###### Abstract

Galaxy mergers provide a mechanism for galaxies to effectively funnel gas and
materials toward their nuclei and fuel the central starbursts and accretion of
supermassive black holes. In turn, the active nuclei drive galactic-scale
outflows that subsequently impact the evolution of the host galaxies. The
details of this transformative process as they pertain to the supermassive
black holes remain ambiguous, partially due to the central obscuration
commonly found in the dust-reddened merger hosts, and also because there are
relatively few laboratories in the nearby universe where the process can be
studied in depth. This review highlights the current state of the literature
on the role of accreting supermassive black holes in local luminous infrared
galaxies as seen from various windows within the electromagnetic spectrum.
Specifically, we discuss the multiwavelength signatures of the active nucleus,
its associated feeding and feedback processes, and the implications of
multiple supermassive black holes found in nearby interacting galaxy systems
for galaxy evolution from the observational perspective. We conclude with a
future outlook on how the topic of active nuclei in low- and high-redshift
galaxy mergers will benefit from the advent of next-generation observing
facilities with unparalleled resolving power and sensitivity in the coming
decade.

###### keywords:

Luminous infrared galaxies; Galaxy mergers; Seyfert galaxies; Starburst
galaxies; Active galactic nuclei; Supermassive black holes; Galaxy
interactions; Kinematics and dynamics

8 8 392 Academic Editors: Anna Sajina and Asantha R. Cooray 25 June 2022 20
July 2022 26 July 2022 https://doi.org/10.3390/universe8080392 The Role of AGN
in Luminous Infrared Galaxies from the Multiwavelength Perspective The Role of
AGN in Luminous Infrared Galaxies from the Multiwavelength Perspective Vivian
U U, VivianU, V.

##  1 Introduction

Galaxies, the fundamental building blocks of the universe, form and evolve
over cosmic time. They might simply have cold gas accretion from the cosmic
web onto their galactic disks to form stars (Martin et al., 2016). However,
the $\Lambda$ cold dark matter (CDM) cosmological model dictates that
structure grows hierarchically (White and Rees, 1978; Blumenthal et al.,
1984), so merging seems to be an inevitable stage within a massive galaxy’s
lifetime. Often in field or loose group environments, spiral galaxies
encounter a near-equal mass companion with which they engage in a major
merger. Such collision events bring about dramatic changes to their stellar
content, gas content, central supermassive black hole (SMBH), and their
overall fate, as stars and gas lose angular momentum and funnel toward the
dynamical center (Sanders et al., 1988; Hopkins et al., 2008). How the
properties of these individual components are affected by the encounter
depends on the specific circumstances associated with the interaction, but one
common phase for these colliding systems is when they display extreme infrared
excess from dust emission and high rates of massive star formation (Rieke and
Lebofsky, 1979; Lonsdale et al., 1984). This infrared excess leads to an
observable bias, dubbing this population of galaxies—“infrared galaxies”.

The study of infrared galaxies surged with the launch of the Infrared
Astronomical Satellite (_IRAS_) (Neugebauer et al., 1984) in 1983, which
enabled the first unbiased, sensitive, all-sky infrared survey (Bei, 1988).
This survey brought about, among an assortment of catalogs and atlases, the
_IRAS_ Bright Galaxy Sample (Soifer et al., 1987), a complete sample of the
brightest, far-infrared 60-$\upmu$m selected galaxies. Over the past three
decades, luminous infrared galaxies (LIRGs, with the empirical definition
$L_{\mathrm{IR}}$[8–1000 $\upmu$m] $\geq$ 10${}^{11}L_{\odot}$) and their
ultraluminous counterparts (ULIRGs, with $L_{\mathrm{IR}}\geq
10^{12}L_{\odot}$) have been studied extensively. Comparing their luminosity
functions to those of normal galaxies (Schechter, 1976), starbursts, Seyferts
(Huchra, 1977), and Palomar–Green quasars (Schmidt and Green, 1983) reveals
that LIRGs dominate luminosities above $\sim$$2\times 10^{11}L_{\odot}$ in the
local universe, matching the space densities of Seyferts at the lower
luminosities and surpassing those of quasars at the high end (Soifer et al.,
1987; Sanders and Mirabel, 1996). The bolometric luminosities and space
densities of ULIRGs are found to be at least a factor of $\sim$1.5–2 times
larger than those of the optically-selected quasi-stellar objects (QSOs; Kim
and Sanders (1998); Surace (1998)), though they are dynamically similar
(Rothberg et al., 2013).

Redshift surveys conducted using the _Infrared Space Observatory_ (Kessler et
al., 1996), SCUBA on the James Clerk Maxwell Telescope (Holland et al., 1999),
and the _Spitzer Space Telescope_ (Werner et al., 2004) find very strong
evolution in the luminosity function of (U)LIRGs (Le Floc’h et al., 2005),
dominating star formation activity beyond redshifts $z>0.7$ (Casey et al.,
2014). Given their prominence at the peak of cosmic history ($z\sim 2$), and
that a substantial fraction of galaxies pass through this intense infrared
emission stage (Soifer et al., 1987), (U)LIRGs are believed to play a
significant role in driving the cosmic star formation rate (SFR) density with
which their cosmic histories coincide (Madau and Dickinson, 2014). Results
from high-redshift (U)LIRG studies conducted using deep surveys, such as the
Cosmic Evolution Survey (Scoville et al., 2007), the Great Observatories
Origins Deep Survey (Elbaz et al., 2011), and the Cosmic Assembly Near-
Infrared Dark Energy Legacy Survey (Grogin et al., 2011; Koekemoer et al.,
2011) suggest that interactions and infrared luminosities are intimately
linked (Zheng et al., 2004; Bell et al., 2005; Bridge et al., 2007; Kartaltepe
et al., 2010, 2012). This is particularly true at low redshifts where all of
the ULIRGs, and many of the LIRGs, are gas-rich major mergers (Sanders et al.,
1988; Larson et al., 2020).

While it is clear that (U)LIRGs are an important population of objects to
understand given their place in cosmic history, whether galaxies of similar
luminosity at high redshifts are analogs of those locally is much more
nuanced. If low-$z$ and high-$z$ (U)LIRGs are alike, what physical processes
connect these two populations given that galaxies evolve? The fundamental link
between the nearby objects and their distant cousins may not be the absolute
value of the infrared luminosity but the amount of gas content, which dictates
SFR and feedback efficiencies for these galaxies that lie above the star
formation main sequence (SFMS) (see Figure 1; Wuyts et al. (2011); Speagle et
al. (2014); Linden et al. (2019)). Thus, it is pertinent to conduct in-depth
studies of the physical processes taking place within local (U)LIRGs where we
can resolve details to appreciate how gas ultimately drives galaxy evolution
through the merger mechanism.

Figure 1: (Left) The specific SFR (=SFR/M∗) distribution as a function of the
stellar mass shows that the galaxy-integrated (orange) and extranuclear star-
forming regions (purple) in GOALS LIRGs are above local star-forming galaxies
(blue; Skibba et al. (2011)) or the global fit from Speagle et al. Speagle et
al. (2014). (Right) The SFMS for the GOALS star-forming regions plotted
against the normalized resolved galaxy main sequence from other surveys (Cano-
Díaz et al., 2016; Hall et al., 2018; Medling et al., 2018). Figure from
Linden et al. Linden et al. (2019); © AAS, reproduced with permission.

The motivation to understand the nature of (U)LIRGs and their role in galaxy
evolution prompted the Great Observatories All-sky LIRGs Survey (GOALS; Armus
et al. (2009)). Comprising the 200 brightest 60 $\upmu$m-selected (U)LIRGs in
the nearby ($z\leq 0.08$) universe, the GOALS sample stemmed from the _IRAS_
Revised Bright Galaxy Sample (Sanders et al., 2003) that includes 629
extragalactic objects with $f_{60}$ $>$ 5.24 Jy at galactic latitudes
$|b|>5^{\circ}$. The data repository for GOALS is extensive across the
electromagnetic spectrum and is growing, encompassing data taken by the
_Nuclear Spectroscopic Telescope Array_ (_NuSTAR_), the _Chandra X-ray
Observatory_ , the _Galaxy Evolution Explorer_ (_GALEX_), the _Hubble Space
Telescope_ (_HST_), the Keck Telescopes, the Very Large Telescopes, the
_Spitzer Space Telescope_ , the Atacama Large Millimeter/submillimeter Array
(ALMA), the Karl G. Jansky Very Large Array (VLA), and imminently, the _James
Webb Space Telescope_ (_JWST_). The rich ancillary data set has facilitated a
large number of studies on the properties of star formation,the interstellar
medium (ISM), dust, and SMBHs in (U)LIRGs and enriched the literature with
results on star cluster formation and destruction, ionization and shocks,
nuclear obscuration, and others. This review highlights many of the SMBH-
related results from GOALS as well as other non-GOALS (U)LIRG studies.

Complementing other articles in this Special Issue—“Infrared Spectral Energy
Distribution and Variability of Active Galactic Nuclei: Clues to the Structure
of Circumnuclear Material” (Lyu and Rieke 2022 (Lyu and Rieke, 2022)), “The
past and future of mid-infrared studies of AGN” (Sajina, Lacy, and Pope 2022
(Sajina et al., 2022)), “Molecular gas heating, star-formation rate relations
and AGN feedback in Ultraluminous Infrared Galaxies” (Farrah 2022), among
others—this review focuses on the coevolution of SMBHs and galaxies. In
particular, we emphasize the role the central SMBH (often found in an actively
accreting phase as an active galactic nucleus–nuclei (AGN) during the late
stages of merging) plays in influencing its host galaxy. An outline is laid
out as follows: In Section 2, we present the conditions of the inner
kiloparsec environment and the nature of the nuclear energy source in infrared
galaxies as viewed from the multiwavelength perspective. We then discuss the
interplay between the AGN and the circumnuclear interstellar medium (ISM) and
beyond in Section 3. In Section 4, the implications of the multiple
interacting systems and the significance of the merging population in the
multi-messenger era are discussed. Finally, the future observational outlook
with selected telescope facilities is summarized in Section 5 while the key
conclusions are given in Section 6.

##  2 The Central Regions of (U)LIRGs

Although infrared galaxies are characterized by their infrared excess, or
characteristic infrared bump peaking at $\sim$100 $\upmu$m (Sanders and
Mirabel, 1996; U et al., 2012; Paspaliaris et al., 2021) due to the large dust
component (Surace, 1998), they are multiwavelength objects by nature—emission
from the various subcomponents of the galaxy system radiates in different
regions of the electromagnetic spectrum (Figure 2). As a result, locating the
true dynamical center where the SMBH resides is nontrivial—particularly for
interacting mergers engaged in a closely-separated pair with clearly distorted
host morphology. The challenge arises partly because the relative contribution
to the total luminosity from the SMBH versus star formation is difficult to
distinguish at resolved scales (e.g., dusty star-forming regions or AGN dusty
torus) (Ramos Padilla et al., 2020), and partially because many nuclei appear
clumpy (Medling et al., 2014; Larson et al., 2020; Linden et al., 2021).
Furthermore, the nature of the SMBH is difficult to discern in these
transitioning systems: quiescent SMBHs may be found in the early stages of
mergers before gas accretion into the central region; deeply buried AGN
(Imanishi et al., 2008, 2010; Lee et al., 2012) may be expected in mid-stage
mergers where the SMBH is actively accreting gas but hidden behind a thick
cocoon of dust, or actively accreting AGN that has emerged after blowing out
the dust through radiation pressure in the late stages of merging (Figure 3).
At any time point, one or both of the SMBHs might be experiencing these
different states. Thus, multiwavelength diagnostic tools have been employed to
verify the presence of AGN in (U)LIRGs. Here, we provide a broad overview of
some recent observational results for AGN identification and characterization
among local (U)LIRGs.

Figure 2: Mrk 266 in multiwavelength light—(Left) _HST_ composite image
showing major structures; (Right, Top) soft X-ray contours (magenta contours)
compared to the _HST_ $F435W$-band image and VLA radio continuum (orange
contours); (Right, Bottom) CO ($1-0$) channel maps (red contours), integrated
HCN ($1-0$) emission (cyan contours), and integrated HCO+ ($1-0$) emission
(yellow contours) overlaid on _HST_ 1.6 $\upmu$m images. This dual AGN system
exhibits an earlier dust blow-out phase and more rapid gas consumption than
predicted from simulations, revealing insight into the evolutionary timeline
of large-scale superwinds. This study illustrates the complexity of dust and
gas kinematics that can only be teased apart with detailed analyses of high-
resolution multiwavelength data. Figure adapted from Mazzarella et al.
Mazzarella et al. (2012); © AAS, reproduced with permission. Figure 3:
Schematic of plausible black hole evolutionary paths along the galaxy merger
sequence. (1) In early-stage mergers, the SMBH is quiescent and the nuclear
activity is dominated by star formation. As interaction proceeds and gas is
funneled toward the accretion disk of the SMBH, the AGN phase is
triggered—though it may be (2a) a low-luminosity AGN partially obscured by a
dusty torus or (2b) a fully buried Compton Thick AGN. (3) In the blow-out
phase, the AGN luminosity may then dominate the nuclear energetics. It is
unclear at which time point one or more of the SMBHs in merging progenitors
achieve these different stages, which likely depend on the local conditions of
the ISM as well as the geometry of the encounter or flickering of the central
engine due to the AGN duty cycle. Putting together multiwavelength data sets
for a statistical sample of galaxy systems at different merger stages will
help constrain the simultaneity of AGN-triggering and shed insight into dual
AGN fractions on merger timescales.

###  2.1 High-Energy Observations in the X-ray

X-ray observations are regarded as robust tools for identifying AGN because
the energetic photons from AGN can penetrate gas and dust more easily and are
better distinguished from those of stars (Teng and Veilleux, 2010; Iwasawa et
al., 2011; Ricci et al., 2017). This is a particularly notable attribute since
(U)LIRGs are known to host high levels of dust extinction in the nuclei where
AGN are likely to reside. Analyzing _Chandra’s_ observations of 44 (U)LIRGs
with $\log(L_{\mathrm{IR}}/L_{\odot})=11.73-12.50$, Iwasawa et al. Iwasawa et
al. (2011) identified AGN in 37 $\pm$ 6% of the sample using an X-ray hardness
ratio ($HR=(H-S)/(H+S)>-0.3$; where $H$ and $S$ are counts in the 2$-$8 keV
and 0.5$-$2 keV bands, respectively (Iwasawa et al., 2009)) and the presence
of the Fe K line at 6.4 keV (Iwasawa and Comastri, 1998; Vignati et al., 1999;
Ikebe et al., 2000; Komossa et al., 2003; Imanishi et al., 2003; Armus et al.,
2009). When inspecting additional _Chandra_ data of similar depths for lower-
luminosity LIRGs, Torres-Albà et al. Torres-Albà et al. (2018) found a lower
AGN fraction (31 $\pm$ 5%) using X-ray and mid-infrared criteria, suggesting
that AGN accretion grows with infrared luminosity. Intriguingly, Pereira-
Santaella et al. Pereira-Santaella et al. (2011) explored _Chandra_ and _XMM-
Newton_ observations of 27 low-luminosity LIRGs and determined that $\sim$15%
of the non-Seyfert LIRGs had excessive hard-X-ray emissions relative to those
expected from star formation, suggesting the presence of an obscured AGN. This
result has interesting implications for the timescale of when the obscured AGN
phase begins in the nuclei of (U)LIRGs.

Torres-Albà et al. Torres-Albà et al. (2018) also found that the fraction of
dual AGN in systems that host at least one AGN is 29 $\pm$ 14% (2 of 7),
nearly three times higher than the 11 $\pm$ 10% (1 of 9) found for the higher-
luminosity sample. While these statistics are limited by the small sample
sizes, the dual AGN fraction of the high-luminosity sample (Iwasawa et al.,
2011) falls short of previous theoretical (Capelo et al., 2017) and
observational X-rays selected (Koss et al., 2012) and other merger studies
(Ellison et al., 2011; Satyapal et al., 2017). The diminished dual AGN
fraction among late-stage mergers may be partially explained by heavy
obscuration given the insufficient sensitivity of the _Chandra_ data to detect
the Fe K line, or its inability to resolve AGN pairs at separations
$d_{\mathrm{sep}}\lesssim 200-300$ pc. This result has strong implications for
the timescale of AGN triggering as well as merging the nuclei in the case of
progenitor gravitational wave sources that will be further explored in Section
4.

