGN having spectra (i.e., 858/(858 AGN+7
unknown/confused sources)). There are, however, some additional considerations
in terms of uniformity and completeness for the BASS survey, which we briefly
mention here and review in more detail in Koss et al. (2022a). The 70-month
BAT survey reaches a 14–195 keVflux sensitivity level of $1.34\times 10^{-11}$
erg cm-2 s-1 over 90% of the sky (assuming a power-law spectral shape fixed to
the Crab $\Gamma\sim$2.15; Baumgartner et al., 2013) but $1.03\times 10^{-11}$
erg cm-2 s-1 over 50% of the sky, meaning there is some small ($\sim$20%)
variation in sensitivity across the sky. Near the Galactic plane, and
particularly in the crowded field of the Galactic center with strong Galactic
X-ray background radiation, the source density is considerably lower, and the
sensitivity of the BAT survey is a factor of two lower (Markwardt et al.,
2005).

The all-sky BAT survey also suffers from biases against harder and/or highly
obscured AGN. Due to the BAT detection method, which is calibrated to the Crab
($\Gamma=2.15$), AGN with harder intrinsic X-ray spectral energy distributions
(SEDs; e.g. $\Gamma=1.7$ vs. $\Gamma=2.1$) will suffer roughly 10% reduced
sensitivity (Koss et al., 2013). A more significant completeness correction is
needed for highly obscured AGN, where BAT detects 90% of the flux for an AGN
with $N_{\rm H}=3\times 10^{23}\,{\rm cm}^{-2}$, 50% for $N_{\rm H}=2\times
10^{24}\,{\rm cm}^{-2}$, and $\sim$10% (or less, depending on models) at
$N_{\rm H}=10^{25}\,{\rm cm}^{-2}$ (e.g., Koss et al., 2016).

##  3 DR2 Papers, Catalogs, and Key Science Results

Table 1: Summary of BASS DR2 Papers

BASS # | Short Title | Major Measurements/Science | Ref.
---|---|---|---
XXI | Overview |  $M_{\rm BH}$ and $L_{\rm bol}/L_{\rm Edd}$ vs. other surveys | This paper
XXII | Catalog | Counterparts, z, Spectra, Best $M_{\rm BH}$ | Koss et al. (2022a)
XXIII | MIR Diagnostic for Absorption | Obscuration in MIR | Pfeifle et al. (2022)
XXIV | Narrow line measurements | BPT Diagnostics | Oh et al. (2022)
XXV | Broad-line measurements | Sy 1-Sy 1.9 BLR vs. $N_{\rm H}$, Broad lines | Mejía-Restrepo et al. (2022)
XXVI | Stellar Velocity Dispersions | Sy 1.9 and Sy 2 $\sigma_{*}$ $M_{\rm BH}$ | Koss et al. (2022b)
XXVIII | NIR High-Ionization and Broad Lines | NIR spectra, NIR lines | Brok et al. (2022)
XXIX | NIR view of the BLR Effects of Obscuration | NIR BLR vs. $N_{\rm H}$ | Ricci et al. (2022)
XXX | Distribution Function of Eddington Ratios | XLF, ERDF, BHMF | Ananna et al. (2022)

Note. — List of all papers in the BASS DR2 release and associated major
measurements and science. BASS XXVII was published separately from the DR2
release.

The DR2 papers Table 1 provide a combination of large datasets of measurements
and derived quantities, and scientific results for either the entire DR2
sample or considerable subsets of data. We provide a short review of these
papers here, to explain the different measurements and catalogs provided in
the various papers and to highlight key scientific findings. In some papers
(e.g., narrow emission line measurements) a very wide spectral range was
critical (3200–10000 Å), while in others (e.g., velocity dispersions) high
spectral resolutions were critical; therefore, sometimes spectra with high
spectral resolution but narrow wavelength range were used (e.g., for the
calcium triplet, CaT, 8498, 8542, and 8662 Å). All DR2 spectra will be
provided at the BASS website.444http://www.bass-survey.com

The main, detailed DR2 catalog paper (BASS XXII, Koss et al., 2022a) gives an
updated list of counterparts and a complete summary of observing
characteristics and reduction procedures for the 1449 optical spectra of the
858 AGN from the BAT 70-month sample. It discusses various issues for basic
AGN observational studies (source confusion, chance alignment of multiple AGN
leading to flux boosting, dual AGN, etc.), including a revised list of beamed
and lensed AGN identifications. This paper also provides a set of overall
best-derived measurements (e.g., redshifts and distances, bolometric
luminosities, and $M_{\rm BH}$) since multiple BH mass estimates are available
for some sources from various tracers (i.e., different broad lines, velocity
dispersions, and/or high-quality literature measurements). The overall best
available measurements from this catalog are then used consistently in
subsequent scientific papers.

Narrow optical emission line measurements over the range 3200–10000 Å (e.g.,
[Ne v]$\,\lambda 3426$, H$\beta$, [O iii] $\lambda 5007$, H$\alpha$, [N
ii]$\,\lambda 6583$, [S ii] $\lambda 9531$) are presented in BASS XXIV (Oh et
al., 2022). In that paper, we employ a full-range spectral fitting procedure,
incorporating both stellar population synthesis models and empirical stellar
templates, which deblends complex nebular emission-line features from the
stellar components. We also study AGN subtypes as a function of X-ray column
density; strong-line ratio diagnostic diagrams (BPT) and their links to
Eddington ratio; and line width comparisons between X-ray BAT AGN and optical
SDSS AGN.

Broad emission line measurements (H$\alpha$, H$\beta$, Mg ii $\lambda 2798$,
and C iv $\lambda 1549$) and derived quantities are the focus of BASS XXV
(Mejía-Restrepo et al., 2022). This paper includes virial estimates of BH mass
($M_{\rm BH}$) which are used as the best black hole mass measurement
throughout DR2 and also allow estimates of the Eddington ratio ($L_{\rm
bol}/L_{\rm Edd}$). The use of Mg ii $\lambda 2798$ and C iv $\lambda 1549$ is
reserved for the beamed AGN with discernible broad lines, which are at higher
redshifts (e.g., $z\gtrsim 1$), where rest-frame H$\beta$ falls outside of the
optical range. This paper concludes that the innermost part of the broad-line
region (BLR), which contributes the highest velocity emission, is
preferentially absorbed in obscured AGN ($\log(N_{\rm H}/{\rm cm}^{-2})>22$)
and/or Sy1.9. This leads to a significant underestimation of the line flux
compared to unobscured sources, which then strongly underestimates BH mass.
These discrepancies typically exceed 1 dex and may reach 2 dex for heavily
obscured AGN ($\log(N_{\rm H}/{\rm cm}^{-2})\geq 24$). We provide some
prescriptions for corrections.

Central velocity dispersion measurements (e.g., from Ca H+K$\,\lambda
3935,3968$, Mg i, or CaT regions) of obscured systems (Sy 1.9 and Sy 2) are
investigated in BASS XXVI (Koss et al., 2022b). This paper finds that BASS AGN
have much higher velocity dispersions than the more numerous optically
selected narrow-line AGN (i.e., $\approx$150 vs. $\approx$100 ${\rm\thinspace
km}{\rm\thinspace s}^{-1}\,$), but also that BASS AGN are not biased toward
the highest velocity dispersions seen in massive ellipticals (i.e., $>$250
${\rm\thinspace km}{\rm\thinspace s}^{-1}\,$). Additionally, despite
sufficient spectral resolution to resolve the velocity dispersions associated
with the bulges of relatively small SMBHs ($\sim 10^{4}-10^{5}$ $M_{\odot}$),
we do not find a significant population of such AGN, which (given the BAT flux
limit) would have presented super-Eddington accretion rates.