The launch of the hard X-ray observatory _NuSTAR_ (Harrison et al., 2013)
offers a way to locate the most obscured Compton Thick (CT,
$N_{\mathrm{H}}\geq 10^{24}$ cm) sources buried in late-stage mergers, which
turn out to be 65${}^{+12}_{-13}$% among AGN-hosting (U)LIRGs (Ricci et al.,
2017). These ULIRGs lie along the column density $N_{H}\sim 10^{24}$ cm-2
track on the Lutz et al. Lutz et al. (2004) relation, which is a well-
established empirical correlation between 2$-$10 keV and 5.8 $\upmu$m
luminosity for both Type-1 and absorption-corrected Type-2 AGN (Bauer et al.,
2010) (Figure 4, left). A recent statistical _NuSTAR_ study of 60 (U)LIRGs
showed that the fraction of CT AGN peaks at 74${}^{+14}_{-19}$% at small
nuclear separations ($d_{\mathrm{sep}}\sim$ 0.4–6 kpc; (Ricci et al., 2021),
see also Figure 4, right). This fraction is consistent with results from
recent simulations showing that much of AGN activity remains undetected even
in late-stage mergers (Blecha et al., 2018), despite using common mid-infrared
selection techniques meant to overcome heavy obscuration. There is growing
evidence that the obscuration properties of AGN in mergers differ from those
of AGN in isolated galaxies; see recent reviews by Ramos Almeida and Ricci
(2017) and Hickox and Alexander (2018). High-resolution multiwavelength
tracers measuring the properties of the obscuring materials at resolved scales
will be needed to assess the nuclear properties of the extraordinary nuclear
environments in late-stage mergers (Gao, U et al., in preparation).

Figure 4: (Left) The rest frame 2$-$10 keV luminosity versus 5.8 $\upmu$m AGN
continuum luminosity for several CT (or nearly CT) AGN-hosting ULIRGs. The
gray shaded region indicates the range of unobscured AGN based on Type-1
sources, while the black dotted (dashed) lines demonstrate the effects of
X-ray absorption by column densities of $N_{H}$ (with mid-infrared extinction
by $A_{6\mu m}$). The observed X-ray luminosities for the illustrated local
ULIRGs residing around the $N_{H}\sim 10^{24}$ cm-2 line lend evidence that
ULIRGs are in fact highly obscured. Figure adapted from Bauer et al. Bauer et
al. (2010); © AAS, reproduced with permission. (Right) Fraction of CT AGN as a
function of projected nuclear separations with a peak found at a separation of
a few kpc. The red continuous line marks the fraction of heavily obscured
sources among _Swift_ /BAT AGN (Ricci et al., 2015, 2017). The resolution
limit at which two nuclei could be separated at the distance of the depicted
sample is $d_{\mathrm{sep}}\sim$ 0.4 kpc. Figure adapted from Ricci et al.
Ricci et al. (2021).

###  2.2 Spectroscopic Diagnostics Using Optical and Near-Infrared Emission
Lines

There are several widely-adopted optical diagnostics for detecting and
characterizing AGN using spectroscopic measurements: the Baldwin, Phillips,
and Terlevich (BPT hereafter; Baldwin et al. (1981))/Veilleux-Osterbrock
(Veilleux and Osterbrock, 1987) diagrams based on excitation levels deduced
from emission line ratios (see the in-depth discussion in the recent review by
Kewley et al. Kewley et al. (2019)), and the Seyfert classifications based on
the line profiles and widths of the hydrogen recombination lines. The BPT
diagrams segregate galaxies or regions within galaxies by their energy source:
theoretical (Kewley et al., 2001) and empirical (Kauffmann et al., 2003)
limits for H II regions as well as the low-ionization nuclear emission-line
region (LINER) and Seyfert division established by Kewley et al. Kewley et al.
(2001, 2006) and Kauffmann et al. Kauffmann et al. (2003). Applying this
classification scheme to the 1 Jy ULIRG sample (Veilleux et al., 1999) and
LIRGs in the Bright Galaxy Survey sample (Veilleux et al., 1995), Yuan et al.
Yuan et al. (2010) found that there is a trend in the increasing ratio of
Seyfert nuclei to H II regions with increasing infrared luminosity. The
composite region between starburst- and AGN-dominated ionization was largely
interpreted as being powered by a mix of star formation and low-luminosity
AGN, but Rich et al. Rich et al. (2011) studied two late-stage merging LIRGs
with wide-field integral-field spectroscopy (IFS) and found that the composite
emission can be explained by a combination of star formation and shocks
without invoking AGN.

Investigating a large, representative sample of 27 (U)LIRGs in GOALS, Rich et
al. Rich et al. (2014, 2015) concluded that galaxy interaction–induced tidal
forces result in galaxy-scale shock excitations (Figure 5). Shocks constitute
a non-negligible fraction of the H$\alpha$ luminosity and may potentially be
responsible for compressing the molecular gas and triggering starbursts at
shock fronts throughout the merger. An optical IFU study of 11 lower-
luminosity LIRGs found large regions of enhanced [N ii]/H$\alpha$ and [S
ii]/H$\alpha$ ratios associated with low surface-brightness H ii regions, with
diffuse emission detected in H$\alpha$ and Pa$\alpha$ (Alonso-Herrero et al.,
2009, 2010). There are likely mechanisms apart from young stars that are
ionizing the gas, and these regions of enhanced low-ionization emission can be
associated with the AGN ionization cone (García-Marín et al., 2006). While the
prominence of galaxy-scale shocks is elucidated, the number of Seyfert nuclei
identified using optical spectroscopic methods is likely a lower limit because
infrared galaxies have dusty cores that would mask a broad feature in
H$\alpha$ and H$\beta$, or masquerade as dusty star-forming galaxies by BPT
criteria. Multiwavelength data (particularly in extinction-free regimes) are
needed to assess the effect of dust and the true contribution from low-
luminosity or obscured AGN.

Figure 5: (Left) BPT classification of the spectral type as a function of
infrared luminosity for (U)LIRG samples from Veilleux et al. Veilleux et al.
(1995, 1999). Seyfert nuclei dominate over H II regions as $L_{\mathrm{IR}}$
increases. Figure from Yuan et al. Yuan et al. (2010). (Right) The
contribution from shocks to the H$\alpha$ luminosity is plotted against the
merger stage, from isolated galaxies to early-stage and late-stage mergers.
The data points are color-coded by the H$\alpha$ velocity dispersion, where
the dispersion cutoffs correspond to H II region velocity dispersions on the
low end, and more turbulent and shock-dominated dispersions on the high end.
The figure from Rich et al. Rich et al. (2015). Both panels © AAS, reproduced
with permission.  Figure 6: Two near-infrared diagnostic diagrams to
distinguish between excitation from starburst (SB; blue) and AGN (orange) as
well as the high line ratio (HLR). The left panel shows the [Fe ii]/Pa$\beta$
versus H2/Br$\gamma$ diagram in the logarithmic scale, as derived from Riffel
et al. (2013, 2021). The right panel shows the [Fe ii]/Br$\gamma$ versus
H2/Br$\gamma$ diagram in the logarithmic scale, following Colina et al.
(2015). These boundaries are derived from spatially-resolved observations of
nearby Seyfert and LIRGs, respectively.

The infrared wavelength range offers an advantage over the bluer regime in its
ability to peer through dust, particularly important in the nuclear regions of
merging galaxies. The extinction in the $K$ band measured using
Br$\gamma$/Br$\delta$ by U et al. U et al. (2019) is $A_{K}\sim$ 2.5–4 mag,
which corresponds to $A_{V}\sim$ 3–17 mag and is consistent with the
$A_{V}\sim$ 5–40 mag found in other adaptive optics (AO) studies of LIRG
nuclei (Mattila et al., 2007; Väisänen et al., 2017). _HST_ NICMOS
observations of 30 (U)LIRGs reveal that the Pa$\alpha$ morphology is dominated
by the nuclear emission with sizes of $\lesssim$1 kpc, which may be related to
the AGN in some cases (Alonso-Herrero et al., 2006). A similar, larger study
of Pa$\beta$ emission in 48 local (U)LIRGs observed with _HST_ WFC3 finds
resolved nuclear clumps down to the resolution limit of 90 pc pixel-1, with
the majority having effective radii of $\sim$400 pc and consistent with being
nuclear star clusters (Larson et al., 2020).

The high-resolution resolving capability of AO-assisted integral-field unit
(IFU) instruments is key to accurate measurements of the Brackett and other
emission lines at the very center over globally integrated values. Similar to
the optical BPT diagram, empirical emission-line studies have produced near-
infrared diagnostic diagrams that assess the excitation and ionization
conditions of the gas (Larkin et al., 1998; Rodríguez-Ardila et al., 2004,
2005; Riffel et al., 2006, 2013; Colina et al., 2015). Since the iron in the
ISM is highly depleted onto grains, strong [Fe ii] emission is often
associated with shock-excited gas where the grains have been processed by
winds, supernovae, or other ionizing sources. Figure 6 illustrates the use of
line ratios, such as [Fe ii] (1.26 $\upmu$m)/Pa$\beta$ and H2 1$-$0
S(1)/Br$\gamma$ to differentiate among starbursts, AGN, and high-line-ratio
(HLR) objects (Riffel et al., 2021). The HLR region corresponds to where the
values of both [Fe ii]/Pa$\beta$ and H2/Br$\gamma$ are high, as defined by
Riffel et al. (2013). The combination of H2/Br$\gamma$ and kinematics using
resolved IFS can further identify shocked gas and outflows (U et al., 2019).
Measurements of these molecular lines will become increasingly common with the
advent of _JWST_ ; detailed theoretical models that integrate the complexities
of molecular physics to radiative transfer will be needed to interpret the
line-ratio results.

Near-infrared coronal lines, such as [Si vi]$\lambda$1.96 $\upmu$m, [Al ix]
$\lambda$2.04 $\upmu$m, and [Ca viii] $\lambda$2.32 $\upmu$m have been
detected in circumnuclear regions around Seyfert nuclei (Müller Sánchez et
al., 2006; Müller-Sánchez et al., 2011; Piqueras López et al., 2012; Bohn et
al., 2021). As high-ionization lines with ionization potential $>$ 100 eV,
these lines can be produced by fast shocks and/or a hard continuum,
originating from a region in between the broad and narrow line regions (NLRs).
They are considered robust indicators of AGN-driven outflows, though caution
should be exercised in associating coronal line detection with a central point
source in merger nuclei where the nuclear structure may be more complex in
nature. In cases where a late-stage merger may harbor a mixture of star
clusters and potentially (multiple) remnant nuclei, the mere detection of a
coronal line from integrated-light spectroscopy may not correctly indicate
where the outflow-driving AGN is located. This is exemplified in the
ultraluminous infrared late-stage merger Mrk 273: the identified outflow based
on the detection of [Si vi] emission did not come from the X-ray AGN but
rather appears to originate from, and helps lend evidence to the presence of,
a second, obscured CT AGN (Iwasawa et al., 2011; U et al., 2013) whose AGN
nature was only confirmed later with deep Chandra data (Liu et al., 2019).
Despite its documented detection in several (U)LIRGs (Müller-Sánchez et al.,
2011; Rodríguez-Ardila et al., 2011; U et al., 2013), the [Si vi] line was not
detected in a single-slit spectroscopic survey of 42 GOALS LIRGs (Borish,
2017) using TripleSpec (Herter et al., 2008) on the ARC 3.5-m Telescope and
the Palomar 200-inch Hale Telescope. A likely explanation is that the [Si vi]
outflow is compact (e.g., $\sim$400 pc in the case of Mrk 273 (U et al.,
2013)) and its signal is thus diluted in large-aperture ($\sim$1′′)
spectroscopy as opposed to subarcsecond resolved observations of the central
regions.

###  2.3 Probing the Infrared Excess

The mid- and far-infrared are crucial for diagnosing AGN at various
obscuration levels for a complete census of AGN activity in (U)LIRGs. For
focused discussions of the characteristics of mid-infrared AGN properties and
the observational studies of (U)LIRGs using the _Spitzer Space Telescope_ , we
refer the readers to recent reviews by Lacy and Sajina (2020); Armus et al.
(2020), and Sajina et al. (Sajina et al., 2022) in this Special Issue. Here,
we briefly mention some _Spitzer_ and _Herschel_ results on GOALS (U)LIRGs,
including the AGN fraction to the mid-infrared and total luminosity measured
by Díaz-Santos et al. (2017) and AGN properties from high ionization potential
lines, such as [Ne v] presented by Petric et al. (2011).

Among the GOALS sample, 18% of all LIRGs contain an AGN based on [Ne v]
detection (Petric et al., 2011). Diagnostics using the [Ne v]/[Ne ii], [O
iv]/[Ne ii] line ratios, the 6.2 $\upmu$m PAH equivalent width, and the mid-
infrared continuum have been used to compute the mid-infrared and bolometric
AGN strengths (Díaz-Santos et al., 2017) and indicate that in 10% of the
sources, the AGN contribution to the total infrared luminosity exceeds 50%
(Petric et al., 2011). This is consistent with another study that finds the
AGN bolometric contribution to the infrared luminosities for 70% of (U)LIRGs
to be generally small: $L_{\mathrm{AGNbol}}/L_{\mathrm{IR}}\leq 0.05$; only
$\sim$8% of the targeted sources have a significant contribution
$L_{\mathrm{AGNbol}}/L_{\mathrm{IR}}>0.25$ (Alonso-Herrero et al., 2012). In
the local universe, the AGN bolometric contribution to the infrared luminosity
does in fact increase with the infrared luminosity (Veilleux et al., 1995;
Desai et al., 2007; Petric et al., 2011), even if the impact of the AGN on the
far-infrared luminosity of the mid-infrared dominated AGN LIRGs is very
limited (Díaz-Santos et al., 2013). The presence of an AGN compacts the
spatial extent of the mid-infrared host dust continuum but has a negligible
effect on the PAH-emitting regions (Díaz-Santos et al., 2011). IRAC color
selection techniques Stern et al. (2005); Lacy et al. (2004) might miss 36–50%
of the AGN-dominated (U)LIRGs or include false positives in starburst-
dominated ones; photometric selections based on the _Wide-field Infrared
Survey Explorer (WISE)_ colors (Satyapal et al., 2018; Pfeifle et al., 2021)
and high ionization lines (Satyapal et al., 2021) will be useful in selecting
obscured or CT AGN, in particular. These mid-infrared identification
techniques will be effective with _JWST_ ’s superb resolution and sensitivity
over _Spitzer_ ’s, where the emission from compact point sources might be
diluted within the beam or nuclear sources below the confusion limit appeared
blended.

###  2.4 The Submillimeter and Radio Regimes with ALMA and VLA

In the extinction-free submillimeter and radio wavelength ranges, excellent
spatial resolution can be achieved with interferometry—down to $\sim$
0$.\\!\\!^{\prime\prime}$01 (0$.\\!\\!^{\prime\prime}$04) in the high-
frequency bands for ALMA (VLA). This capability allows us to bypass the dust
columns and resolve the energetically dominant regions in the dusty cores of
local (U)LIRGs, distinguishing between recent star formation and AGN activity.
Often, star formation is taken to emit free–free, or “thermal” emission while
AGN radiates synchrotron or “non-thermal” emissions. However, the non-thermal
radio emission can be a tracer for star formation activity due to synchrotron
resulting from particle acceleration in supernovae, whereas the non-thermal
emission of AGN may be a mixture of synchrotron emission and that of an
accretion disk, which radiate thermally. Complementing multiwavelength data
sets, a combination of the location of the radio emission peak (though this is
complicated by the interaction mechanisms, which could displace the central
SMBH), morphology, compactness, and brightness temperature is often used to
help verify the AGN nature of the radio source. The physics of ULIRGs with
MUSE and ALMA (PUMA; Pereira-Santaella et al. (2021)) project observed 220 GHz
continuum and CO(2-1) gas in 22 $z<0.165$ ULIRGs (32 individual nuclei) to
address whether ULIRGs are powered by AGN. Determining the nuclear
$L_{\mathrm{IR}}$ and cold molecular gas surface densities to be
$\Sigma_{L_{\mathrm{IR}}}=10^{11.5-14.3}L_{\odot}$ kpc-2 and
$\Sigma_{H_{2}}=10^{2.9-4.2}M_{\odot}$ pc-2, respectively, the derived
$\Sigma_{SFR}$ would imply extremely short depletion times and unphysical
star-formation efficiencies for 70% of the sample, arguing in favor of the
presence of obscured AGN in these nuclei.