Ananna et al. (2022) use the highly complete set of BASS DR2 measurements to
derive the intrinsic XLF, BH mass function (BHMF) and Eddington ratio
distribution function (ERDF), for both obscured and unobscured low-redshift
AGN using the BAT sample. It employs an elaborate forward-modeling approach to
derive the intrinsic XLF, BHMF and BHMF from the observed distributions, while
accounting for various selection effects. We find that the intrinsic ERDF of
narrow-line (Type 2) AGN is significantly skewed toward lower Eddington ratios
than that of broad-line (Type 1) AGN, while the BHMFs of these subsamples are
consistent with each other. This result offers insights into the geometric
structure of the obscuring “torus” and lends support to the radiation-
regulated unification scenario (Ricci et al., 2017b), which suggests that
radiation pressure dictates the geometry of the dusty obscuring structure
around an AGN. The XLF, BHMF, and ERDF are also used to investigate the AGN
duty cycle in the low-redshift universe.

Pfeifle et al. (2022) investigate the relationship between MIR colors and
X-ray column density. Heavily obscured BAT AGN are found to be more heavily
X-ray suppressed, displaying lower ratios of $L^{\rm obs}_{\mathrm{2-10\
keV}}$ $/L_{\mathrm{12\,{\mbox{$\mu{\rm m}$}}}}$, and they display “redder”
MIR colors compared to unobscured AGN. This paper develops diagnostic criteria
that are designed to select both highly complete and highly reliable samples
of heavily obscured AGN ($\log(N_{\rm H}/{\rm cm}^{-2})>23.5$). We also derive
expressions relating the luminosity ratios and column density, to predict the
AGN column density in lower count-rate X-ray SEDs, where detailed spectral
modeling is impossible. These diagnostics could be used on future samples of
AGN, such as those being discovered by eROSITA (Predehl et al., 2021), to
efficiently distinguish between heavily obscured and unobscured AGN.

Brok et al. (2022) provide a detailed analysis of the NIR coronal lines (CLs,
ionization potential $\chi>100$ eV) and test their usage as indicators of AGN
activity by comparing their strength, in particular that of [Si vi] $\lambda
19640$, to the X-ray flux. A key finding is that CLs correlate more tightly
(i.e., smaller scatter) with the X-ray fluxes than with the optical [O iii]
$\lambda 5007$ line fluxes. Even in these bright AGN, in only about half of
the sources is a CL detected, limiting the extent to which CLs can be used as
tracers of AGN activity. This study finds a clear trend of line blueshifts
with increasing ionization potential in several CLs, such as [Si vi] $\lambda
19640$, [Si x] $\lambda 14300$, [S viii] $\lambda 9915$, and [S ix] $\lambda
12520$, which elucidates the radial structure of the CL region.

Finally, Ricci et al. (2022) investigate the NIR BLR using Pa $\alpha$, Pa
$\beta$, and He i$\,\lambda 10830$, and associated virial BH mass estimates.
The NIR regime is less affected by dust than the optical and can thus trace
the innermost and fastest-moving BLR gas — even in the presence of mild
obscuration. The study finds that the velocities of the BLR gas as estimated
from the FWHMs of H$\alpha$ and the NIR lines in Sy 1–1.9 agree and are
independent of the level of BLR extinction or obscuration (for $\log(N_{\rm
H}/{\rm cm}^{-2})<23.5$), but the broad line luminosities are suppressed with
increasing obscuration, biasing virial-based $M_{\rm BH}$ estimates. The
latter finding is in agreement with the conclusion of (Mejía-Restrepo et al.,
2022, see above). The line luminosity decrement and the obscuration level at
which it occurs change as a function of wavelength, with H$\alpha$
experiencing a higher decrement than Pa$\alpha$ (above $\log(N_{\rm H}/{\rm
cm}^{-2})\simeq 21.0$ and $21.9$, respectively). Thus, we caution against
relying solely on H$\alpha$-based single-epoch BH mass estimates when
$\log(N_{\rm H}/{\rm cm}^{-2})\gtrsim 21$, and on NIR lines when $\log(N_{\rm
H}/{\rm cm}^{-2})\simeq 22$. A less biased proxy for the BLR radius in virial-
based $M_{\rm BH}$ should be used at higher $N_{\rm H}$, such as $L_{X}$.

##  4 Other BASS Observing Campaigns

Beyond the complete coverage with optical spectroscopy and the extensive NIR
spectroscopy that are the main components of the BASS DR2, the BASS project
aims to obtain and analyze large multiwavelength data sets for the BAT AGN, in
the X-ray, UV, optical, IR, FIR/submillimeter, and radio regimes. These
include ongoing legacy BASS campaigns, past observations through BASS and the
community, and various all-sky surveys (e.g. GALEX, GAIA, 2MASS, WISE, Akari,
IRAS). The status of targeted observational programs for the 70-month AGN as
of 2021 July is summarized in Table 3 and on the BASS
website.555http://www.bass-survey.com

In addition to this, partial sky coverage exists from several wide-field
surveys for hundreds of AGN. Multiband high-quality optical imaging ($<$2′′)
exists for the majority ($>$80%) of BASS AGN from the SDSS, the Panoramic
Survey Telescope and Rapid Response System (Pan-STARRS Chambers et al., 2019),
the DESI Legacy Imaging Surveys (Dey et al., 2019), The VLT Survey Telescope
ATLAS (Shanks et al., 2015), and targeted studies like Koss et al. (2011a). In
the NIR, coverage exists in the VISTA Hemisphere Survey (VHS; McMahon et al.,
2013) and the UKIRT Hemisphere Survey (UHS; Dye et al., 2018). In the radio
coverage at 1.4 GHz exists for more than half of the BAT AGN (Wong et al.,
2016) from the Faint Images of the Radio Sky at Twenty Centimeters (FIRST;
Becker et al., 1995) and NRAO VLA Sky survey (NVSS; Condon et al., 1998).

The BAT AGN have the largest coverage in the X-rays and UV, with all 858
observed with Swift/XRT and also observed with Swift/UVOT in the UV and
optical. This results in a vast database of simultaneous X-ray and UV
observations, obtained since the launch of Swift in 2004. For instance, for
broad-line (Sy1) AGN there are 32,184 distinct observations, due to the
slewing nature of Swift and their intensive coverage in legacy observations of
BAT AGN.

As the brightest ultra-hard-X-ray-selected AGN in the sky, many archival
observations are available from several other X-ray telescopes. In particular,
many BAT AGN have also been observed as part of 20 ks filler observations in
the NuSTAR BAT Legacy
Survey,666https://www.nustar.caltech.edu/page/legacy_surveys which continues
to observe approximately six BASS sources each month. A Chandra Cool Target
program,777Chandra-BASS (C-BASS);
https://cxc.harvard.edu/target_lists/CCTS.html which started in 2019 January,
is also observing nearby BAT AGN ($z<0.1$).