The claim that ULIRGs host obscured AGN is further evidenced by ALMA
observations of dense molecular gas tracers HCN $J=3-2$ and
HCO${}^{+}~{}J=3-2$, where the enhanced HCN emission in the nuclear region may
be explained by the presence of a luminous buried AGN, high molecular gas
density and temperature, and/or mechanical heating by spatially compact
nuclear outflows (Imanishi et al., 2019). The link between a heightened
${L}_{\mathrm{HCN(1-0)}}^{\prime}/{L}_{\mathrm{{HCO^{+}}(1-0)}}^{\prime}$
ratio (HCN/HCO+ hereafter) and the presence of AGN in (U)LIRGs has previously
been investigated by Kohno et al. (2003); Costagliola et al. (2011) and Privon
et al. (2015). While AGN generally has high HCN/HCO+, the reverse does not
seem to be true: high HCN/HCO+ values could be driven by sources other than
the AGN (Privon et al., 2015, 2020). Hard X-ray observations of four mid-
infrared classified starburst galaxies with high HCN/HCO+ ratios did not show
that they harbor heavily obscured AGN, and for hard X-ray-identified AGN, the
inferred AGN luminosity and AGN fractions were uncorrelated with HCN/HCO+
ratios (Privon et al., 2020). One viable mechanism for driving the increased
HCN/HCO+ ratio may be mechanical heating from shocks or outflows, which could
conceivably originate from an AGN—but the lack of a direct link between high
HCN/HCO+ sources and AGN hosts suggests that the mechanical heating may have
plausibly come from starburst-driven outflows.

High-resolution ($\leq$ 0$.\\!\\!^{\prime\prime}$1, or 30 pc) studies of dense
gas enabled by ALMA and VLA have facilitated the search for compact obscured
nuclei (CONs) in (U)LIRGs (Sakamoto et al., 2013). Using the rotational line
of HCN in its vibrationally excited $v_{2}=1$ state (hereafter HCN-vib), whose
first extragalactic detection was in LIRG NGC 4418 by Sakamoto et al. (2010)),
Falstad et al. (2021) searched for HCN-vib in 46 (U)LIRGs and identified CONs
in 38${}^{18}_{13}$% of the ULIRGs, 21${}^{12}_{6}$% of the LIRGs, and
0${}^{9}_{0}$% of the lower luminosity galaxies. The HCN-vib emission is too
luminous with respect to the infrared luminosity to represent a normal cool
mode of star formation (Aalto et al., 2015). Instead, the intense HCN-vib
emission may come from CT AGN or from an embedded, compact, hot ($T>200K$),
and opaque starburst (Andrews and Thompson, 2011). Although the jury is still
out on the exact nature of CONs, the evidence that CONs seem to be exclusively
hosted in (U)LIRGs suggests that they may be an extreme example of obscured
AGN. In a case study by LIRG IC 860, Aalto et al. (2019) notes that the
infrared core (within $r<10$ pc) is consistent with a buried, efficiently
accreting SMBH, or an extreme, O-star only starburst. Whether and how HCN-vib
in CONs may correlate with outflow properties is relevant for understanding
the nuclear feedback mechanism in the context of CONs being a heavily obscured
stage of evolution Falstad et al. (2019), a topic we will revisit in Section
3.

A recent study on the radio properties of nearby ULIRGs using the VLA
suggested that ULIRGs have common radiative processes regardless of the
presence of optical AGN (Hayashi et al., 2021). Based on high angular
resolution (0$.\\!\\!^{\prime\prime}$05$-$0$.\\!\\!^{\prime\prime}$2), imaging
of 22 (U)LIRGs at 33 GHz, nearly half of the samples were found to host single
bright compact sources that contribute $>$ 50% of the total emission (Barcos-
Muñoz et al., 2017), and nearly half of the subsamples are either confirmed
AGN or potential AGN candidates, given their resolved compact sizes. The fact
that the sample selection here focused on ULIRGs whose AGN were not identified
by an optical signature ($\sim$70% of all the ULIRGs) lends support to the
idea that the AGN might contribute equally to every ULIRG. Multiwavelength
diagnosis combining radio and far-infrared, for instance, offers further
insight into the nature of the compact sources by assessing whether the
infrared light powered by starbursts or AGN would correspond to free–free or
synchrotron emission.

###  2.5 Spectral Energy Distributions and Multiwavelength Comparisons

Given the presence of AGN, stars, and dust at play in the complex nuclei of
(U)LIRGs, cross-wavelength calibration is useful to evaluate the effectiveness
and complementarity of individual AGN-selection approaches and to achieve a
complete census of active nuclei within the (U)LIRG population. Spectral
energy distribution (SED) studies of nearby (U)LIRGs provide the means to
investigate the multiband emission from various galactic subcomponents as the
galaxies progress through the merger sequence (U et al., 2012; Paspaliaris et
al., 2021). With a sample of 65 (U)LIRGs from GOALS, U et al. (2012) amassed
the aperture-matched X-ray to radio-integrated photometry and compared several
of the multiwavelength indicators for AGN:

  * •

The correlation between far-infrared and radio emissions as a way to
distinguish radio-loud AGN from starbursts (Condon et al., 1991; Ji et al.,
2000; Donley et al., 2005); while radio excess (infrared-excess) sources with
extremely low (high) values of $q$ (as defined in Table 1) could both contain
an AGN or obscured AGN, there are more likely to be starburst contaminants
among the high $q$ sources (Yun et al., 2001; Vardoulaki et al., 2015). This
indicator has since been refined by adopting 24 $\upmu$m flux instead of far-
infrared flux (Appleton et al., 2004; Donley et al., 2005).

  * •

The X-ray luminosity threshold or hardness ratio based on relative
contributions from the hard (2–8 keV) and soft (0.5–2 keV) bands (Iwasawa and
Taniguchi, 1993; Mushotzky et al., 1993); the X-ray—radio excess criterion due
to AGN activity (Alberts et al., 2020);

  * •

Mid-infrared power-law index and _Spitzer_ /IRAC colors (see Sajina et al.
2022 (Sajina et al., 2022) and Lyu and Rieke 2022 (Lyu and Rieke, 2022) in
this Special Issue for more in-depth discussions); and

  * •

Optical spectral classification based on the BPT diagrams and line widths of
the hydrogen recombination emission features (Yuan et al., 2010)

The multiwavelength indicators along with the associated AGN criteria are
summarized in Table 1. The results from the cross-wavelength comparison of
these various selection techniques show that they complement each other (see
also Lyu et al. in the review). Approximately 60% of the ULIRGs and 25% of the
LIRGs were classified to host at least one AGN by at least one of the
aforementioned criteria (Figure 7; (U et al., 2012)).

While a luminous AGN, if present at the center of a galaxy, tends to dominate
the light of the host galaxy if its luminosity is above $10^{44}$ erg s-1
(Prieto et al., 2010), (U)LIRGs may harbor lower-luminosity or dust-obscured
AGN whose emissions may contribute only negligibly to global SEDs. A more
definitive approach to understanding the growth pathway of a SMBH through the
merging sequence would be a resolved SED study that isolates the nuclear
emission with an aperture size that minimizes contamination from the host
galaxy (Gao, U et al., in preparation). An even more powerful capability is
the NIRSpec IFU, where the presence of hot dust can be isolated spatially and
through the dilution of the stellar absorption lines. The unprecedented high-
resolution imaging and spectroscopic capability of _JWST_ in the near- and
mid-infrared will be valuable in constraining the AGN hot dust emission for
such studies.

Figure 7: Radio-infrared flux ratio $q$ vs. $\nu L_{\mathrm{IR}}$ plots are
shown for a sample of local (U)LIRGs along with the distribution in $q$ on the
right panels. The solid line indicates the median value ($q$ = 2.41) of the
sample, with $\sigma$ = 0.29. The dotted lines indicate the radio excess ($q$
$<$ 1.64) and infrared-excess ($q$ $>$ 3.0) criteria from Yun et al. (2001).
From top to bottom are four different AGN indicators (filled
circles/histograms): (red) X-ray luminosity and hardness ratio, (blue) mid-
infrared power-law slope, (orange) IRAC colors, and (green) optical spectral
classification. Figure from U et al. (2012); reproduced with permission from
the author.  Table 1: A summary table for multiwavelength AGN diagnostics in
(U)LIRGs

Multiwavelength Indicator |  AGN Criteria |  Ref.
---|---|---
Radio-infrared flux ratio |  |
$q\equiv\log\left(\frac{\mathrm{FIR}}{3.75\,\times\,10^{12}~{}\mathrm{Wm}^{-2}}\right)-\log\left(\frac{S_{\mathrm{1.4~{}GHz}}}{\mathrm{Wm}^{-2}\mathrm{Hz}^{-1}}\right)$ |  $q<1.64$: radio-excess |  (Condon et al., 1991; Yun et al., 2001; Vardoulaki et al., 2015)
FIR $\equiv 1.26\times 10^{-14}(2.58~{}S_{60\upmu\text{m}}+S_{100\upmu\text{m}})\mathrm{Wm}^{-2}$ |  |
or |  |
$q\equiv\log(f_{24\upmu\text{m}}/f_{\mathrm{1.4GHz}})$ |  $q<0$ |  (Appleton et al., 2004; Donley et al., 2005)
X-ray Detection |  |  (Kartaltepe et al., 2010)
|  $L_{\mathrm{2-10~{}keV}}>10^{42}$ erg s-1 |
X-ray Hardness Ratio |  |  (Iwasawa et al., 2011)
| HR $=(H-S)/(H+S)$
---
$H$ ($S$): counts in 2$-$8 (0.5$-$2) keV
HR $>-0.3$ |
X-ray Radio Excess |  |  (Alberts et al., 2020)
|  $L_{\mathrm{0.5-7~{}keV}}/L_{\mathrm{6GHz}}>1.2\times 10^{19}$ |
Mid-infrared Power-law Index |  |  (Kartaltepe et al., 2010; Alonso-Herrero et al., 2006; Donley et al., 2007)
|  $\log(\nu L_{4.5})-\log(\nu L_{2.2})>0$ |
IRAC Colors |  |  (Donley et al., 2012)
$x=\log\left(\frac{f_{5.8\upmu\text{m}}}{f_{3.6\upmu\text{m}}}\right)$, $y=\log\left(\frac{f_{8.0\upmu\text{m}}}{f_{4.5\upmu\text{m}}}\right)$ |  | $x\geqslant 0.08\wedge y\geqslant 0.15$
---
$\wedge y\geqslant(1.21\times x)-0.27\wedge y\leqslant(1.21\times x)+0.27$
$\wedge f_{4.5\upmu\text{m}}>f_{3.6\upmu\text{m}}\wedge
f_{5.8\upmu\text{m}}>f_{4.5\upmu\text{m}}\wedge
f_{8.0\upmu\text{m}}>f_{5.8\upmu\text{m}}$
Optical Spectral Classification |  |  (Yuan et al., 2010; Kewley et al., 2001)
|  Seyfert 1: H$\alpha$ (FWHM) $>5\times 10^{3}$ km s-1 |
|  Seyfert 2: lie above Kewley et al. (2001) theoretical lines in BPT diagrams |

##  3 The Coevolution of SMBHs and the Host Galaxies

The theoretical and observational studies of black hole feeding and feedback
have provided key insights toward understanding galaxy evolution as part of
the cosmic ecosystem. These fueling and feedback processes are responsible for
many of the correlations between the observed black hole mass, SFR, and other
galaxy properties seen in statistical studies of SMBHs and their host
galaxies. Building on the assumption that SMBHs and their hosts are linked,
how gas of different phases and temperatures may be shocked, compressed, or
entrained throughout the ISM at different scales comes into the spotlight
(Figure 8; see also Gaspari and Sądowski (2017)). Here, we review recent
observational results from integral-field spectrographs and interferometry
arrays that focus on nearby (U)LIRGs, many of which prominently feature these
energetic events to be examined in detail.

Figure 8: Schematic showing the multiphase, multiscale fueling, and feedback
processes in a self-regulated cycle. This diagram captures several major
crucial phases from SMBH scales (sub-pc) to the virial radius of a galaxy
(Mpc). The arrows illustrate the anticipated motions of the multiphase gas
with different temperatures (red: hot plasma at $T\sim 10^{6}-10^{8}$K;
yellow: warm gas at $T\sim 10^{3}-10^{6}$ K; blue: cold gas at $T\sim
10^{0}-10^{3}$ K). This review features results from observational studies on
gas accretion onto SMBH and entrained, multiphase outflows from the nuclear
region to galactic scales in nearby (U)LIRGs. Figure from Gaspari et al.
(2020); reprinted with permission.

###  3.1 AGN Fueling

####  3.1.1 Transporting Gas to the Center

SMBHs feed on gas that may have originated from the galactic halo at 50 kpc
scales via cold accretion flows (Gaspari et al., 2017). Simulations of merging
gas-rich disk galaxies have shown that major mergers are generally more
effective in transporting gas to the nuclei than their higher-mass ratio
merger counterparts, resulting in 50% as opposed to 20–30% of the gas being
funneled to the center of the system (Naab and Burkert, 2001). More recent
galaxy merger simulation suites using hydrodynamic simulations coupled with
dust radiative transfer have uncovered obscured signatures of merger-triggered
AGN fueling (Blecha et al., 2018). The obscuration phase coincides with the
peak of BH fueling, such that merger-triggered AGN fueling could be missed
among late-stage mergers. While mid-infrared AGN selection might include
contribution from star formation, gas that fuels starbursts also fuels AGN
simultaneously in merger nuclei. Using the Stars and MUltiphase Gas in
GaLaxiEs (SMUGGLE (Marinacci et al., 2019)) model that incorporates the
multiphase ISM and stellar evolution, Sivasankaran et al. (2022) demonstrated
that interaction-induced inflows cause peak gas densities near the SMBH to
increase by orders of magnitude, subsequently resulting in elevated and bursty
star formation as well as enhanced accretion. The instantaneous accretion rate
depends strongly on the conditions of the local ISM, the constraints for which
will be derived from high-resolution observations of galactic nuclei.

Specifically, in nearby infrared galaxies, large reservoirs of cold and warm
molecular gases have been detected in late-stage interacting galaxies as
amassed by the merging mechanism. Submillimeter interferometry studies of
merger remnants have found large amounts of CO gases (10${}^{7-11}M_{\odot}$)
that reside in rotation-supported turbulent disks varying from 1.1 to 9.3 kpc
in size (Ueda et al., 2014). Within the central kpc regions, young stellar and
warm H2 disks are nearly ubiquitous among ULIRGs. With stellar ages $<$ 30
Myr, masses of 10${}^{8-10}M_{\odot}$, and effective radii
$r_{\mathrm{eff}}\approx$ 200–1800 pc (Medling et al., 2014), these
circumnuclear molecular gas disks have also been seen in simulations of galaxy
mergers hosting inflow rates as high as 10${}^{4}M_{\odot}$ year-1 (Mayer et
al., 2010; Mayer, 2013). The presence of this gas signals sufficient fueling
for forming stars in situ during the merger, and this gas would likely be
driven toward the center to aid the final coalescence of binary black holes in
short timescales ($<$$10^{7}M_{\odot}$) (Escala et al., 2005; Dotti et al.,
2007; Mayer et al., 2007).