Another subset of observing programs focuses on high spatial resolution
imaging ($\sim$100 pc) that can be achieved for nearby BASS AGN ($z<0.1$).
This includes a recent HST SNAP program with the Advanced Camera for Surveys
(ACS), which has obtained $i$-band (F814W) images for 154 DR1 BAT AGN at
$z<0.1$ (Kim et al., 2021). A large HST SNAP program, approved through 2022,
aims to obtain near-UV (${<}$3000 Å) imaging of BASS AGN, followed up with
simultaneous X-ray and UV/optical observations of the nuclear AGN emission
with Swift and ground-based optical imaging in $griz$. Koss et al. (2018)
published 98 Keck/NIRC2 AO-assisted NIR observations (in the $Kp$ band) for a
volume-limited sample of AGN ($z<0.1$), along with many archival HST NIR
observations of more nearby galaxies available from earlier HST/NICMOS studies
(e.g. Hunt & Malkan, 2004). Finally, approved high-resolution AO NIR imaging
and spectroscopy are also underway through 2022 using Keck/NIRC2, Keck/OSIRIS,
and Gemini/GSAOI, with a focus on hidden galaxy mergers and dual, small-
separation AGN in obscured systems.

Finally, another set of survey programs are broadly focused on connecting the
key star-formation-related properties of the AGN hosts, such as SFR and
molecular and atomic gas, with AGN activity in nearby AGN, using high-
resolution and high-sensitivity observations over the IR-millimeter-radio
spectral regimes. These programs include earlier studies carried out with
Spitzer (e.g., Weaver et al., 2010) and Herschel (e.g., Mushotzky et al.,
2014), which focused on nearby BAT AGN and were then followed up in the
submillimeter and radio. The Herschel program to measure star formation
observed 317 of the nearest BAT AGN ($z<0.05$), which were later followed up
with more recent measurements of host galaxy molecular gas using CO lines
(Koss et al., 2021). and 22 GHz (e.g., Smith et al., 2020) observations with
the JVLA. A yet nearer-distance sample ($z<0.025$) is being targeted for HI
mapping using the JVLA. More molecular gas observations using APEX have also
been approved for these sources, through 2022. A program to obtain 100 pc
resolution CO(2-1) measurements using ALMA was done for 33 nearby and luminous
AGN. High spatial resolution radio observations (0.2–0.5 pc resolution) that
form a complete volume-limited sample out to 40 Mpc for AGN above $L^{\rm
obs}_{\mathrm{14-195\ keV}}$ $>10^{42}$ ${\rm\thinspace erg}{\rm\thinspace
s}^{-1}\,$, has also been done for a sample of 37 objects using C-band Very
Long Baseline Array (VLBA).

Table 2: Summary of BASS DR2 Data

Telescope | Instrument | Total | Range (Å) | Slit Width (′′) | $R$
---|---|---|---|---|---
(1) | (2) | (3) | (4) | (5) | (6)
Palomar | DBSP | 502 | 3150-10500 | 1.5 | 1220/1730
VLT | X-Shooter | 233 | 2990-10200 | 1.6/1.5 | 3850/6000
APO | SDSS | 177 | 3830-9180 | 3 | 1760/2490
du Pont | BC | 119 | 3000-9070 | 1 | 480
Archivalaafootnotemark:  | Various | 90 | Various | Various | Various
VLT | FORS2 | 70 | 3400-6100 | 1 | 830
SOAR | Goodman | 67 | 4560-8690 | 1.2 | 890/1630
Keck | LRIS | 21 | 3200-10280 | 1 | 1280/1810
Magellan | MAGE | 12 | 3300-10010 | 1 | 3850
VLT | MUSE | 6 | 4800-9300 | 2 | 1850/3150
Velocity Dispersion Setupsbbfootnotemark:  |  |  |  |  |
SOAR | Goodman | 86 | 7900-9070 | 1.2 | 4720
Palomar | DBSP | 66 | 3970-5499/8050-9600 | 2 | 2170/4720
NIR DR2 |  |  |  |  |
VLT | X-SHOOTER | 168 | 10240-24800 | 0.9 | 5400
Magellan | FIRE | 65 | 8000-25000 | 0.6 | 6000

aafootnotemark:

The archival sample is from earlier surveys that were not included in the DR1,
including ROSAT AGN that overlap with BASS in unpublished or published (Grupe
et al., 2004) works, from the Palermo surveys of Swift BAT AGN (Rojas et al.,
2017), or as part of an atlas of low redshift AGN (Ho & Kim, 2009). While not
typically used in catalog measurements because of new DR2 data, we include
these spectra for long-term studies of changing-look AGN (e.g., MacLeod et
al., 2019). bbfootnotemark: These setups were done for velocity dispersion
measurements of obscured AGN (e.g., Sy1.9 and Sy 2).

Note. — Column (1): telescope. Column (2): instrument. Column (3): total
number of DR2 spectra observed with this setup. Column (4): wavelength range
for the most common setup with the telescope. Column (5–6): slit width and
resolving power for the most common setup. In some cases larger or smaller
slit widths (e.g., 1$\farcs$5 vs. 2′′) were used resulting in different
resolutions. See Koss et al. (2022a) for a detailed list of instrument setups.
Two values are listed when the instrument had both a blue and red arm with
different settings. Resolving power is wavelength dependent in some cases and
so the values are given at 5000 Å and 8500 Å depending on the spectral range.

Table 3: Additional BASS and Archival Multiwavelength Data

Data Set / Telescope | Spectral Bands | $N_{\rm AGN}$ | Focus | Investigators
---|---|---|---|---
(1) | (2) | (3) | (4) | (5)
NuSTAR | 3-70 keV | 527 |  | Ricci, Koss, Archival
Swift XRT | 0.5-10 keV | 858 |  | Ricci, Koss, Archival
XMM-Newton | 0.5-10 keV | 386 |  | Ricci, Koss, Archival
Chandra | 0.5-8 keV | 384 |  | Koss et al., Archival
Suzaku | 0.3-10 keV | 210 |  | Archival
HST | F225W | 54+ | Sy1, $z{<}0.1$ | Koss et al.
Swift UVOT | UV ($W2$, $M1$, $W1$)/ $UBV$ | 812 |  | Ricci, Koss, Archival
XMM OM | UV ($W2$, $M1$, $W1$)/ $UBV$ | 342 |  | Ricci, Koss, Archival
SNIFS IFU | 3200-10200 Å | 46 | $z{<}0.05$ | Koss et al.
MUSE IFU | 4800-9300 Å | 84 |  | Archival
HST | F606W | 157 |  | Archival
HST | F814W | 243 | $z{<}0.1$ | Barth, Archival
HST | F160W | 104 |  | Archival
NIR AO (Keck/NIRC2, Gemini/GSAOI) |  $H$, $K$ | 98+ | $z{<}0.1$ | Koss, Treister et al.
NIR AO IFU (Keck/OSIRIS, VLT/SINFONI) |  $H$, $K$ | 108+ |  | Koss et al.
VLT/VISIR AO | 8-13 $\mu{\rm m}$ | 125 | $z{<}0.01$ | Asmus et al.
Spitzer IRS low res. | 5.3-35 $\mu{\rm m}$ | 175 |  | Archival
Spitzer IRS high res. | 10-37 $\mu{\rm m}$ | 140 | $z{<}0.05$ | Weaver et al.
Herschel | 70, 160, 250, 350, 500 $\mu{\rm m}$ | 317 | $z{<}0.05$ | Mushotzky, Shimizu et al.
JCMT/Scuba 2 | 450, 850 $\mu{\rm m}$ | 63 | $z{<}0.05$ | Koss et al.
ALMA | 100 GHz | 99 |  | Archival
APEX/IRAM/JCMT | CO 1-0/CO 2-1 | 305+ | z$<$0.05 | Koss, Shimizu, et al.
ALMA | CO 1-0/CO 2-1 | 156 |  $<$100 Mpc | Izumi et al., Archival
(J)VLA | 22 GHz | 232 | $z{<}0.05$ | Smith, Mushotzky, et al.
VLBA | 5 GHz | 37 |  $<$40 Mpc | Secrest et al.
GBT | HI | 96 | $z{<}0.05$ | Winter et al.
ATCA/(J)VLA/WSRT/GMRT | HI mapping | 98 |  $<$120 Mpc | Chung, Wong, et al.