####  3.1.2 Black Hole Masses in Galaxy Mergers

The presence of nuclear disks provides a means to determine dynamical black
hole masses where the sphere of influence is resolved. Local galaxies ($<$20
Mpc) are prime candidates for these dynamical measurements, given the
resolving capability of the current integral-field spectrographs and
interferometric facilities. The nearby (U)LIRGs are slightly further, but they
are also more massive (with mean stellar mass
$\log(M_{\star}/M_{\odot})=10.79\pm 0.040$; (U et al., 2012)) with expectedly
larger black holes with larger spheres of influence, so dynamical measurements
are still plausible up to $z\lesssim 0.05$, or 200 Mpc. Modeling the high-
resolution integral-field data that demonstrate the primarily Keplerian
rotation of the central point source in molecular and ionized gas, Medling et
al. (2015) suggest that late-stage merging (U)LIRGs tend to harbor overmassive
black holes relative to the black hole–galaxy relationships based on normal
galaxies: $M_{\mathrm{BH}}-\sigma$ (see the left panel in Figure 9),
$M_{\mathrm{BH}}-L_{\mathrm{bulge}}$, and $M_{\mathrm{BH}}-M_{\star}$
(Ferrarese and Merritt, 2000; Gebhardt et al., 2000; Tremaine et al., 2002;
Merritt and Ferrarese, 2001; Kormendy and Gebhardt, 2001; Marconi and Hunt,
2003; Magorrian et al., 1998; Gültekin et al., 2009; McConnell and Ma, 2013).

Figure 9: (Left) The $M_{\mathrm{BH}}-\sigma$ relation for normal galaxies
from McConnell and Ma (2013) (black) and merging (U)LIRGs (colored) from
different mass determination methods. (U)LIRGs appear to lie on top of or
above the scaling relation. Figure from Medling et al. (2015). (Right) The
same plot but now the black hole mass measurement for NGC 6240 has been
updated to incorporate correction based on the submillimeter measurement of
the gas mass. Figure adapted from Medling et al. (2019). Both panels © AAS,
reproduced with permission.

Whether SMBH growth truly precedes that of the bulge remains nuanced (c.f.
(Kormendy and Ho, 2013)), where the observed relationship between the two
might depend on the sample selection (e.g., isolated vs. mergers), host
properties (e.g., dry mergers), or measurement uncertainties (e.g.,
systematics in measuring black hole masses and/or bulge velocity dispersion).
In fact, one caveat with reporting the stellar velocity dispersion of the
bulge in galaxy mergers is that it is difficult to define what is considered
the bulge when the progenitors are intertwined. Simulations have also shown
that the velocity dispersion oscillates during the course of the merger
(Stickley and Canalizo, 2014), easily introducing uncertainties to the scaling
relation. The accuracy of the black hole mass measurement is also susceptible
to systematics associated with the various mass determination methods.

Further investigating the phenomenon of overmassive black holes, whose masses
are postulated to include excess gas mass within the sphere of influence that
has yet to be accreted onto the central source, Medling et al. (2019) observe
one such merging system—NGC 6240—with high-resolution ALMA Band 6 observations
that probe at the physical scale of 15$-$30 pc. This study computes gas mass
via two approaches: 1. using the gas mass calculation technique based on the
dust continuum from Scoville et al. (2015), and 2. using a resolved-calibrated
$\alpha_{\mathrm{CO}}$ measurement. They found cold CO molecular gas
reservoirs of a few times 107 and 10${}^{8}M_{\odot}$ for the north and south
nuclei, respectively. These measurements provide a black hole mass correction
that potentially brings the dynamical measurement based on near-infrared gas
and star tracers closer to the M-$\sigma$ relation (see the right panel in
Figure 9). If the detected CO gas mass has yet to be accreted onto the central
SMBH, it is thus posited that the black hole may not, after all, be
overmassive relative to the property of the bulge. Rather, we can deduce the
timescale by which the gas is accreted given finer time sampling of black hole
mass measurements along the merger sequence. While evidence for merger-induced
black hole fueling is present, SMBH studies with larger samples of mergers
will be needed to resolve the scaling relations conundrum.

####  3.1.3 Emergence of AGN in ULIRGs

The GOALS sample encompasses (U)LIRGs with $\log L_{\mathrm{IR}}=11.0-12.5$;
among which the brightest object is Mrk 231, the only bonafide QSO in the
sample (Armus et al., 2009). Because of their relatively moderate luminosities
compared to those of quasars, plus the mixed presence of starbursts and AGN
that sometimes obfuscates the dominant energy driver, LIRGs are often
overlooked in the context of luminous AGN studies. GOALS was designed to probe
primarily the merger phases that bridge the normal star-forming galaxies to
the extreme QSO populations. Section 2 presents the detection rate of AGN and
obscured AGN activity in (U)LIRGs amidst a discussion of multiwavelength
identification techniques, but the AGN dominates the host galaxy light, mostly
after the dust blow-out phase in the merger.

At the higher luminosities, the average AGN contribution in ULIRGs ranges from
$\sim$15%–35% among optically-classified H ii-like and LINER ULIRGs to
$\sim$$75\%$ among Seyfert 1 ULIRGs, reaching $\sim$$80\%$ in QSOs (Veilleux
et al., 2009). Such a comparison benefits from well-matched samples, such as
those in the Quasar-ULIRG Evolution STudy (QUEST; (Schweitzer et al., 2006;
Veilleux, 2008, 2012)) survey, which consists of ULIRGs from the  _IRAS_ 1-Jy
Survey of ULIRGs (Kim and Sanders, 1998) and QSOs from the Palomar–Green (PG)
quasar survey (Schmidt and Green, 1983). A similar trend in increasing AGN
fraction with infrared luminosity was noted by Alonso-Herrero et al. (2012),
with a jump from $\sim$$50\%$ to $\sim$$80\%$ occurring at $\log
L_{\mathrm{IR}}=12.5$. A recent simulation of a galaxy merger demonstrates
that the black hole accretion rate peaks correlate with pericenter encounters,
such that the second and third pericenter passes cause an enhancement in the
Eddington limit (Prieto et al., 2021). The result of AGN fueling is that the
high mass accretion rate onto compact objects will trigger AGN activity.
Further evidence of AGN prominence in ULIRGs lies in high-velocity outflows
detected in sources with large AGN luminosities (Sturm et al., 2011), the
effect of which on the host galaxies will be examined in the next section.

###  3.2 AGN Feedback

Energetic feedback from stars, supernovae, and accreting supermassive black
holes regulates the baryonic content of a galaxy by injecting energy and
momentum into its ISM and driving powerful winds from the nucleus to galactic
scales Heckman et al. (1995); Veilleux et al. (2005). In the canonical picture
of galaxy evolution, these winds play a key role in the chemical evolution of
galaxies through metal enrichment and redistribution, and the suppression of
star formation throughout the galaxy and out into the circumgalactic medium
(CGM). AGN feedback in particular has been invoked to explain the discrepancy
between the galaxy luminosity function predicted by $\Lambda$CDM and that
observed at the high-luminosity end. Readers are referred to articles by
Veilleux et al. (2005); Fabian (2012); Rupke (2018); Veilleux et al. (2020)
for in-depth discussions of the physics of feedback and associated
investigations in quasars, Seyferts, and star-forming galaxies. In this
review, we focus on key questions and results from studies of feedback in
(U)LIRGs and mergers.

####  3.2.1 Feedback: A Multiphased Phenomenon

As powerful sources hosting both starbursts and AGN, (U)LIRGs (as a
population) present an intriguing albeit challenging environment to
disentangle the feedback effect on the ISM. Theoretically, the “Feedback In
Realistic Environments-2” (FIRE-2) model (Hopkins et al., 2018) resolves star
formation, feedback processes, and the multiphase structure of the ISM in an
extensive suite of parsec-scale galaxy merger simulations. This framework
subsequently enables the study of interaction-induced ISM conditions and their
dependence on orbital geometry as well as the SFR-enhancement or SFR-
suppression in different local environments—underscoring the need for large,
spatially-resolved galaxy merger surveys (Moreno et al., 2020).

Observationally, massive outflows in nearby ULIRGs, often associated with
late-stage mergers, appear to be ubiquitous (Yamada et al., 2021; Pereira-
Santaella et al., 2021). These transformative processes have been detected in
ionized and neutral atomic gas (Soto et al., 2012; Rupke and Veilleux, 2013;
Leung et al., 2021), warm molecular gas (U et al., 2013; Medling et al., 2015;
U et al., 2019), and cold molecular gas (Cicone et al., 2014; Alatalo, 2015;
García-Burillo et al., 2015; Izumi et al., 2020). Relative to AGN studies
where outflows that correlate with decreased SFR are taken as evidence for
negative feedback, the topic is much more nuanced among (U)LIRGs given the
presence of both intense star formation and AGN in the nuclei, dust
obscuration masking the intrinsic state of the AGN, and merger interaction
that alters the ISM conditions on dynamic timescales. With the ability to
resolve stellar and gaseous emissions on circumnuclear scales, U et al. (2019)
determined dust-corrected nuclear SFRs for 21 (U)LIRGs in the GOALS sample and
found that nuclear star formation exhibits a trend with the merging stage and
diminishing projected nuclear separation (Figure 10). The trends support the
notion of positive feedback at small scales, consistent with the picture of
merger-induced starbursts in the center of galaxy mergers (U et al., 2019).

Figure 10: Dust-corrected nuclear SFR on logarithmic scales as a function of
the merger class (left) and nuclear separation $\log r_{\mathrm{sep}}$
(right). The correlation coefficient $\tau$ is given in each panel; data
points reproduced here are taken from U et al. (2019) with authors’
permission. The merger classification scheme was adopted from Haan et al.
(2011) and Kim et al. (2013) as follows: (0) a single galaxy with no obvious
companion, (1) separate galaxies with symmetric disks and no tidal tails, (2)
distinguishable progenitor galaxies with asymmetric disks and/or tidal tails,
(3) two distinct nuclei sharing a common envelope, (4) closely-separated
nuclei with visible tidal tails, (5) a single nucleus with prominent tails,
and (6) a post-merger remnant.

On top of the complexities associated with multiscale investigations of
outflows, the usual challenges of constraining AGN feedback remain:
determining the energetics, mass, metallicity, and physical extent of the
outflowing gas will rely on resolved IFU studies (Kakkad et al., 2018, 2020).
Sources for errors in calculating mass loss rates include uncertainties in the
outflow geometry and electron density in the absence of high-sensitivity and
spatially-resolved data (Cicone et al., 2018). The outflowing gas can
experience phase changes in different ISM environments and may be
intrinsically multiphase, sharing similar kinematics but different mass,
energy, and momentum contributions (Pereira-Santaella et al., 2018). With the
advent of IFUs and interferometry among both ground- and space-based
facilities, proper calibrations between integrated and resolved measurements
will be useful. Here we discuss a few in-depth case studies of feedback in
(U)LIRGs as well as conclusions drawn from emerging surveys.

####  3.2.2 A Case Study: Tracing Outflows in Mrk 273 from the Core to the
Halo

Among the most nearby ULIRGs, Mrk 273 is a late-stage merger that features
multiphase high-velocity AGN-driven outflows with particularly complex
kinematics within its nuclear region (Figure 11). Based on infrared
observations from _HST_ NICMOS, the nuclear complex appears clumpy harboring
at least one SMBH (Scoville et al., 2000). High-resolution radio observations
resolved multiple sources that were identified to be nuclear star cluster
candidates. _Chandra_ X-ray observations of 44-ks depth revealed an X-ray AGN
in the southern nucleus and offered constraints that suggested the northern
nucleus to be CT (see Figure 1 in Iwasawa et al. (2011)). The dual-AGN nature
of Mrk 273 was not confirmed until (U et al., 2013) analyzed high-resolution
AO near-infrared observations of the nuclear gas kinematics. In the central
kiloparsec region, the warm shocked molecular hydrogen gas is found in
biconical outflows emanating from the northern nucleus, where a black hole of
mass $M_{\mathrm{BH}}=1\times 10^{9}M_{\odot}$ has been located using the gas
dynamical measurement (U et al., 2013). This measurement agrees very well with
that from OH maser (Klöckner and Baan, 2004), a golden standard among black
hole mass measurement techniques. The inferred density from this massive
compact object exceeded that of most known stellar clusters, arguing in favor
of this being a remnant nucleus from the progenitor merging galaxy. The dual
AGN nature of Mrk 273 was further verified by deep 200-ks _Chandra_ data
subsequently obtained where the hard X-ray emission was successfully resolved
(Liu et al., 2019). There are even speculations that a gas clump shock-heated
by the outflow in the [Si vi] emission might actually be the third nucleus in
this system (Rodríguez Zaurín et al., 2014). The implication brought by this
interesting system will be discussed in the broader context of dual SMBHs in
Section 4.

On the galactic scale, the large nebular bubble 20 kpc to the northeast
direction of the nuclear region in Mrk 273 has been imaged in ionized gas via
_HST_ narrow band imaging (Rodríguez Zaurín et al., 2014), and more recently,
ground-based IFU observations (Leung et al., 2021) (Figure 11). The mixing of
AGN photoionization and shock excitation is detected in both the nuclear
region and the NE nebula, but the relative contribution from each is assessed
to be different, spatially but connected, suggesting a common origin for both.
The advent of wide-field IFU observations facilitates not only kinematic
studies but also resolved studies of key diagnostic line ratios. Line ratio
diagnostics coupled with modeling can be a powerful technique to disentangle
the excitation conditions and the intricate driving mechanisms often seen in
galaxy mergers (for example, of another in-depth study of LIRG NGC 6240
Medling et al. (2021)).

Figure 11: (a) Mrk 273 as seen in visible light from _HST_ /ACS _F435W_ and
_F814W_ imaging; north points up and a 10 kpc ($\sim$13′′) scale bar is shown.
Purple and orange boxes correspond to insets (b,c), respectively; (b) from
left to right: a) _HST_ /ACS _F814W_ , b) _HST_ /NICMOS _F160W_ , and c)
composite of the previous two overlaid with the VLA 8.4 GHz image (Condon et
al., 1991) in contours, figure adapted from Iwasawa et al. (2011), reproduced
with permission from © ESO; (c) from left to right: flux, velocity, and
velocity dispersion of the H2 1$-$0 S(1) overlaid with the $K$ band continuum
as observed with Keck OSIRIS, figure from U et al. (2013); (d) _HST_ /ACS
continuum (red) + [O iii] (blue) from Rodríguez Zaurín et al. (2014),
reproduced with permission from © ESO; green box indicates the zoomed-in
region for (e); (e) emission line maps highlighting the extent of ionized
gasses ([O ii], [O iii], [Ne v], and He ii) well beyond the nuclear region in
Mrk 273; figure adapted from Leung et al. (2021); © AAS, reproduced with
permission.

####  3.2.3 How Does AGN drive outflows in (U)LIRGs?

Not only are outflows multiphased and multiscaled—often requiring observations
from different facilities to fully sample the processes involved—but the
dynamics of mergers plus the sometimes obscured state of the AGN add
challenges to putting together the pieces of the feedback puzzle. Fortunately,
the picture is becoming clearer with the advent of recent multiwavelength
studies being devoted to investigating the detailed mechanisms for this
phenomenon in nearby (U)LIRGs. A _NuSTAR_ and Swift/BAT study found that the
most luminous AGN (with $L_{\mathrm{bol,AGN}}\sim 10^{46}$ erg s-1) have
relatively small column densities ($\log N_{H}\leq 23\,\mathrm{cm}^{-2}$)
(Yamada et al., 2021). The proposed scenario depicts strong multiphase
outflows triggered by chaotic quasi-spherical inflows but the AGN in these
late-stage mergers might appear X-ray weak and CT due to obscuration by
inflowing or outflowing material (Figure 12). This finding is consistent with
that from a far-infrared _Herschel_ -based study of OH, where the highest
molecular outflow velocities are found in buried sources (González-Alfonso et
al., 2017). Using high-resolution ALMA images, the PUMA survey found that two-
thirds of the surveyed ULIRGs have nuclear radiation pressures above the
Eddington limit, giving weight to the role that radiation pressure plays in
the outflow launching process (Pereira-Santaella et al., 2021). Most of the
outflowing gas mass is loaded from within the central few hundred parsec of
the galactic cores. Outflow depletion timescales of $<10^{8}$ years are found
to be anti-correlated with AGN luminosity and much shorter than the gas-
consumption-by-star-formation timescales (González-Alfonso et al., 2017;
Gowardhan et al., 2018; Pereira-Santaella et al., 2018).