Note. — Column (1): telescope or instrument for the survey data. If the data
are substantially similar (e.g., AO imaging in the same filter), we have
grouped telescopes. Column (2): wavelength, frequency, line, filter, or energy
band. Column (3): total number of unique AGN from the Swift BAT 70-month
catalog, which includes a total of 858 AGN. This number does not include
105-month AGN which will be released in subsequent catalogs (BASS DRs). The
“+” sign indicates approved and/or ongoing additional observations. Column
(4): indicates whether (part of) the observations were focused on a volume-
limited sample, and/or particular AGN subxclass. Column (5): main
investigators for survey data. “Archival” indicates that the majority of
corresponding data are from disparate observing programs.

##  5 Overall Survey Results

The BASS survey is a spectroscopically complete for 100% (858/858) of the AGN
identified in the 70-month BAT all-sky survey outside of 7 sources deep within
the Galactic plane ($\lvert b\rvert{<}3^{\circ}$) which we were unable to
target. The BASS DR2 reports redshifts for 99.9% (857/858) of the AGN,
excluding only one continuum dominated blazar with a foreground Galactic star.
This includes 47 redshifts reported for the first time. Outside of the
Galactic plane ($\lvert b\rvert{>}10^{\circ}$) the survey completeness in BH
mass measurements from broad lines or stellar velocity dispersion is $98\%$
for all unbeamed AGN because of the typically lower extinction in these
regions. The remaining sources without BH mass measurements are mostly double-
peaked/and or assymetric broad-line AGN and high redshift Sy 2 ($z>0.1$),
where high-quality velocity dispersion measurements are difficult. For beamed
AGN we report black hole masses only for the BZQ class based on broad lines;
for continuum-dominated blazars (BZB) velocity dispersion measurements are
difficult to the AGN-dominated continuum.

###  5.1 Survey Completeness and Measurables

In this section, we briefly illustrate the distributions of some of the key
AGN-related properties of the BAT 70-month AGN, as determined through the BASS
DR2 measurements We also briefly discuss the reliability and limitations of
the measurements. A more elaborate forward-modeling approach accounting for
various survey selection effects associated with a flux-limited survey with
various levels of obscuration is described in Ananna et al. (2022).

Summaries of the typical bolometric luminosities, BH masses, Eddington ratios,
and line-of-sight hydrogen column densities are provided in Table 4 and Table
5 for unbeamed and beamed AGN, respectively. Their BH mass vs. Eddington ratio
parameter space is shown in Figure 3. Overall, the completeness quite similar
for Sy 1, Sy 1.9, and Sy 2 ($\sim 98\%$) outside the Galactic plane.

As a survey of the nearest and brightest hard X-ray-selected AGN in the sky,
the measurement-associated uncertainties on BASS measurements are typically
small. Apart from the beamed sources (e.g., blazars or “BZB”) that lack
emission lines and a handful of sources that are located in extremely crowded
(Galactic plane) regions, the spectra of all BASS sources have multiple
emission lines for robust redshift measurements. The uncertainties on BH mass
determinations from $\sigma_{*}$ are dominated by the systematics on the
$M_{\rm BH}$-$\sigma_{*}$ scaling relation (e.g., $\sim$0.3-0.5 dex, Marsden
et al., 2020) rather than on the scatter found in repeat observations (Koss et
al., 2022b, $\sim$0.1-0.2 dex). Similarly, the uncertainties on BH masses
derived through spectral analysis of broad lines may reach $\sim$0.3-0.4 dex
(Peterson, 2014) whereas the measurement uncertainty is much lower (i.e., 0.1
dex Mejía-Restrepo et al., 2022). The uncertainties on $N_{\rm H}$ are
$\sim$0.05 dex for $\log(N_{\rm H}/{\rm cm}^{-2})$$<$23.5 and $\sim$0.3 for
$\log(N_{\rm H}/{\rm cm}^{-2})$$>$23.5 (Ricci et al., 2017a). For intrinsic
X-ray luminosity, the errors are typically $<0.1$ dex (Lanz et al., 2019),
unless the AGN are CT, in which case the typical errors can reach $\approx
0.4$ dex (Ricci et al., 2015). The bolometric luminosities are calculated from
the intrinsic luminosities in the 14–150 keV range as shown in Ricci et al.
(2017a, see their Table 12), using a bolometric correction of 8 (see, e.g.,
Koss et al., 2022a). In this case, the uncertainties are in the range of 0.2
dex (e.g., Trakhtenbrot et al., 2017).

We also looked at relationships between the line-of-sight column density, as
measured from X-ray data, and BH mass and Eddington ratio, as determined from
optical spectroscopy (Figure 4). One clear takeaway is that unobscured (Sy 1)
AGNs occupy a region of higher Eddington ratios compared to obscured (Sy 1.9s
and 2) AGN. This echoes the previous BASS DR1-based finding by Ricci et al.
(2017b), which supports a scenario where radiation pressure is the main driver
of the geometry of the (dusty) circumnuclear gas. There are no obvious trends
between $N_{\rm H}$ and either $M_{\rm BH}$ or $L_{\rm bol}/L_{\rm Edd}$
within each AGN optical subclass.