Figure 12: Schematic of AGN in the late-stage merger explaining how the AGN
may end up deeply buried. Galaxy interaction induces chaotic quasi-spherical
inflows (light pink) toward the dusty torus (dark pink), which block the X-ray
emission from the corona, and in turn, launch the UV-line driven winds (blue
arrows) and drive outflowing materials (orange). These inflows and outflows
are responsible for keeping the AGN obscured within molecular outflows found
at larger radii. Figure adapted from Yamada et al. (2021); © AAS, reproduced
with permission.

In a study of near-infrared H2 in 21 GOAL (U)LIRGs, the shocked molecular gas
is preferentially found in the ultraluminous systems but is also seen to be
triggered at a lower-luminosity, earlier merging stage (U et al., 2019). In a
separate study of CO and H2 in 6 (U)LIRGs with Seyfert nuclei, those galaxies
undergoing major mergers are found to exhibit shallower temperature
distributions and, thus, a higher relative contribution of shock excitation
with respect to the photodissociation region, for the warm molecular gas than
the normal spirals (Pereira-Santaella et al., 2014). These findings raise the
question of whether AGN—also commonly found in the more luminous systems—are
responsible for shock-heating the gas. On circumnuclear scales, AG nuclei have
strong effects on heating the molecular gas, but the shocks do not strictly
correlate with AGN strength. Such a link may be muddled by the presence of
patchy dust, torus orientation, varying gas density, or other factors (U et
al., 2019).

####  3.2.4 The AGN–Outflow–Merger–ULIRG Connection

At the population level, multiphase winds occur in almost all (U)LIRGs with
SFR above 10 $M_{\odot}$ year-1 (Rupke et al., 2005a, b; Rupke and Veilleux,
2005). How much of this wind is powered by AGN versus the underlying starburst
population requires detailed investigations of the wind geometry (i.e.,
opening angles) as well as momentum and energy injection into the ISM among
Seyfert 1, Seyfert 2, and starburst-dominated ULIRG systems (Rupke et al.,
2005). In a systematic search for OH molecular outflows in 43 nearby
ultraluminous infrared galaxy mergers, Veilleux et al. (2013) measured higher
outflow velocities in systems with large AGN fractions and high AGN
luminosities [$\log(L_{\mathrm{AGN}}/L_{\odot})\geq 11.8\pm 0.3$]. However,
the most AGN-dominated systems having relatively modest OH line widths might
suggest an evolutionary scenario where molecular outflows subside at the
quasar blow-out phase.

The scenario that AGN are more likely to be buried, or less luminous, during
late-stage mergers associated with the highest molecular outflow velocities
and short gas depletion timescales seem contradictory to studies that found
the fastest ionized winds in the most luminous AGN. For instance, Harrison et
al. (2014) compared the velocities of ionized outflows in AGN, ULIRGs, and
ULIRG-AGN composites and found that the most extreme ionized gas velocities
($v\gtrsim 2000$ km s-1) are preferentially found in quasar-ULIRG composite
galaxies (Figure 13 (Harrison et al., 2014)). This is likely because AGN and
highest-SFR starbursts are confounded in late-stage merging ULIRGs, where
distinguishing between the two as the origin of the outflows is non-trivial.
In this case, the starburst may contribute a non-negligible (if not dominant)
amount to the ionized outflows seen at the kiloparsec scale. A recent study of
highly-ionized [O vi] and [N v] outflows in 19 ULIRGs found that outflows are
more frequently detected in the X-ray weak or absorbed sources, which also
feature higher-velocity outflows (Liu et al., 2022). The seemingly contentious
point rests on whether ULIRGs host deeply buried AGN or luminous quasars, and
may be reconciled if the obscuration timescales and properties in ULIRGs can
be better quantified with imminently available high-resolution _JWST_
observations for large samples of local ULIRGs.

Figure 13: Outflow velocity vs. infrared luminosity from star formation
(left), AGN luminosity (middle), and $q_{\mathrm{IR}}$ (right) for nearby AGN
(red stars; Harrison et al. (2014)), ULIRGs (green triangles; Rupke and
Veilleux (2013)), and ULIRG-AGN composites (magenta squares; Rodríguez Zaurín
et al. (2013)). The most extreme velocities appear to be preferentially found
in ULIRGs that host quasars. Figure adapted from Harrison et al. (2014) with
permission.

##  4 Dual SMBHs in Merger Systems

###  4.1 The Observational Challenges and Significance of Constraining the
SMBH Pair Population

In the era of multi-messenger astrophysics, the science case for studying
merging SMBHs is prominent given the anticipated detection of gravitational
wave signatures from this massive regime. Understanding the statistical
properties of the merging SMBH population or the nuclear conditions of the
progenitor galaxies has great implications for the expected detection rate of
gravitational wave signals. Naturally, close SMBH pairs are expected to be
found within the circumnuclear disks in late-stage or post-mergers. A large
number of dual AGN candidates with nuclear separations of 1 kpc or greater
have been discovered using optical methods (Liu et al., 2011), but the
$r_{\mathrm{sep}}<1$ kpc regime has presented many more challenges. While
several binary SMBH candidates have been posited (Fu et al., 2011; Kharb et
al., 2017; Dey et al., 2018), only one appears robust to date (with
$r_{\mathrm{sep}}=7.3$ pc in between the two SMBHs as resolved by VLBI
(Rodriguez et al., 2006)). A compilation of candidate AGN pairs discovered is
summarized in Figure 14.

Figure 14: A compilation of candidate AGN pairs to date shown in projected
nuclear separation—redshift space. The dual AGN are indicated by their
discovery method: optical/near-infrared (dots, from Liu et al. (2011)), X-ray
(circles), and radio (crosses). The colored dashed and thick curves represent
the resolution limits of the ngVLA at different frequency–baseline
configurations with the VLA (10 GHz, A array; thin black line) as a
comparison. Critical stages in binary formation and evolution are marked as
horizontal lines. Only a handful of duals and binaries have been identified
with nuclear separation below 1 kpc, so a key area of research would be to
locate these SMBH pairs in mergers and post-mergers using new multiwavelength
facilities with enhanced resolving power in the coming decade. Figure adapted
from Burke-Spolaor et al. (2018) with permission.

Observational challenges to finding merging SMBHs are several-fold:
scientifically, close SMBH pairs might be missed from traditional optical
searches if they reside in the dust-obscured nuclei of late-stage mergers,
where intense starbursts and other sources of extreme energetics might
contribute to the contamination rate of AGN detection. The two AGN may also
engage in asynchronous triggering, causing the quiescent black hole in the
pair to be off the radar. On the technical front, the best-case resolving
capability—the diffraction limit $\theta$—scales with the aperture size $D$ of
an observing facility for a given wavelength of interest $\lambda$ in the form
of $\theta=1.22\lambda/D$. Specific detection methods may also introduce
biases and blind spots. Hence, the multi-messenger search of true inspiraling
SMBH binaries prompts multiple validations, for instance, using a combination
of gravitational wave detectors, such as the pulsar timing arrays (PTAs
(Foster and Backer, 1990; Hobbs et al., 2010)) and large time-domain surveys
providing multi-epoch light curves to verify the periodicity of the merging
pair.

In the massive black hole binary regime, observational evidence for
identification includes the following: emission of gravitational waves, direct
imaging of double nuclei using VLBI, quasi-periodicity in light curves of AGN
and quasars, and double-peaked emission lines in AGN and quasar spectra. For
more details, we refer the readers to articles dedicated to the search for
binary SMBHs by Burke-Spolaor et al. (2019); De Rosa et al. (2019); Bogdanovic
et al. (2021). In the dual regime with larger nuclear separation, additional
identification and verification techniques may include determining the
dynamics around black holes and directly resolving hard X-ray sources given
the larger separation allowance. The key is to be able to 1) resolve the two
(or more) point sources within the distance limitation, and 2) be able to
unambiguously confirm the SMBH nature for each of the point sources at the
wavelength(s) of interest.

Our discussion here highlights observational results of dual SMBHs with
projected separation $r_{\mathrm{sep}}\approx 10-1000$ pc residing in the
center of merging infrared galaxies. The duals are scientifically relevant
because they are the precursors to the gravitationally-bound binaries and an
understanding of the environment in which they are located will give us
insights into the dual AGN discoverability and the dynamical timescale at the
population level. The amount of infalling gas into the center triggered by the
merger’s orbital dynamics may have huge implications on whether the binary
SMBHs will eventually coalesce and produce gravitational wave signals or if
they will instead be stalled (e.g., the “final parsec problem” (Milosavljević
and Merritt, 2003)).

###  4.2 The Multiwavelength Searches for Duals in Mergers

Much of the search for double AGN has taken advantage of the high spatial
resolution capability of interferometry in the radio regime. Thanks to their
long baseline configurations, extended radio interferometry arrays, such as
the Very Long Baseline Interferometry (VLBI), can probe down to 1 $\times$
$10^{-6}$ ArcSec (Bartel et al., 1988), or equivalently, the sub-parsec regime
in local sources. Given the power of the radio spectral index to detect high
brightness temperature and synchrotron emission from AGN in even dusty nuclei
(see Section 2), a technique using VLBI to resolve and identify multiple point
sources promises to be very feasible. Such a blind search was conducted using
archival VLBI data of radio-luminous AGN (Burke-Spolaor, 2011). However, only
one binary black hole candidate was confirmed among 3114 sources examined:
0402+370, or 4C +37.11, which hosts two nuclei separated at 7 pc as previously
discovered by Rodriguez et al. (2006). Taking into account the limitations in
the spatial resolution of the large redshift range ($z=0.000113-4.715$, where
projected separation from sub-parsec up to 8.5 pc can be resolved) probed in
the sample, the implications from this statistical result place constraints on
the physics of merging SMBH pairs and the broader cosmological context of the
merger population. Whether both binary SMBHs are unlikely to be found in a
radioactive state simultaneously or if the timescale for an inspiraling SMBH
pair to have $<$10 pc separation is very short will require further
investigations, possibly with the future square kilometer array.

Optical long-slit spectroscopy can be useful in helping to verify or rule out
the nature of dual AGN given the diagnostic power of BPT or equivalent line
ratio diagrams (see Section 2). Dual AGN selection using the presence of a
double-peaked [O iii] $\lambda$5007 narrow emission line has been favored
given the abundance of rest-frame optical spectra of galaxies and quasars
available through the Sloan Digital Sky Survey (SDSS) and other large
spectroscopic surveys. A double-peaked [O iii] line may originate from the two
distinct NLRs surrounding the two black holes in a dual or binary system
(Boroson and Lauer, 2009; Wang et al., 2009; Comerford et al., 2009, 2013;
McGurk et al., 2011; Barrows et al., 2012; Fu et al., 2012), but it may also
be produced by biconical or one-sided outflows in a single black hole
(Veilleux et al., 2001; Fischer et al., 2011; Rupke and Veilleux, 2013; Liu et
al., 2018) or a rotating disk of NLR associated with a single black hole
(Greene and Ho, 2005; Liu et al., 2010; Smith et al., 2012). Additional
scrutiny is needed to vet the contaminants. Using either IFU or a combination
of high-resolution imaging plus resolved spectroscopy, McGurk et al. (2015)
studied a sample of 140 SDSS DR7 sources with double-peaked narrow [O iii]
lines and found 30% having two spatial components. A follow-up in-depth study
of 12 candidates within this subset substantiated only one dual AGN system (at
separation 6.6 kpc) with additional outflow or other candidates (McGurk et
al., 2015). A larger, more recent study of 1271 SDSS quasars at $z<0.25$ with
multiwavelength verification methods found a similar success rate (77 dual
candidates, or 6%) (Kim et al., 2020).

While hard X-ray observations provide a robust tool for locating highly
obscured sources that are most likely to be close SMBH pairs during the late
stages of mergers, current X-ray facilities with photon-counting techniques
tend not to have high enough spatial resolution to resolve close pairs at less
than 0$.\\!\\!^{\prime\prime}$5 separation, though deep X-ray observations of
putative duals can help confirm their candidacy (Comerford et al., 2015).
Combining a hard X-ray selection with systematic high-resolution AO-assisted
near-infrared imaging of 96 low-redshift (0.01 $<z<$ 0.075) Swift/BAT-detected
sources, Koss et al. (2018) confirmed the excess of nuclear obscured duals
with separations $<$ 3 kpc among late-stage mergers (17.6%) compared to a
control sample of inactive galaxies (1.1%), consistent with theoretical
predictions (Figure 15). Indeed, another X-ray + near-infrared spectra +
_WISE_ study of advanced mergers with projected separations $<$ 10 kpc
confirmed that optical studies alone miss a significant fraction of single and
dual AGN in advanced mergers, advocating that mid-infrared pre-selection can
highly enhance the success rate of identifying dual AGN (Satyapal et al.,
2017; Pfeifle et al., 2019).

Figure 15: Distributions of close mergers at the $<$3 kpc and 3–10 kpc
separations, respectively, as categorized by the presence of broad H
recombination lines from optical spectroscopy and bolometric luminosity as
computed from X-ray emission. There is clearly an excess of nuclear mergers
among obscured AGN at close separations. Figure adapted from Koss et al.
(2018) with permission.

A general note of caution is that resolving multiple point sources is a
necessary but insufficient criterion for identifying dual AGN, particularly in
galaxy mergers where the central regions tend to be complex due to the
presence of dust, nuclear star clusters, and the corresponding entangled
dynamics. Sometimes it might be difficult to define where the “center” of an
interacting system is, such as in II Zw 96, where the most infrared bright
source appears off-nuclear (Inami et al., 2010)). Medling et al. (2014)
presented the nuclear stellar and gas kinematics for a sample of 17 (U)LIRGs
at a high ($\leq$ 0$.\\!\\!^{\prime\prime}$1) resolution: dynamically-rotating
stellar and gaseous disks are nearly ubiquitous, but in most cases, nuclei
with resolved point sources do not display a superposition of two (or more)
dynamical disks. While not completely ruled out, those sources are more likely
to host a remnant nucleus with a nearby star cluster that appears close-by by
chance projection than a circumbinary disk with merging black holes.

###  4.3 Implications from Duals in Merging (U)LIRGs

The current list of confirmed dual AGN with $r_{\mathrm{sep}}\lesssim 1$ kpc
among local (U)LIRGs consists of only a handful of sources: NGC 6240 (1 kpc;
(Komossa et al., 2003)), Arp 299 (900 pc; (Ballo et al., 2004)), and Mrk 273
(800 pc; (Iwasawa et al., 2011; U et al., 2013; Liu et al., 2019)). (U)LIRGs
that feature dual AGN with slightly larger separations include Mrk 266 (6 kpc;
(Mazzarella et al., 2012)), Mrk 463 (3.8 kpc; (Bianchi et al., 2008; Treister
et al., 2018; Yamada et al., 2018), and borderline-LIRG Mrk 739 (3.4 kpc;
(Koss et al., 2011)). NGC 7674, a moderately-luminous LIRG with a companion 20
kpc away from the main infrared emission source, was identified as a binary
SMBH host candidate within its nucleus based on VLBI results (Kharb et al.,
2017), but the signal-to-noise ratio of the radio imaging is marginal and its
candidacy was more recently challenged (Burke-Spolaor et al., 2019).

Nevertheless, the identification of close SMBH pairs in these (U)LIRGs
suggests that they might represent the best bet for locating progenitor
merging SMBHs in order to understand the small-scale environments that favor
pairs. Ongoing galaxy mergers may preferentially be found in group
environments where galaxies are more susceptible to interacting with one or
more partner(s) in the recent past, so characterizing the SMBH-pair rate in
these systems at the population level could place constraints on our
understanding of the evolution of the merger fraction or the timescale of
synchronous AGN-triggering. Detailed dynamical modeling of merging systems
(Figure 16; (Privon et al., 2013)) coupled with the detection of dual SMBHs
could address these questions, though accomplishing this task at the
population level could be resource intensive.