Figure 3: Distribution of BH masses ($M_{\rm BH}$) and Eddington ratios
($L_{\rm bol}/L_{\rm Edd}$) for the entire sample of BASS AGNs for which
redshift measurements are available, including unbeamed AGN (e.g., Sy 1, Sy
1.9, and Sy 2) and beamed AGN (BZQs). The lower limits on $M_{\rm BH}$ and
$L_{\rm bol}/L_{\rm Edd}$ due to the survey flux limits are illustrated with
dashed (antidiagonal) lines, for various redshifts corresponding to $z{=}0.01$
($L_{\rm bol}$${=}1.8{\times}10^{43}$${\rm\thinspace erg}{\rm\thinspace
s}^{-1}\,$), $z{=}0.04$ ($L_{\rm bol}$${=}3.2{\times}10^{44}$${\rm\thinspace
erg}{\rm\thinspace s}^{-1}\,$), and $z{=}0.1$ ($L_{\rm
bol}$${=}2.0{\times}10^{45}$${\rm\thinspace erg}{\rm\thinspace s}^{-1}\,$).
Errors in $M_{\rm BH}$ are of order 0.5 dex owing to systematic uncertainties
in virial and $\sigma_{*}$-based scaling relations (e.g., Ricci et al., 2022).
The BASS unbeamed AGN occupy narrow, redshift-dependent slices of the $M_{\rm
BH}-L_{\rm bol}/L_{\rm Edd}$ plane due to the Eddington limit and survey flux
limits (see text for details). Interestingly, we find that the $\log$($L_{\rm
bol}/L_{\rm Edd}$) tends to be bounded at -2.5 to -3 possibly associated with
BAT identifying disk accretion primarily rather than inefficient accretion and
the upper bounds at the Eddington limit (except for beamed AGN). The lower
bound of the BH mass distribution corresponds to $\log\left(M_{\rm
BH}/M_{\odot}\right)$=5, where the range pushes into intermediate-mass BHs and
would only be sensitive to Eddington/super-Eddington accretion, if it exists.
The upper limit at $\log\left(M_{\rm BH}/M_{\odot}\right)$=10 is largely owing
to the space density of massive BHs.

Figure 4: Summary scatter plots of the line-of-sight hydrogen column density
($N_{\rm H}$, an obscuration indicator) vs. BH mass (top) and Eddington ratio
(bottom). Different symbols mark subclasses of unbeamed BASS AGN (e.g., Sy 1,
Sy1.9, and Sy 2). X-ray measurements of $N_{\rm H}=10^{20}\,{\rm cm}^{-2}$ are
essentially upper limits and represent sources with no sign of obscuration.
The large squares indicate the binned medians for each subclass. Error bars on
the plotted median values are equivalent to 1$\sigma$ and calculated based on
a bootstrap procedure with 100 realizations. The bin sizes were constructed to
have equal numbers of sources in each bin. The Sy 1 AGN tend to have higher
Eddington ratios than the narrow-line AGN (Sy 1.9 and Sy 2). Typical 90%
errors in $N_{\rm H}$ are $<0.2$ dex based on X-ray modeling (Ricci et al.,
2017a), but higher for heavily obscured AGN log $\log(N_{\rm H}/{\rm
cm}^{-2})$$>24.5$. Table 4: Summary of Unbeamed AGN Properties

Type | $N$ | $N$ | $z$ | $N_{M_{\rm BH}}$ | $N_{M_{\rm BH}}$ | % Meas $M_{\rm BH}$ | % Meas |  $\log$ $M_{\rm BH}$ |  $\log$ $L_{\rm bol}$ |  $\log$ $L_{\rm bol}/L_{\rm Edd}$ |  $\log$ $N_{\rm H}$
---|---|---|---|---|---|---|---|---|---|---|---
|  | $\lvert b\rvert>10^{\circ}$ |  |  | $\lvert b\rvert>10^{\circ}$ |  | $\lvert b\rvert>10^{\circ}$ | ($M_{\odot}$) | (${\rm\thinspace erg}{\rm\thinspace s}^{-1}\,$) |  | (cm-2)
(1) | (2) | (3) | (4) | (5) | (6) | (7) | (8) | (9) | (10) | (11) | (12)
Sy1 | 359 | 318 | 0.050$\pm$0.003 | 350 | 311 | 97 | 98 | 7.81$\pm$0.04 | 44.87$\pm$0.04 | -1.17$\pm$0.03 | 20.0$\pm$0.05
Sy1.9 | 101 | 86 | 0.030$\pm$0.004 | 97 | 84 | 96 | 98 | 7.98$\pm$0.06 | 44.59$\pm$0.08 | -1.61$\pm$0.09 | 22.28$\pm$0.13
Sy2 | 292 | 259 | 0.029$\pm$0.003 | 275 | 253 | 94 | 98 | 8.06$\pm$0.04 | 44.50$\pm$0.04 | -1.71$\pm$0.04 | 23.27$\pm$0.05
Total | 752 | 663 | 0.038$\pm 0.002$ | 722 | 648 | 96 | 98 | 7.96$\pm$0.03 | 44.67$\pm$0.03 | -1.42$\pm$0.03 | 21.98$\pm$0.06

Note. — Summary of the medians and standard error of the median for different
populations of unbeamed AGN. Column (1): AGN optical type based on the
presence of broad H$\beta$ and H$\alpha$. Column (2): total for the whole
sample. Column (3): total excluding the Galactic plane region $\lvert
b\rvert<10^{\circ}$ where high optical extinction makes measurements more
difficult. Column (4): median redshift from optical lines. Columns (5–8):
number of unique AGN with $M_{\rm BH}$ measurements and excluding the Galactic
plane region $\lvert b\rvert<10^{\circ}$ where high optical extinction makes
measurements more difficult; also listed as percentages. Columns (9-12):
median $M_{\rm BH}$, $L_{\rm bol}$, $L_{\rm bol}/L_{\rm Edd}$, and
$\log(N_{\rm H}/{\rm cm}^{-2})$ for the sample.

Table 5: Summary of Beamed AGN Properties

Type | $N$ |  $N$, $\lvert b\rvert>10^{\circ}$ | $z$ | $N_{M_{\rm BH}}$ | % Meas $M_{\rm BH}$ |  $\log$ $M_{\rm BH}$ |  $\log$ $L_{\rm bol}$ |  $\log$ $L_{\rm bol}/L_{\rm Edd}$ |  $\log$ $N_{\rm H}$
---|---|---|---|---|---|---|---|---|---
(1) | (2) | (3) | (4) | (5) | (6) | (7) | (8) | (9) | (10)
BZQ | 74 | 63 | 0.88$\pm$0.12 | 67 | 91 | 8.83$\pm$0.09 | 47.66$\pm$0.16 | 0.38$\pm$0.12 | 20$\pm$0.11
BZB | 22 | 18 | 0.13$\pm$0.02 |  |  |  | 45.81$\pm$0.12 |  | 20.57$\pm$0.12
BZG | 8 | 6 | 0.07$\pm$0.02 |  |  |  | 45.11$\pm$0.20 |  | 20.81$\pm$0.11
Sy1/lensed | 1 | 1 | 0.65 | 1 | 100 | 8.79 | 47.18 | 0.21 | 20
BZQ/lensed | 1 | 0 | 2.51 |  |  |  | 49.49 |  | 22.77
Total | 106 | 88 | 0.33$\pm$0.10 | 68 |  | 8.83$\pm$0.09 | 46.53$\pm$0.14 | 0.38$\pm$0.12 | 20.54$\pm$0.08

Note. — Summary of the medians and standard error of the median for different
populations of beamed and/or lensed AGN. Column (1): AGN optical type based on
presence of broad lines (BZQ), only host galaxy features lacking broad lines
(BZG), or traditional continuum dominated blazars with no emission lines
(BZB), or lensing. Column (2): Total for the whole sample. Column (3) total
excluding the Galactic plane region $\lvert b\rvert<10^{\circ}$ where high
optical extinction makes measurements more difficult. Colun (4): median
redshift from optical lines. Column (5–6): number of unique AGN with $M_{\rm
BH}$ measurements and percentages. Column (7–10): median $M_{\rm BH}$, $L_{\rm
bol}$, $L_{\rm bol}/L_{\rm Edd}$, and, $\log(N_{\rm H}/{\rm cm}^{-2})$ for the
sample.

###  5.2 Redshift Survey Biases

Looking more carefully into Figure 3, the unbeamed BASS AGN typically occupy
narrow regions in the BH mass—Eddington ratio plane, in different redshift
intervals. These could be easily understood as a combination of survey (flux)
limits, AGN physics, and demographics. For example, the most distant AGN in
our sample ($z{>}0.1$) include almost no sources with small BHs ($M_{\rm
BH}{\lesssim}10^{7}$ $M_{\odot}$) because these would have to be super-
Eddington to be detected.888That is, $L_{\rm bol}\propto M_{\rm BH}\times
L_{\rm bol}/L_{\rm Edd}$ for a high-$z$ source would result in a low flux.
Likewise, the lowest Eddington ratios sources ($L_{\rm bol}/L_{\rm
Edd}{\lesssim}10^{-3}$) are also almost exclusively at the highest-mass and
lowest-redshift ($z{<}0.01$) systems. However, while our sample is affected by
strong low-mass and low accretion rate biases, there are no biases against
super-Eddington non-beamed AGN. Notably, such sources are extremely rare in
the survey, suggesting that the Eddington limit remains meaningful despite the
simplifications in its derivation.999The alternative is that super-Eddington
accretion in SMBHs is extremely X-ray weak (e.g., Laurenti et al., 2022).

We show the Eddington ratio vs. redshift in the top panel of Figure 5. Despite
the aforementioned possible biases, Sy 1 sources tend to have higher Eddington
ratios (on average) than Sy 1.9 or Sy 2 sources, even when matched in
redshift, though this difference decreases towards the highest redshifts in
the sample ($z{>}0.05$). The Sy 1.9 and Sy 2 classes follow the same
distribution in $L_{\rm bol}/L_{\rm Edd}$ rising sharply with redshift from
$L_{\rm bol}/L_{\rm Edd}\approx 10^{-2}$ at $z{=}0.01$ to $L_{\rm bol}/L_{\rm
Edd}\approx 6{\times}10^{-2}$. Conversely, the median Eddington ratio of Sy 1s
is nearly flat with redshift ($L_{\rm bol}/L_{\rm
Edd}\approx(7-9){\times}10^{-2}$).

###  5.3 Comparison to Other Surveys

A comparison to other distant AGN X-ray surveys is shown in Figure 1. Due to
its all-sky coverage, the BAT AGN include the highest number of sources at
${z}<$0.1, the most luminous unbeamed QSOs at $0.1<z<0.6$, and a population of
high-redshift beamed AGN between $z{=}1-3.5$. In luminosity space the survey
overlaps well with medium-deep surveys (e.g. XMM XXL, Stripe 82, and COSMOS)
at $0.5<z<3$.

Among all-sky X-ray surveys, it is useful to compare the BAT AGN to the past
ROSAT survey in the soft X-ray band ($0.1-2.4$ keV; Truemper, 1982). The more
recent reprocessing of the second ROSAT all-sky survey (2RXS) source catalog
(Boller et al., 2016) had a flux limit of $\sim 10^{-13}$ erg cm-2 s-1at
0.1-2.4 keV, with $\sim$130,000 sources. Assuming $\Gamma=1.8$, this flux
limit corresponds to $\sim 2\times 10^{-13}$ over the BAT 14-195 keV band,
which is $\sim$50$\times$ deeper than the 70-month BAT survey. Of these, 7005
ROSAT sources were cross-matched with the SDSS Data Release 5 (Anderson et
al., 2007). Due to the soft X-ray sensitivity of ROSAT, the vast majority of
these sources (6224/7005, or 89%) were broad-line AGN whereas the fraction
within BASS for such (unbeamed) sources is 42%. In addition, the median
redshift of the (subset of) ROSAT sources was $z=0.42$, which is more than a
factor of 10 more distant than the unbeamed BASS AGN ($z\simeq 0.037$). The
BASS overlap with ROSAT is 95% for Sy 1 sources, down to 53% for Sy2, and only
30% for LINERS (see, e.g., Oh et al., 2022, for further details). A more
comprehensive comparison between the 2RXS and the BAT AGNs is also available
in Oh et al. (2018).

The concurrent eROSITA mission and its all-sky survey (0.2-8 keV Predehl et
al., 2021) are expected to eventually yield a few million AGN and be roughly a
factor of 100 times deeper (${\sim}10^{-15}$ erg cm-2 s-1) than ROSAT, which
means a particularly larger number of higher-redshift sources. The eROSITA
Final Equatorial Depth Survey (eFEDS), with a depth of ${\sim}10^{-14}$ erg
cm-2 s-1at 0.5-2 keV (Wolf et al., 2021) over 140 deg2 provides some early
insight into what one could expect for the AGN population to be surveys by
eROSITA. Specifically, 90% are unobscured ($\log(N_{\rm H}/{\rm
cm}^{-2})$$<$21.5) and the redshift distribution peaks at around $z\simeq 1$
(Liu et al., 2021), though redshift determination for the majority of sources
is problematic until larger spectroscopic surveys are completed (e.g., via
SDSS-V, Kollmeier et al. 2017 and/or VISTA/4MOST, Salvato et al. 2021) Thus,
we expect that BASS will provide a bright complement of well-understood
luminous nearby sources ($z\simeq 0.037$) that is less biased with regard to
obscuration, but also missing the numerous distant AGN to be detected by
eROSITA.

To put the BASS sample in perspective, we also compare it to several other
optical surveys of nearby luminous AGN, including the SDSS quasars (Shen et
al., 2011), SDSS Sy2 AGN (Greene & Ho, 2005), the PG quasars (Boroson & Green,
1992), and Type 2 Quasars selected using [O iii] $\lambda 5007$ emission (Kong
& Ho, 2018). Compared to these samples, the BAT-selected AGN are typically
found at lower redshifts ($z{<}0.1$). The Eddington ratios of broad-line (Sy
1) AGN are above the SDSS Sy 2 AGN, consistent with those of SDSS and PG
quasars, but below the SDSS Type 2 quasars. Among Sy 1.9 and Sy 2 types, only
the highest redshift BAT sources ($z{>}0.08$) have similar Eddington ratios as
the SDSS quasars and PG quasars. However, the SDSS Type 2 quasars tend to have
significantly higher Eddington ratios than BAT Sy1.9 or Sy 2 types, despite
overlap in redshift at $z{=}0.1$. Finally, BASS Sy 2 AGN tend to have higher
Eddington ratios than SDSS Sy 2 AGN.

The most luminous quasars in our sample are generally not found in other
quasar samples. We investigated whether the most luminous BAT AGN were
selected by the SDSS quasars and Type 2 quasars samples and found virtually no
BAT AGN in these samples. We focused specifically on the range of $z{<}0.3$
and $L_{\rm bol}{>}10^{46}\,{\rm erg\,s}^{-1}$, which includes 18 unbeamed BAT
AGN, of which 8 are found within the SDSS footprint. Of these, only one source
(SWIFTJ1547.5+2050 aka 3C323.1) is selected by the SDSS quasar sample. The
other seven AGN were not targeted for SDSS spectroscopy. In five cases the
quasar is classified as a star in terms of colors, and in two cases the
obscured (Sy2) AGN are classified as galaxies, but no spectra were taken.