Figure 16: (Top) Nuclear separation as a function of time for four merging
systems (from left to right: NGC 2623, Antennae, The Mice, and NGC 5257/8) as
determined from Identikit (Barnes and Hibbard, 2009) dynamical modeling of Hi
and optical imaging. The vertical line at the time of zero represents the
current viewing time. Colored arrows mark the corresponding smoothing length
in kpc for each system, below which the spatial resolution of the simulations
may not be reliable. (Bottom) Example snapshots of how LIRG NGC 2623 evolved
as a function of time. The grey lines show the initial Keplerian orbits of the
galaxies, and the times shown are relative to now. Figures adapted from Privon
et al. (2013); © AAS, reproduced with permission.

The conundrum that (U)LIRGs in the local universe are relatively rare and yet
our current state-of-the-art resolving capability limits our electromagnetic
search for duals and binaries to the local universe means that our next
breakthrough will be instigated by the next-generation observing facilities.
More cutting-edge technology across the electromagnetic spectrum pushing the
angular resolution limit will hopefully render the search for SMBH merger
progenitors more fruitful within the next decades. With the onset of PTAs and
other next-generation gravitational wave detectors, such as LISA, prompted to
detect gravitational wave signals from the merging of SMBHs, it is more timely
than ever that we investigate the nuclear environments within (U)LIRGs that
lead to black hole mergers.

##  5 Future Outlook from the Observational Perspective

In this review, we covered recent observational results in identifying AGN via
a multiwavelength approach, investigating AGN fueling and feedback from the
galactic core to halo, and locating close SMBH pairs within merging infrared
galaxies. These science cases align primarily with one of the three priority
science themes identified by the Astro2020 Decadal Survey—understanding Cosmic
Ecosystems through unveiling the drivers of galaxy growth and exploring the
interconnectedness of different processes responsible for galaxy evolution. A
large part of this multiscale problem lies in understanding the physics of
AGN, driving investments in facilities that provide high spatial resolution.
Here, we discuss how several of the upcoming observatories and facilities with
unparalleled technical capabilities will help achieve breakthrough science
cases in our understanding of AGN’s role in infrared galaxies.

###  5.1 Peering through the Dust

The _JWST_ just started science operations at the time of this manuscript’s
preparation. As a NASA flagship, _JWST_ is expected to extend the legacy of
both _HST_ and _Spitzer_ as a space telescope with near- and mid-infrared
(0.8–28 $\upmu$m) wavelength coverage. Its mid-infrared wavelength reach
coupled with its 6.5-m mirror will surpass _Spitzer_ ’s sensitivity and
spatial resolution (with a 50$\times$ smaller PSF area), enabling observations
to sample the rest frame UV and optical light from high-redshift galaxies and
probes far into the early universe. To facilitate the search for CT AGN, the
mid-infra-red instrument (MIRI) will be able to observe the forbidden neon
lines with ionization potentials (IPs) ranging from 21.6 ([Ne ii]) to 126 eV
([Ne vi]) (Alberts et al., 2020) or the highly-ionized [Mg iv] $\lambda$4.487
$\upmu$m (IP of 80 eV) and [Ar vi] $\lambda$4.528 $\upmu$m (IP of 75 eV)
(Satyapal et al., 2021). These lines are associated with the extra hard-
ionizing radiation from the AGN given their high ionization potentials. For
instance, the enhanced [Ne iii] $\lambda$15.56 $\upmu$m/[Ne ii] $\lambda$12.81
$\upmu$m ratio measured in three Seyfert galaxies relative to the median value
of that for a LIRG sample showing no AGN signatures demonstrates their utility
as robust AGN indicators (Pereira-Santaella et al., 2010). MIRI MRS has the
potential to acquire mid-infrared spectra of (U)LIRGs up to $z\sim 1-2$ that
includes [Ne vi] $\lambda$7.65 $\upmu$m. The synergy of the MIRI imager and
NIRCam will enable us to detect heavily obscured and CT AGN while possibly
associating them to mergers at high redshifts. See Alberts et al. (2020) and
Satyapal et al. (2021) for more quantitative discussions.

As for nearby infrared galaxies, early release science, guaranteed time
observations, and general observer programs in Cycle 1 will capitalize on MIRI
being the first mid-infrared IFU in space to investigate the kinematics of gas
and dust in the dusty cores of (U)LIRGs. With a pixel scale of $\sim$
0$.\\!\\!^{\prime\prime}$2 pixel-1, MIRI in MRS mode is well-suited to study
the resolved morphology of the cool ($\sim$1000 K) molecular gas, silicate
absorption, and PAH features often found in (U)LIRGs (Figure 17). MIRI’s
imaging mode has a plate scale of 0$.\\!\\!^{\prime\prime}$11 pixel-1, fully
sampling the JWST point spread function at 5.6 $\upmu$m. It will be able to
image the AGN dusty torus at $<$10 pc resolutions within the Seyfert nuclei of
the nearest AGN, such as LIRG NGC 1068.

Figure 17: _Spitzer_ /IRS spectra for several (U)LIRGs in GOALS (Stierwalt et
al., 2013). A full suite of crucial mid-infrared diagnostic lines are
accessible and will probe excitation mechanisms, extincted star formation,
dust and metal content, and AGN strength and ionization state in typically
heavily obscured regions otherwise opaque to optical studies. The spatial and
spectral resolutions of the IRS spectra (R$\sim$60–600) will be superseded by
MIRI MRS (R$\sim$2500). Dotted vertical lines indicate the spectral region
covered by the zoomed-in inset. (Inset) New MIRI MRS engineering calibration
spectrum of Seyfert galaxy NGC 6552 (red), shown in comparison with a lower
spectral resolution IRS spectrum of a similar galaxy in the same spectral
region (blue). The strong emission feature at rest frame 9.66 $\upmu$m comes
from molecular hydrogen H2 S(3), while the weaker feature is likely [Fe vii]
$\lambda$9.53 $\upmu$m. Inset figure from NASA, ESA, and the MIRI Consortium
(courtesy of David Law and Alvaro Labiano).

In the near-infrared regime, _JWST_ excels in sensitivity given its large
6.5-m mirror in space, while the angular resolution afforded by its near-
infrared camera NIRCam
(0$.\\!\\!^{\prime\prime}$031–0$.\\!\\!^{\prime\prime}$063 pixel-1) and IFU
NIRSpec (0$.\\!\\!^{\prime\prime}$1 pixel-1) is matched by diffraction-limited
AO observations from large, 8- to 10-m class ground-based facilities. The Keck
Telescopes, for instance, host AO-assisted near-infrared IFU (OSIRIS) and
camera (NIRC2) whose smallest plate scales are 0$.\\!\\!^{\prime\prime}$020
and 0$.\\!\\!^{\prime\prime}$01, respectively, with a typical FWHM of
0$.\\!\\!^{\prime\prime}$05 under good seeing conditions. These observations
have been instrumental in mapping the nuclear dynamics surrounding the AGN in
nearby (U)LIRGs and finding dual AGN within the dusty cores (see Sections 3
and 4 for examples). Existing natural and laser guide star AO systems have
been phenomenal in resolved galactic nuclei studies, and novel techniques
(such as multi-conjugate AO) that allow multiplexing, ground-layer, and all-
sky AO that strive to improve sky coverage, Strehl ratio, or image quality,
will greatly enhance the number of galaxy systems that can be targeted from
the ground to broaden the demographics (Figure 18; see Davies and Kasper
(2012) for an in-depth discussion of different AO techniques).

Figure 18: Conceptual illustrations of several AO techniques that aim to
improve the sky coverage, Strehl ratio, or image quality of ground-based
observations. Figure adapted from Davies and Kasper (2012) with permission.

###  5.2 A Leap in Resolution

The next step in technological advances is to push the frontier of AO
diffraction-limited observations to the visible wavelength range. This goal is
currently being realized by an ongoing mission named ORCAS, or the Orbiting
Configurable Artificial Star. As a hybrid space and ground observatory, the
ORCAS missionhttps://asd.gsfc.nasa.gov/orcas/about/ (accessed on 24 June 2022)
(PI E. Peretz) beams down a laser to the ground-based observatory and provides
AO corrections and flux calibrations over the wavelength range 0.5–2 $\upmu$m.
Coupled with the 10-m mirror at Keck Observatory, ORCAS will enable near-
diffraction limited performance (high angular resolution ($\sim$12–40 mas);
high spectral resolution (R $\lesssim$ 10,000); and high sensitivity (29th
magnitude)) from the ground at visible wavelengths, detecting AGN feeding and
feedback signatures within the inner pc-scale regions and resolving SMBH
binaries in the near universe. At its best, ORCAS$+$Keck will be able to
resolve close SMBH pairs with separations down to $\sim$100 pc up to redshift
$z\sim 2$, where the (U)LIRG population dominates the star formation rate
density (Casey et al., 2014). The visible-light imager ORCAS-Keck Instrument
Demonstration (ORKID; currently being commissioned in Semester 2022B),
together with a near-diffraction-limited visible IFU in the works, will place
critical constraints on the number of optically-selected dual AGN pairs among
SMBHs and measure the merger fraction throughout cosmic history.

The end of the 2020s and the 2030s will welcome the 30-m class extremely large
telescopes (ELTs), including the European-ELT (E-ELT), the Giant Magellan
Telescope (GMT), and the Thirty-Meter Telescope (TMT). These ELTs will be
revolutionary in providing optical and near-infrared observations from the
ground at superb sensitivity and resolution. Coupled with the large glass,
advanced IFU instruments, such as the high angular resolution monolithic
optical and near-infrared integral-field spectrograph (HARMONI) on the ELT or
the infra-red imaging spectrograph (IRIS) on the TMT will reach resolutions of
0$.\\!\\!^{\prime\prime}$009 at $\sim$1–2 $\upmu$m, ideal for probing down to
the dusty tori around AGN. Figure 19 shows the capability of TMT/IRIS in
resolving the sphere of influence for black holes of certain masses at a range
of distances. The IFU ability to determine kinematics around black holes will
allow not just black hole mass determination, but also putative dual AGN
identification as well as a direct probe into the feeding and feedback
mechanisms around one or more AGN in infrared galaxies beyond the local
universe.

Figure 19: (Top) The projected radius of influence for a range of black hole
masses (black lines) plotted as a function of distance. Overlaid are the
angular resolution limits of current typical IFU observations (blue dashed)
and those anticipated for IRIS at $K$ (red) and $Z$ (green) bands,
respectively. The enhancement in resolving capability brought about by 30-m
class telescopes will push the frontier of black hole mass measurements to
further distances. (Bottom) Lower limits on the black hole mass measurements
are shown as a function of distance. Figure from Do et al. Do et al. (2014); ©
AAS, reproduced with permission.

##  6 Concluding Remarks

In this review, we presented recent multiwavelength observational results
related to the detection of one (or more AGN) in nearby (U)LIRGs, which are
mostly gas-rich major mergers, in addition to the fueling and feedback roles
they play in regulating galaxy growth. Unlike using homogeneous quasars or
Seyfert samples, (U)LIRG studies present additional challenges in finding the
true location and verifying the nature of the nuclei within merging systems,
thanks both to the galaxy interaction and dust obscuration. Detailed
understanding of the nuclear regions requires high spatial resolution
observations, which are limited to the local universe where there are
relatively rare (U)LIRGs. Nevertheless, we as a community have built up
statistical samples that altogether begin to shed light on (U)LIRGs’ place
within galaxy evolution. Upcoming space- and ground-based facilities will
provide a leap in resolution across the electromagnetic spectrum and allow us
to expand our investigations both deeper within the galactic nuclei and
broader to a larger population at higher redshifts. The coming decade will be
a golden age for studying AGN in infrared galaxies.

This research was funded by National Aeronautics and Space Administration
Astrophysics Data Analysis Program grant number 80NSSC20K0450.

###### Acknowledgements.

V.U. thanks A. Sajina, G. Rieke, S. Yamada, L. Barcos-Muñoz, G. Privon, A.
Medling, J. Mazzarella, D. Sanders, A. Stacey, A. Evans, and the two anonymous
referees for insightful comments that contributed to the framing and content
of this review. This research has made use of the NASA/IPAC Extragalactic
Database, which is funded by the National Aeronautics and Space Administration
and operated by the California Institute of Technology. The funders had no
role in the design of the study; in the collection, analyses, or
interpretation of data; in the writing of the manuscript, or in the decision
to publish the results. Abbreviations The following abbreviations are used in
this manuscript:
AGN | Active Galactic Nuclei
---|---
ALMA | Atacama Large Millimeter/submillimeter Array
AO | Adaptive Optics
BAT | Burst Alert Telescope
CON | Compact Obscured Nuclei
CT | Compton Thick
ELT | Extremely Large Telescope
GOALS | Great Observatories All-sky LIRGs Survey
HST | Hubble Space Telescope
IFU(S) | Integral Field Unit (Spectroscopy)
IRAS | Infrared Astronomical Satellite
ISM | Interstellar Medium
IP | Ionization Potential
JWST | James Webb Space Telescope
LIRG | Luminous Infrared Galaxy
MIRI | Mid-Infra-Red Instrument
MRS | Medium Resolution Spectroscopy
MUSE | Multi Unit Spectroscopic Explorer
NIRCam | Near Infrared Camera
NIRSpec | Near Infrared Spectrograph
NLR | Narrow Line Region
PG | Palomar-Green (quasar)
PUMA | Physics of ULIRGs with MUSE and ALMA
PTA | Pulsar Timing Arrays
QSO | Quasi Stellar Object
QUEST | Quasars/ULIRGs Evolutionary STudy
SDSS | Sloan Digital Sky Survey
SFR | Star Formation Rate
SFMS | Star Formation Main Sequence
SMBH | Supermassive Black Hole
ULIRG | Ultraluminous Infrared Galaxy
VLA | Very Large Array
VLBI | Very Long Baseline Interferometry
WISE | Wide-field Infrared Survey Explorer (WiSE)
[custom] References

## References

  * Martin et al. (2016) Martin, D.C.; Matuszewski, M.; Morrissey, P.; Neill, J.D.; Moore, A.; Steidel, C.C.; Trainor, R.  A Newly Forming Cold Flow Protogalactic Disk, a Signature of Cold Accretion from the Cosmic Web.  Astrophys. J. Lett. 2016, 824, L5.  https://doi.org/black10.3847/2041-8205/824/1/L5.
  * White and Rees (1978) White, S.D.M.; Rees, M.J.  Core condensation in heavy halos: A two-stage theory for galaxy formation and clustering.  Mon. Not. R. Astron. Soc. 1978, 183, 341–358.  https://doi.org/black10.1093/mnras/183.3.341.
  * Blumenthal et al. (1984) Blumenthal, G.R.; Faber, S.M.; Primack, J.R.; Rees, M.J.  Formation of galaxies and large-scale structure with cold dark matter.  Nature 1984, 311, 517–525.  https://doi.org/black10.1038/311517a0.
  * Sanders et al. (1988) Sanders, D.B.; Soifer, B.T.; Elias, J.H.; Madore, B.F.; Matthews, K.; Neugebauer, G.; Scoville, N.Z.  Ultraluminous infrared galaxies and the origin of quasars.  Astrophys. J. 1988, 325, 74–91.  https://doi.org/black10.1086/165983.
  * Hopkins et al. (2008) Hopkins, P.F.; Hernquist, L.; Cox, T.J.; Kereš, D.  A Cosmological Framework for the Co-evolution of Quasars, Supermassive Black Holes, and Elliptical Galaxies. I. Galaxy Mergers and Quasar Activity.  Astrophys. J. Suppl. Ser. 2008, 175, 356–389.  https://doi.org/black10.1086/524362.
  * Rieke and Lebofsky (1979) Rieke, G.H.; Lebofsky, M.J.  Infrared emission of extragalactic sources.  Annu. Rev. Astron. Astrophys. 1979, 17, 477–511.  https://doi.org/black10.1146/annurev.aa.17.090179.002401.
  * Lonsdale et al. (1984) Lonsdale, C.J.; Persson, S.E.; Matthews, K.  Infrared observations of interacting/merging galaxies.  Astrophys. J. 1984, 287, 95–107.  https://doi.org/black10.1086/162666.
  * Neugebauer et al. (1984) Neugebauer, G.; Habing, H.J.; van Duinen, R.; Aumann, H.H.; Baud, B.; Beichman, C.A.; Beintema, D.A.; Boggess, N.; Clegg, P.E.; de Jong, T.; et al.  The Infrared Astronomical Satellite (IRAS) mission.  Astrophys. J. Lett. 1984, 278, L1–L6.  https://doi.org/black10.1086/184209.
  * Bei (1988) Beichman, C.A.; Neugebauer, G.; Habing, H.J.; Clegg, P.E.; Chester, T.J. Infrared Astronomical Satellite (IRAS) Catalogs and Atlases. Volume 1: Explanatory Supplement; _IRAS_ Project Office, California Institute of Technology, Pasadena, CA., 1988.
  * Soifer et al. (1987) Soifer, B.T.; Sanders, D.B.; Madore, B.F.; Neugebauer, G.; Danielson, G.E.; Elias, J.H.; Lonsdale, C.J.; Rice, W.L.  The IRAS Bright Galaxy Sample. II. The Sample and Luminosity Function.  Astrophys. J. 1987, 320, 238.  https://doi.org/black10.1086/165536.
  * Schechter (1976) Schechter, P.  An analytic expression for the luminosity function for galaxies.  Astrophys. J. 1976, 203, 297–306. https://doi.org/10.1086/154079.
  * Huchra (1977) Huchra, J.P.  The nature of Markarian galaxies.  Astrophys. J. Suppl. Ser. 1977, 35, 171–195.  https://doi.org/black10.1086/190474.
  * Schmidt and Green (1983) Schmidt, M.; Green, R.F.  Quasar evolution derived from the Palomar bright quasar survey and other complete quasar surveys.  Astrophys. J. 1983, 269, 352–374.  https://doi.org/black10.1086/161048.
  * Sanders and Mirabel (1996) Sanders, D.B.; Mirabel, I.F.  LUMINOUS INFRARED GALAXIES.  Annu. Rev. Astron. Astrophys. 1996, 34, 749–792.  https://doi.org/black10.1146/annurev.astro.34.1.749.
  * Kim and Sanders (1998) Kim, D.C.; Sanders, D.B.  The IRAS 1 Jy Survey of Ultraluminous Infrared Galaxies. I. The Sample and Luminosity Function.  Astrophys. J. Suppl. Ser. 1998, 119, 41–58.  https://doi.org/black10.1086/313148.
  * Surace (1998) Surace, J.A.  A High-Resolution Near-Infrared Study of the Evolutionary Link between Ultraluminous Infrared Galaxies and Optical QSOS.  Ph.D. Thesis, University of Hawaii, Manoa, Institute for Astronomy, Honolulu, HI, USA, 1998.
  * Rothberg et al. (2013) Rothberg, B.; Fischer, J.; Rodrigues, M.; Sanders, D.B.  Unveiling the $\sigma$-discrepancy. II. Revisiting the Evolution of ULIRGs and the Origin of Quasars.  Astrophys. J. 2013, 767, 72.  https://doi.org/black10.1088/0004-637X/767/1/72.
  * Kessler et al. (1996) Kessler, M.F.; Steinz, J.A.; Anderegg, M.E.; Clavel, J.; Drechsel, G.; Estaria, P.; Faelker, J.; Riedinger, J.R.; Robson, A.; Taylor, B.G.; et al.  The Infrared Space Observatory (ISO) mission.  Astron. Astrophys. 1996, 315, L27–L31.
  * Holland et al. (1999) Holland, W.S.; Robson, E.I.; Gear, W.K.; Cunningham, C.R.; Lightfoot, J.F.; Jenness, T.; Ivison, R.J.; Stevens, J.A.; Ade, P.A.R.; Griffin, M.J.; et al.  SCUBA: A common-user submillimetre camera operating on the James Clerk Maxwell Telescope.  Mon. Not. R. Astron. Soc. 1999, 303, 659–672.  https://doi.org/black10.1046/j.1365-8711.1999.02111.x.
  * Werner et al. (2004) Werner, M.W.; Roellig, T.L.; Low, F.J.; Rieke, G.H.; Rieke, M.; Hoffmann, W.F.; Young, E.; Houck, J.R.; Brandl, B.; Fazio, G.G.; et al.  The Spitzer Space Telescope Mission.  Astrophys. J. Suppl. Ser. 2004, 154, 1–9.  https://doi.org/black10.1086/422992.
  * Le Floc’h et al. (2005) Le Floc’h, E.; Papovich, C.; Dole, H.; Bell, E.F.; Lagache, G.; Rieke, G.H.; Egami, E.; Pérez-González, P.G.; Alonso-Herrero, A.; Rieke, M.J.; et al.  Infrared Luminosity Functions from the Chandra Deep Field-South: The Spitzer View on the History of Dusty Star Formation at 0 $\lesssim$ z $\lesssim$ 1\.  Astrophys. J. 2005, 632, 169–190.  https://doi.org/black10.1086/432789.
  * Casey et al. (2014) Casey, C.M.; Narayanan, D.; Cooray, A.  Dusty star-forming galaxies at high redshift.  Phys. Rep. 2014, 541, 45–161.  https://doi.org/black10.1016/j.physrep.2014.02.009.
  * Madau and Dickinson (2014) Madau, P.; Dickinson, M.  Cosmic Star-Formation History.  Annu. Rev. Astron. Astrophys. 2014, 52, 415–486. https://doi.org/10.1146/annurev-astro-081811-125615.
  * Scoville et al. (2007) Scoville, N.; Aussel, H.; Brusa, M.; Capak, P.; Carollo, C.M.; Elvis, M.; Giavalisco, M.; Guzzo, L.; Hasinger, G.; Impey, C.; et al.  The Cosmic Evolution Survey (COSMOS): Overview.  Astrophys. J. Suppl. Ser. 2007, 172, 1–8.  https://doi.org/black10.1086/516585.
  * Elbaz et al. (2011) Elbaz, D.; Dickinson, M.; Hwang, H.S.; Díaz-Santos, T.; Magdis, G.; Magnelli, B.; Le Borgne, D.; Galliano, F.; Pannella, M.; Chanial, P.; et al.  GOODS-Herschel: an infrared main sequence for star-forming galaxies.  Astron. Astrophys. 2011, 533, A119.  https://doi.org/black10.1051/0004-6361/201117239.
  * Grogin et al. (2011) Grogin, N.A.; Kocevski, D.D.; Faber, S.M.; Ferguson, H.C.; Koekemoer, A.M.; Riess, A.G.; Acquaviva, V.; Alexander, D.M.; Almaini, O.; Ashby, M.L.N.; et al.  CANDELS: The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey.  Astrophys. J. Suppl. Ser. 2011, 197, 35.  https://doi.org/black10.1088/0067-0049/197/2/35.
  * Koekemoer et al. (2011) Koekemoer, A.M.; Faber, S.M.; Ferguson, H.C.; Grogin, N.A.; Kocevski, D.D.; Koo, D.C.; Lai, K.; Lotz, J.M.; Lucas, R.A.; McGrath, E.J.; et al.  CANDELS: The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey—The Hubble Space Telescope Observations, Imaging Data Products, and Mosaics.  Astrophys. J. Suppl. Ser. 2011, 197, 36.  https://doi.org/black10.1088/0067-0049/197/2/36.
  * Zheng et al. (2004) Zheng, X.Z.; Hammer, F.; Flores, H.; Assémat, F.; Pelat, D.  HST/WFPC2 morphologies and color maps of distant luminous infrared galaxies.  Astron. Astrophys. 2004, 421, 847–862.  https://doi.org/black10.1051/0004-6361:20035784.
  * Bell et al. (2005) Bell, E.F.; Papovich, C.; Wolf, C.; Le Floc’h, E.; Caldwell, J.A.R.; Barden, M.; Egami, E.; McIntosh, D.H.; Meisenheimer, K.; Pérez-González, P.G.; et al.  Toward an Understanding of the Rapid Decline of the Cosmic Star Formation Rate.  Astrophys. J. 2005, 625, 23–36.  https://doi.org/black10.1086/429552.
  * Bridge et al. (2007) Bridge, C.R.; Appleton, P.N.; Conselice, C.J.; Choi, P.I.; Armus, L.; Fadda, D.; Laine, S.; Marleau, F.R.; Carlberg, R.G.; Helou, G.; et al.  The Role of Galaxy Interactions and Mergers in Star Formation at z<=1.3: Mid-Infrared Properties in the Spitzer First Look Survey.  Astrophys. J. 2007, 659, 931–940.  https://doi.org/black10.1086/512029.
  * Kartaltepe et al. (2010) Kartaltepe, J.S.; Sanders, D.B.; Le Floc’h, E.; Frayer, D.T.; Aussel, H.; Arnouts, S.; Ilbert, O.; Salvato, M.; Scoville, N.Z.; Surace, J.; et al.  A Multiwavelength Study of a Sample of 70 $\upmu$m Selected Galaxies in the COSMOS Field. II. The Role of Mergers in Galaxy Evolution.  Astrophys. J. 2010, 721, 98–123.  https://doi.org/black10.1088/0004-637X/721/1/98.
  * Kartaltepe et al. (2012) Kartaltepe, J.S.; Dickinson, M.; Alexander, D.M.; Bell, E.F.; Dahlen, T.; Elbaz, D.; Faber, S.M.; Lotz, J.; McIntosh, D.H.; Wiklind, T.; et al.  GOODS-Herschel and CANDELS: The Morphologies of Ultraluminous Infrared Galaxies at z$\sim$2\.  Astrophys. J. 2012, 757, 23.  https://doi.org/black10.1088/0004-637X/757/1/23.
  * Larson et al. (2020) Larson, K.L.; Díaz-Santos, T.; Armus, L.; Privon, G.C.; Linden, S.T.; Evans, A.S.; Howell, J.; Charmandaris, V.; U, V.; Sanders, D.B.; et al.  Star-forming Clumps in Local Luminous Infrared Galaxies.  Astrophys. J. 2020, 888, 92.  https://doi.org/black10.3847/1538-4357/ab5dc3.
  * Wuyts et al. (2011) Wuyts, S.; Förster Schreiber, N.M.; van der Wel, A.; Magnelli, B.; Guo, Y.; Genzel, R.; Lutz, D.; Aussel, H.; Barro, G.; Berta, S.; et al.  Galaxy Structure and Mode of Star Formation in the SFR-Mass Plane from z $\sim$ 2.5 to z $\sim$ 0.1.  Astrophys. J. 2011, 742, 96.  https://doi.org/black10.1088/0004-637X/742/2/96.
  * Speagle et al. (2014) Speagle, J.S.; Steinhardt, C.L.; Capak, P.L.; Silverman, J.D.  A Highly Consistent Framework for the Evolution of the Star-Forming “Main Sequence” from z $\sim$ 0–6.  Astrophys. J. Suppl. Ser. 2014, 214, 15.  https://doi.org/black10.1088/0067-0049/214/2/15.
  * Linden et al. (2019) Linden, S.T.; Song, Y.; Evans, A.S.; Murphy, E.J.; Armus, L.; Barcos-Muñoz, L.; Larson, K.; Díaz-Santos, T.; Privon, G.C.; Howell, J.; et al.  A Very Large Array Survey of Luminous Extranuclear Star-forming Regions in Luminous Infrared Galaxies in GOALS.  Astrophys. J. 2019, 881, 70.  https://doi.org/black10.3847/1538-4357/ab2872.
  * Skibba et al. (2011) Skibba, R.A.; Engelbracht, C.W.; Dale, D.; Hinz, J.; Zibetti, S.; Crocker, A.; Groves, B.; Hunt, L.; Johnson, B.D.; Meidt, S.; et al.  The Emission by Dust and Stars of Nearby Galaxies in the Herschel KINGFISH Survey.  Astrophys. J. 2011, 738, 89.  https://doi.org/black10.1088/0004-637X/738/1/89.
  * Cano-Díaz et al. (2016) Cano-Díaz, M.; Sánchez, S.F.; Zibetti, S.; Ascasibar, Y.; Bland-Hawthorn, J.; Ziegler, B.; González Delgado, R.M.; Walcher, C.J.; García-Benito, R.; Mast, D.; et al.  Spatially Resolved Star Formation Main Sequence of Galaxies in the CALIFA Survey.  Astrophys. J. Lett. 2016, 821, L26.  https://doi.org/black10.3847/2041-8205/821/2/L26.
  * Hall et al. (2018) Hall, C.; Courteau, S.; Jarrett, T.; Cluver, M.; Meurer, G.; Carignan, C.; Audcent-Ross, F.  Unique Tracks Drive the Scatter of the Spatially Resolved Star Formation Main Sequence.  Astrophys. J. 2018, 865, 154.  https://doi.org/black10.3847/1538-4357/aadee1.
  * Medling et al. (2018) Medling, A.M.; Cortese, L.; Croom, S.M.; Green, A.W.; Groves, B.; Hampton, E.; Ho, I.T.; Davies, L.J.M.; Kewley, L.J.; Moffett, A.J.; et al.  The SAMI Galaxy Survey: spatially resolving the main sequence of star formation.  Mon. Not. R. Astron. Soc. 2018, 475, 5194–5214.  https://doi.org/black10.1093/mnras/sty127.
  * Armus et al. (2009) Armus, L.; Mazzarella, J.M.; Evans, A.S.; Surace, J.A.; Sanders, D.B.; Iwasawa, K.; Frayer, D.T.; Howell, J.H.; Chan, B.; Petric, A.; et al.  GOALS: The Great Observatories All-Sky LIRG Survey.  Publ. Astron. Soc. Pac. 2009, 121, 559–576.  https://doi.org/black10.1086/600092.
  * Sanders et al. (2003) Sanders, D.B.; Mazzarella, J.M.; Kim, D.C.; Surace, J.A.; Soifer, B.T.  The IRAS Revised Bright Galaxy Sample.  Astron. J. 2003, 126, 1607–1664.  https://doi.org/black10.1086/376841.
  * Lyu and Rieke (2022) Lyu, J.; Rieke, G.  Infrared Spectral Energy Distribution and Variability of Active Galactic Nuclei: Clues to the Structure of Circumnuclear Material.  Universe 2022, 8, 304.  https://doi.org/black10.3390/universe8060304.
  * Sajina et al. (2022) Sajina, A.; Lacy, M.; Pope, A.  The Past and Future of Mid-Infrared Studies of AGN.  Universe 2022, 8, 356. https://doi.org/10.3390/universe8070356.
  * U et al. (2012) U, V.; Sanders, D.B.; Mazzarella, J.M.; Evans, A.S.; Howell, J.H.; Surace, J.A.; Armus, L.; Iwasawa, K.; Kim, D.C.; Casey, C.M.; et al.  SPECTRAL ENERGY DISTRIBUTIONS OF LOCAL LUMINOUS AND ULTRALUMINOUS INFRARED GALAXIES.  Astrophys. J. Suppl. Ser. 2012, 203, 9.  https://doi.org/black10.1088/0067-0049/203/1/9.