On the other hand, there are 14 SDSS quasars with $L_{\rm
bol}$${>}10^{46}\,{\rm erg\,s}^{-1}$, which are not detected by the BAT
survey. The BAT detection limit at $z{=}0.3$ for 90% sky coverage is
equivalent to $L_{\rm bol}$$=1.1{\times}10^{46}$ erg s-1, assuming a simple
conversion of $L_{\rm bol}$$=8{\times}$$L^{\rm obs}_{\mathrm{14-195\ keV}}$
${\rm\thinspace erg}{\rm\thinspace s}^{-1}\,$(e.g., Koss et al., 2017). Hence,
all the 14 SDSS quasars should be detected. It is possible that these
undetected quasars may be part of a class of X-ray-weak quasars that have been
found by several campaigns (e.g., Laor et al., 1997; Pu et al., 2020).
Alternatively, the single, constant X-ray bolometric correction, rather than a
luminosity-dependent one (e.g., Duras et al., 2020), may be too low for our
sample.Techniques can be used to study known sources at $\sim 3\times$ fainter
limits within BAT and would be ideal for this population (e.g., Koss et al.,
2013). We reserve further discussion for future detailed studies. For the SDSS
Type 2 Quasars, there is no overlap between the samples, with 26 AGN in the
Type 2 Quasars having $L_{\rm bol}$${>}10^{46}\,{\rm erg\,s}^{-1}$, and 0/26
detected by BAT, when all should be detected.

We further compare the bolometric luminosities and BH masses of the BASS DR2
sample. Only the most luminous quartile of Sy 1.9 and Sy 2 AGN reaches the
average luminosities of the SDSS quasars at similar redshifts ($z{<}0.3$). By
comparison, roughly half of the BASS Sy 1 sample occupies similar
distributions in bolometric luminosity and BH mass as the SDSS quasars. The Sy
1 AGN have similar luminosities as the PG quasars, other than the least
luminous quartile, but somewhat larger BH masses. The SDSS Type 2 quasars
occupy a region significantly above the BAT Sy 1.9 and Sy 2 AGN in $L_{\rm
bol}$, reached only by the most luminous quartile of the Sy 1 AGN.

Finally, the BASS sample has a significant number of low-mass BHs ($M_{\rm
BH}{<}10^{7}\,M_{\odot}$) that are not present in any of the other comparison
samples. This feature of BASS is due to the higher spectral resolution
($R>2500$) in the optical spectroscopy, which allows us to resolve spectral
features tracing smaller BHs; the purely AGN-dominated selection in the $>$14
keV band, which allows to study low-$M_{\rm BH}$ AGNs whose optical emission
is host dominated; and the ability to study even the nearest AGN (i.e.,
$z<0.01$) which are saturated in SDSS imaging and thus excluded from the
spectroscopic follow-up. A small number of nearby ($z{<}0.01$) SMBHs have
Eddington ratios as low as $\simeq 10^{-5}$ (e.g., M81, Devereux, 2019) and
offer an opportunity to study the emission properties of advection-dominated
accretion flows (ADAF, Yuan & Narayan 2014). These radiatively inefficient
accretion flows result in broadband SEDs that are markedly different from
those characterizing standard, thin-disk accretion (see, e.g., Ryan &
MacFadyen 2017).

We finally compare the BASS DR2 sample to MIR-selected AGN. The standard WISE
color cut ($W1-W2>0.8$ Stern et al., 2012) identifies only 56% of the BAT AGN
sample (482/858). The fraction of detections is highly dependent on AGN
luminosity, with a much higher fraction of luminous AGN detected (Figure 6;
see also Ichikawa et al., 2017).

We also compare the number of BASS AGN in the WISE-selected AGN drawn from the
30,093 deg2 of extragalactic sky in the AllWISE Data Release (Assef et al.,
2018). The AllWISE AGN study by Assef et al. (2018) provides an AGN catalog
with 90% reliability (the “R90” catalog), selected purely using the WISE $W1$
and $W2$ bands, but with lower completeness. However, many of the BAT AGN are
excluded by default because they reside in galaxies that are extended in
2MASS, a criterion adopted by the WISE AGN catalog to avoid contamination by
separate, resolved parts of nearby galaxies. We find that only about 74/858
BASS DR2 sources overlap with the WISE R90 AGN catalog (9%), including 37
broad line AGN (Sy 1–1.8), 6 narrow line AGN (Sy 1.9–2), 28 beamed broad line
AGN, and 3 continuum-dominated blazars (BZB). This corresponds to a WISE AGN
detection fraction of 11.7% (37/314) for broad line AGN and only 1.5% for
obscured BAT AGN (6/393), though the detection fraction would be higher if
extended galaxies were included in the WISE AGN catalog. We note again that
the WISE detection fraction is strongly dependent on AGN luminosity, with no
BAT AGN with $L_{\rm bol}{<}5\times 10^{44}\,{\rm erg\,s}^{-1}$ found in the
WISE AGN catalog.

When comparing the $L_{\rm bol}-z$ distribution of BAT- and WISE-selected AGN,
in Figure 6, it appears that the WISE AGN tend toward high redshifts at
similar AGN luminosities, due to the requirement that they are point-like in
2MASS. If we further restrict the WISE AGN to $L_{\rm bol}$${>}10^{46}\,{\rm
erg\,s}^{-1}$ and $z{<}0.3$, there are six WISE AGN above this limit from
(Barrows et al., 2021), five of which overlap with the BAT sample. On the
other hand, we find 6/18 (33%) of the unbeamed BAT quasars with $z{<}0.3$ and
$L_{\rm bol}{>}10^{46}\,{\rm erg\,s}^{-1}$ are in the WISE all-sky AGN
catalog.

In summary, BAT is finding a broad range of nearby AGN in terms of bolometric
luminosity, BH mass, Eddington ratio, and particularly obscuration, including
a significant population of low $L_{\rm bol}/L_{\rm Edd}$, low $M_{\rm BH}$
sources, with a well-characterized selection function and $>$99% complete
spectroscopic coverage making it a unique legacy sample for future AGN
studies. Given the unique selection criteria provided by BAT and the complete,
adaptive optical spectroscopy, it complements other legacy samples of nearby
AGN.

##  6 Summary

We present here an overview of the BASS DR2, with 1449 optical spectra, of
which 1181 are released for the first time, for the 858 ultra-hard-X-ray-
selected AGN in the Swift BAT 70-month sample. In this special issue, we
provide several immediate top-level scientific results and catalogs, including
the following:

  1. 1.

A largely statistically complete sample with 99.6% and 98% of the brightest
858 ultrahard X-ray (14-195 keV) selected AGN outside the Galactic plane
having measured spectroscopic redshifts and BH mass estimates (respectively).
The BH masses are derived from broad emission line (Mejía-Restrepo et al.,
2022) or from stellar velocity dispersion measurements (Koss et al., 2022b).

  2. 2.

The 858 AGN represent a uniquely complete census of nearby AGN ($z<0.3$),
spanning 5-7 orders of magnitude in AGN bolometric luminosity ($L_{\rm
bol}\sim 10^{40}-10^{47}\,{\rm erg\,s}^{-1}$), BH mass ($M_{\rm BH}\sim
10^{5}-10^{10}\,M_{\odot}$), Eddington ratio ($L_{\rm bol}/L_{\rm Edd}\sim
10^{-5}-10$), and obscuration ($N_{\rm H}\sim 10^{20}-10^{25}\,{\rm
cm}^{-2}$). These AGN are largely distinct from those found by other surveys,
specifically with very little overlap even among nearby SDSS quasars or WISE
AGN.

  3. 3.

A large catalog of emission-line measurements from 3200-10000 Å (Oh et al.,
2022) for the 858 AGN and an additional 233 NIR spectroscopic measurements
($1-2$ $\mu{\rm m}$; Brok et al., 2022; Ricci et al., 2022).

  4. 4.

The first directly constrained BHMF and ERDF using both unobscured and
obscured AGN, in addition to a highly robust determination of the XLF (Ananna
et al., 2022).

  5. 5.

The significant bias toward underestimation of BH mass when using H$\beta$ or
H$\alpha$ emission in obscured systems ($\log(N_{\rm H}/{\rm cm}^{-2}){>}21$;
see Mejía-Restrepo et al., 2022; Ricci et al., 2022).

  6. 6.

The ability of MIR emission to recover the X-ray column density (Pfeifle et
al., 2022).

We hope that these initial results are only the beginning of the legacy value
of the BASS project for understanding BH growth in nearby AGN, and that the
data products will be of lasting and general usefulness to the broader
astronomical community. There are a variety of studies that can be done using
this dataset, such as focusing on SFRs, stellar masses, stellar population
ages, dust reddening, metallicities, AGN-driven outflows, weak/faint emission
lines, and links to morphological studies — all of which are not a significant
part of the present data release. The broad wavelength coverage of the BASS
sample is highly conducive to modeling SED with recent modeling tools (e.g.,
X-CIGALE; Yang et al., 2020). The future DR3 will focus in particular on
fainter AGN from the 105-month BAT catalog (Oh et al., 2018), which reaches
flux limits 23% deeper than the 70-month catalog used for DR2, and for which
follow-up observations are currently ongoing. We encourage the community to
engage with the BASS data and team, to maximize the science output of this
unique sample and dataset.

Figure 5: The distribution of BASS DR2 AGNs in the Eddington ratio vs.
redshift (top) and bolometric luminosity vs. BH mass (bottom) planes. The
large squares indicate the binned medians for each AGN subclass with redshift
(top) and $M_{\rm BH}$ (bottom). Error bars on the plotted median values are
equivalent to 1$\sigma$, calculated based on a bootstrap procedure with 100
realizations. The number of bins was fixed to four constructed to have equal
numbers of sources in each bin. For comparison, we plot SDSS quasars at
$z{<}0.3$ (grey contours; Shen et al., 2011) and lower-luminosity SDSS Type 2
Seyferts (purple contours; Greene & Ho, 2005). The solid and dashed contour
covers 68% and 95% of the data, respectively. We also plot the median for PG
Quasars (Boroson & Green, 1992) (black squares) and SDSS Type 2 Quasars (Kong
& Ho, 2018) selected based on their [O iii] $\lambda 5007$ emission (purple
squares). The BASS AGN have roughly similar BH masses and bolometric
luminosities to the different SDSS samples but also extend to lower redshifts
and BH masses. We note that there is essentially no overlap between BASS DR2
and these SDSS-based samples of powerful AGNs (see text for discussion).

Figure 6: The distribution of BASS DR2 in the bolometric luminosity vs. and
redshift (top) and WISE colors vs. bolometric luminosity (bottom) planes. For
comparison, we also plot the distributions of WISE-selected AGN at $z{<}0.3$
(Assef et al., 2018, red contours), with SED fitting and redshifts measured
from the SDSS (Barrows et al., 2021). The solid and dashed contours cover 68%
and 95% of the WISE AGN data, respectively. A black dashed line indicates the
WISE color cut to identify AGN ($W1-W2>0.8$ Stern et al., 2012). The BAT AGN
tend to probe similar luminosities as the higher redshift WISE AGN, but at
lower redshifts.

We thank the reviewer for the constructive comments that helped us improve the
quality of this paper. BASS/DR2 was made possible through the coordinated
efforts of a large team of astronomers, supported by various funding
institutions, and using a variety of facilities. We acknowledge support from
NASA through ADAP award NNH16CT03C (M.K.); the Israel Science Foundation
through grant number 1849/19 (B.T.); the European Research Council (ERC) under
the European Union’s Horizon 2020 research and innovation program, through
grant agreement number 950533 (B.T.); FONDECYT Regular 1190818 (E.T., F.E.B.)
and 1200495 (E.T., F.E.B); ANID grants CATA-Basal AFB-170002 (E.T., F.E.B.),
ACE210002 (E.T., F.E.B.) and FB210003 (C.R., E.T., F.E.B.); ANID Anillo
ACT172033 and Millennium Nucleus NCN19_058 (E.T.); and Millennium Science
Initiative Program – ICN12_009 (F.E.B.); an ESO fellowship (M.H.,J.M.);
Fondecyt Iniciacion grant 11190831 (C.R.); the National Research Foundation of
Korea grant NRF-2020R1C1C1005462 and the Japan Society for the Promotion of
Science ID: 17321 (K.O.); Comunidad de Madrid through the Atracción de Talento
Investigador Grant 2018-T1/TIC-11035 (I.L.); YCAA Prize Postdoctoral
Fellowship (M.B.); from a Clay Fellowship administered by the Smithsonian
Astrophysical Observatory and by the BH Initiative at Harvard University,
which is funded by grants from the John Templeton Foundation and the Gordon
and Betty Moore Foundation (F.P.). This work was performed in part at the
Aspen Center for Physics, which is supported by National Science Foundation
grant PHY-1607611. We acknowledge the work done by the 50+ BASS scientists and
Swift BAT team to make this project possible. We acknowledge the various
telescopes used in this paper. We are tremendously thankful to all the
observing and support staff in all the observatories, and their headquarters,
for their great assistance in planning and conducting the observations that
made BASS/DR2 possible.
Specifically, BASS/DR2 is based on data obtained through the European
Organisation for Astronomical Research in the Southern Hemisphere (ESO); the
Palomar Observatory; the Southern Astrophysical Research (SOAR); the Kitt Peak
National Observatory, managed by the US National Science Foundation’s NOIRLab;
the 6.5 m Magellan and the 2.5 m du Pont telescopes located at Las Campanas
Observatory, Chile; the W.M. Keck Observatory at Maunakea, Hawaii; and the
various stages of the Sloan Digital Sky Survey (SDSS). A full list of
observing facilities, program numbers, and their supporting bodies is provided
in the main DR2 Catalog paper (Koss et al., 2022a, see the acknowledgments
there). A significant part of the BASS observations and work took place during
the COVID-19 crisis. We thank the healthcare experts in communities around the
world, for their tireless efforts to keep us all as safe and healthy as
possible.

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