  * Paspaliaris et al. (2021) Paspaliaris, E.D.; Xilouris, E.M.; Nersesian, A.; Masoura, V.A.; Plionis, M.; Georgantopoulos, I.; Bianchi, S.; Katsioli, S.; Mountrichas, G.  The physical properties of local (U)LIRGs: A comparison with nearby early- and late-type galaxies.  Astron. Astrophys. 2021, 649, A137.  https://doi.org/black10.1051/0004-6361/202038605.
  * Ramos Padilla et al. (2020) Ramos Padilla, A.F.; Ashby, M.L.N.; Smith, H.A.; Martínez-Galarza, J.R.; Beverage, A.G.; Dietrich, J.; Higuera-G., M.A.; Weiner, A.S.  The AGN contribution to the UV-FIR luminosities of interacting galaxies and its role in identifying the main sequence.  Mon. Not. R. Astron. Soc. 2020, 499, 4325–4369.  https://doi.org/black10.1093/mnras/staa2813.
  * Medling et al. (2014) Medling, A.M.; U, V.; Guedes, J.; Max, C.E.; Mayer, L.; Armus, L.; Holden, B.; Roškar, R.; Sanders, D.  STELLAR AND GASEOUS NUCLEAR DISKS OBSERVED IN NEARBY (U)LIRGs.  Astrophys. J. 2014, 784, 70.  https://doi.org/black10.1088/0004-637X/784/1/70.
  * Linden et al. (2021) Linden, S.T.; Evans, A.S.; Larson, K.; Privon, G.C.; Armus, L.; Rich, J.; Díaz-Santos, T.; Murphy, E.J.; Song, Y.; Barcos-Muñoz, L.; et al.  Massive Star Cluster Formation and Destruction in Luminous Infrared Galaxies in GOALS. II. An ACS/WFC3 Survey of Nearby LIRGs.  Astrophys. J. 2021, 923, 278.  https://doi.org/black10.3847/1538-4357/ac2892.
  * Imanishi et al. (2008) Imanishi, M.; Nakagawa, T.; Ohyama, Y.; Shirahata, M.; Wada, T.; Onaka, T.; Oi, N.  Systematic Infrared 2.5–5 $\upmu$m Spectroscopy of Nearby Ultraluminous Infrared Galaxies with AKARI.  Publ. Astron. Soc. Jpn. 2008, 60, S489.  https://doi.org/black10.1093/pasj/60.sp2.S489.
  * Imanishi et al. (2010) Imanishi, M.; Nakagawa, T.; Shirahata, M.; Ohyama, Y.; Onaka, T.  AKARI IRC Infrared 2.5-5 $\upmu$m Spectroscopy of a Large Sample of Luminous Infrared Galaxies.  Astrophys. J. 2010, 721, 1233–1261.  https://doi.org/black10.1088/0004-637X/721/2/1233.
  * Lee et al. (2012) Lee, J.C.; Hwang, H.S.; Lee, M.G.; Kim, M.; Lee, J.H.  AKARI Near-infrared Spectroscopy of Luminous Infrared Galaxies.  Astrophys. J. 2012, 756, 95.  https://doi.org/black10.1088/0004-637X/756/1/95.
  * Mazzarella et al. (2012) Mazzarella, J.M.; Iwasawa, K.; Vavilkin, T.; Armus, L.; Kim, D.C.; Bothun, G.; Evans, A.S.; Spoon, H.W.W.; Haan, S.; Howell, J.H.; et al.  Investigation of Dual Active Nuclei, Outflows, Shock-heated Gas, and Young Star Clusters in Markarian 266.  Astron. J. 2012, 144, 125.  https://doi.org/black10.1088/0004-6256/144/5/125.
  * Teng and Veilleux (2010) Teng, S.H.; Veilleux, S.  X-QUEST: A Comprehensive X-ray Study of Local ULIRGS and QSOs.  Astrophys. J. 2010, 725, 1848–1876.  https://doi.org/black10.1088/0004-637X/725/2/1848.
  * Iwasawa et al. (2011) Iwasawa, K.; Sanders, D.B.; Teng, S.H.; U, V.; Armus, L.; Evans, A.S.; Howell, J.H.; Komossa, S.; Mazzarella, J.M.; Petric, A.O.; et al.  C-GOALS: Chandra observations of a complete sample of luminous infrared galaxies from the IRAS Revised Bright Galaxy Survey.  Astron. Astrophys. 2011, 529, A106.  https://doi.org/black10.1051/0004-6361/201015264.
  * Ricci et al. (2017) Ricci, C.; Bauer, F.E.; Treister, E.; Schawinski, K.; Privon, G.C.; Blecha, L.; Arevalo, P.; Armus, L.; Harrison, F.; Ho, L.C.; et al.  Growing supermassive black holes in the late stages of galaxy mergers are heavily obscured.  Mon. Not. R. Astron. Soc. 2017, _468_ , 1273–1299.  https://doi.org/black10.1093/mnras/stx173.
  * Iwasawa et al. (2009) Iwasawa, K.; Sanders, D.B.; Evans, A.S.; Mazzarella, J.M.; Armus, L.; Surace, J.A.  High-Ionization Fe K Emission From Luminous Infrared Galaxies.  Astrophys. J. Lett. 2009, 695, L103–L106.  https://doi.org/black10.1088/0004-637X/695/1/L103.
  * Iwasawa and Comastri (1998) Iwasawa, K.; Comastri, A.  ASCA spectroscopy of the luminous infrared galaxy NGC 6240: X-ray emission from a starburst and a buried active nucleus.  Mon. Not. R. Astron. Soc. 1998, 297, 1219–1226.  https://doi.org/black10.1046/j.1365-8711.1998.01588.x.
  * Vignati et al. (1999) Vignati, P.; Molendi, S.; Matt, G.; Guainazzi, M.; Antonelli, L.A.; Bassani, L.; Brandt, W.N.; Fabian, A.C.; Iwasawa, K.; Maiolino, R.; et al.  BeppoSAX unveils the nuclear component in NGC 6240.  Astron. Astrophys. 1999, 349, L57–L60.
  * Ikebe et al. (2000) Ikebe, Y.; Leighly, K.; Tanaka, Y.; Nakagawa, T.; Terashima, Y.; Komossa, S.  RXTE observation of NGC 6240: a search for the obscured active nucleus.  Mon. Not. R. Astron. Soc. 2000, 316, 433–441.  https://doi.org/black10.1046/j.1365-8711.2000.03514.x.
  * Komossa et al. (2003) Komossa, S.; Burwitz, V.; Hasinger, G.; Predehl, P.; Kaastra, J.S.; Ikebe, Y.  Discovery of a Binary Active Galactic Nucleus in the Ultraluminous Infrared Galaxy NGC 6240 Using Chandra.  Astrophys. J. Lett. 2003, 582, L15–L19.  https://doi.org/black10.1086/346145.
  * Imanishi et al. (2003) Imanishi, M.; Terashima, Y.; Anabuki, N.; Nakagawa, T.  X-ray Evidence of a Buried Active Galactic Nucleus in UGC 5101.  Astrophys. J. Lett. 2003, 596, L167–L170.  https://doi.org/black10.1086/379503.
  * Torres-Albà et al. (2018) Torres-Albà, N.; Iwasawa, K.; Díaz-Santos, T.; Charmandaris, V.; Ricci, C.; Chu, J.K.; Sanders, D.B.; Armus, L.; Barcos-Muñoz, L.; Evans, A.S.; et al.  C-GOALS. II. Chandra observations of the lower luminosity sample of nearby luminous infrared galaxies in GOALS.  Astron. Astrophys. 2018, 620, A140.  https://doi.org/black10.1051/0004-6361/201834105.
  * Pereira-Santaella et al. (2011) Pereira-Santaella, M.; Alonso-Herrero, A.; Santos-Lleo, M.; Colina, L.; Jiménez-Bailón, E.; Longinotti, A.L.; Rieke, G.H.; Ward, M.; Esquej, P.  The X-ray emission of local luminous infrared galaxies.  Astron. Astrophys. 2011, 535, A93.  https://doi.org/black10.1051/0004-6361/201117420.
  * Capelo et al. (2017) Capelo, P.R.; Dotti, M.; Volonteri, M.; Mayer, L.; Bellovary, J.M.; Shen, S.  A survey of dual active galactic nuclei in simulations of galaxy mergers: frequency and properties.  Mon. Not. R. Astron. Soc. 2017, 469, 4437–4454.  https://doi.org/black10.1093/mnras/stx1067.
  * Koss et al. (2012) Koss, M.; Mushotzky, R.; Treister, E.; Veilleux, S.; Vasudevan, R.; Trippe, M.  Understanding Dual Active Galactic Nucleus Activation in the nearby Universe.  Astrophys. J. Lett. 2012, 746, L22.  https://doi.org/black10.1088/2041-8205/746/2/L22.
  * Ellison et al. (2011) Ellison, S.L.; Patton, D.R.; Mendel, J.T.; Scudder, J.M.  Galaxy pairs in the Sloan Digital Sky Survey—IV. Interactions trigger active galactic nuclei.  Mon. Not. R. Astron. Soc. 2011, 418, 2043–2053.  https://doi.org/black10.1111/j.1365-2966.2011.19624.x.
  * Satyapal et al. (2017) Satyapal, S.; Secrest, N.J.; Ricci, C.; Ellison, S.L.; Rothberg, B.; Blecha, L.; Constantin, A.; Gliozzi, M.; McNulty, P.; Ferguson, J.  Buried AGNs in Advanced Mergers: Mid-infrared Color Selection as a Dual AGN Candidate Finder.  Astrophys. J. 2017, 848, 126.  https://doi.org/black10.3847/1538-4357/aa88ca.
  * Harrison et al. (2013) Harrison, F.A.; Craig, W.W.; Christensen, F.E.; Hailey, C.J.; Zhang, W.W.; Boggs, S.E.; Stern, D.; Cook, W.R.; Forster, K.; Giommi, P.; et al.  The Nuclear Spectroscopic Telescope Array (NuSTAR) High-energy X-ray Mission.  Astrophys. J. 2013, 770, 103.  https://doi.org/black10.1088/0004-637X/770/2/103.
  * Lutz et al. (2004) Lutz, D.; Maiolino, R.; Spoon, H.W.W.; Moorwood, A.F.M.  The relation between AGN hard X-ray emission and mid-infrared continuum from ISO spectra: Scatter and unification aspects.  Astron. Astrophys. 2004, 418, 465–473.  https://doi.org/black10.1051/0004-6361:20035838.
  * Bauer et al. (2010) Bauer, F.E.; Yan, L.; Sajina, A.; Alexander, D.M.  X-ray Constraints on the Active Galactic Nuclei Properties in Spitzer-Infrared Spectrograph Identified z $\sim$ 2 Ultraluminous Infrared Galaxies.  Astrophys. J. 2010, 710, 212–226.  https://doi.org/black10.1088/0004-637X/710/1/212.
  * Ricci et al. (2021) Ricci, C.; Privon, G.C.; Pfeifle, R.W.; Armus, L.; Iwasawa, K.; Torres-Albà, N.; Satyapal, S.; Bauer, F.E.; Treister, E.; Ho, L.C.; et al.  A hard X-ray view of luminous and ultra-luminous infrared galaxies in GOALS—I. AGN obscuration along the merger sequence.  Mon. Not. R. Astron. Soc. 2021, 506, 5935–5950.  https://doi.org/black10.1093/mnras/stab2052.
  * Blecha et al. (2018) Blecha, L.; Snyder, G.F.; Satyapal, S.; Ellison, S.L.  The power of infrared AGN selection in mergers: a theoretical study.  Mon. Not. R. Astron. Soc. 2018, 478, 3056–3071.  https://doi.org/black10.1093/mnras/sty1274.
  * Ramos Almeida and Ricci (2017) Ramos Almeida, C.; Ricci, C.  Nuclear obscuration in active galactic nuclei.  Nat. Astron. 2017, 1, 679–689. https://doi.org/10.1038/s41550-017-0232-z.
  * Hickox and Alexander (2018) Hickox, R.C.; Alexander, D.M.  Obscured Active Galactic Nuclei.  Annu. Rev. Astron. Astrophys. 2018, 56, 625–671. https://doi.org/black10.1146/annurev-astro-081817-051803.
  * Ricci et al. (2015) Ricci, C.; Ueda, Y.; Koss, M.J.; Trakhtenbrot, B.; Bauer, F.E.; Gandhi, P.  Compton-thick Accretion in the Local Universe.  Astrophys. J. Lett. 2015, 815, L13.  https://doi.org/black10.1088/2041-8205/815/1/L13.
  * Ricci et al. (2017) Ricci, C.; Trakhtenbrot, B.; Koss, M.J.; Ueda, Y.; Del Vecchio, I.; Treister, E.; Schawinski, K.; Paltani, S.; Oh, K.; Lamperti, I.; et al.  BAT AGN Spectroscopic Survey. V. X-ray Properties of the Swift/BAT 70-month AGN Catalog.  Astrophys. J. Suppl. Ser. 2017, 233, 17.  https://doi.org/black10.3847/1538-4365/aa96ad.
  * Baldwin et al. (1981) Baldwin, J.A.; Phillips, M.M.; Terlevich, R.  Classification parameters for the emission-line spectra of extragalactic objects.  Publ. Astron. Soc. Pac. 1981, 93, 5–19.  https://doi.org/black10.1086/130766.
  * Veilleux and Osterbrock (1987) Veilleux, S.; Osterbrock, D.E.  Spectral Classification of Emission-Line Galaxies.  Astrophys. J. Suppl. Ser. 1987, 63, 295.  https://doi.org/black10.1086/191166.
  * Kewley et al. (2019) Kewley, L.J.; Nicholls, D.C.; Sutherland, R.S.  Understanding Galaxy Evolution Through Emission Lines.  Annu. Rev. Astron. Astrophys. 2019, 57, 511–570.  https://doi.org/black10.1146/annurev-astro-081817-051832.
  * Kewley et al. (2001) Kewley, L.J.; Dopita, M.A.; Sutherland, R.S.; Heisler, C.A.; Trevena, J.  Theoretical Modeling of Starburst Galaxies.  Astrophys. J. 2001, 556, 121–140.  https://doi.org/black10.1086/321545.
  * Kauffmann et al. (2003) Kauffmann, G.; Heckman, T.M.; Tremonti, C.; Brinchmann, J.; Charlot, S.; White, S.D.M.; Ridgway, S.E.; Brinkmann, J.; Fukugita, M.; Hall, P.B.; et al.  The host galaxies of active galactic nuclei.  Mon. Not. R. Astron. Soc. 2003, 346, 1055–1077.  https://doi.org/black10.1111/j.1365-2966.2003.07154.x.
  * Kewley et al. (2006) Kewley, L.J.; Groves, B.; Kauffmann, G.; Heckman, T.  The host galaxies and classification of active galactic nuclei.  Mon. Not. R. Astron. Soc. 2006, 372, 961–976.  https://doi.org/black10.1111/j.1365-2966.2006.10859.x.
  * Veilleux et al. (1999) Veilleux, S.; Kim, D.C.; Sanders, D.B.  Optical Spectroscopy of the IRAS 1 JY Sample of Ultraluminous Infrared Galaxies.  Astrophys. J. 1999, 522, 113–138.  https://doi.org/black10.1086/307634.
  * Veilleux et al. (1995) Veilleux, S.; Kim, D.C.; Sanders, D.B.; Mazzarella, J.M.; Soifer, B.T.  Optical Spectroscopy of Luminous Infrared Galaxies. II. Analysis of the Nuclear and Long-Slit Data.  Astrophys. J. Suppl. 1995, 98, 171.  https://doi.org/black10.1086/192158.
  * Yuan et al. (2010) Yuan, T.T.; Kewley, L.J.; Sanders, D.B.  The Role of Starburst-Active Galactic Nucleus Composites in Luminous Infrared Galaxy Mergers: Insights from the New Optical Classification Scheme.  Astrophys. J. 2010, 709, 884–911.  https://doi.org/black10.1088/0004-637X/709/2/884.
  * Rich et al. (2011) Rich, J.A.; Kewley, L.J.; Dopita, M.A.  Galaxy-wide Shocks in Late-merger Stage Luminous Infrared Galaxies.  Astrophys. J. 2011, 734, 87.  https://doi.org/black10.1088/0004-637X/734/2/87.
  * Rich et al. (2014) Rich, J.A.; Kewley, L.J.; Dopita, M.A.  Composite Spectra in Merging U/LIRGs Caused by Shocks.  Astrophys. J. Lett. 2014, 781, L12.  https://doi.org/black10.1088/2041-8205/781/1/L12.
  * Rich et al. (2015) Rich, J.A.; Kewley, L.J.; Dopita, M.A.  Galaxy Mergers Drive Shocks: An Integral Field Study of GOALS Galaxies.  Astrophys. J. Suppl. Ser. 2015, 221, 28.  https://doi.org/black10.1088/0067-0049/221/2/28.
  * Alonso-Herrero et al. (2009) Alonso-Herrero, A.; García-Marín, M.; Monreal-Ibero, A.; Colina, L.; Arribas, S.; Alfonso-Garzón, J.; Labiano, A.  PMAS optical integral field spectroscopy of luminous infrared galaxies. I. The atlas.  Astron. Astrophys. 2009, 506, 1541–1562.  https://doi.org/black10.1051/0004-6361/200911813.
  * Alonso-Herrero et al. (2010) Alonso-Herrero, A.; García-Marín, M.; Rodríguez Zaurín, J.; Monreal-Ibero, A.; Colina, L.; Arribas, S.  PMAS optical integral field spectroscopy of luminous infrared galaxies. II. Spatially resolved stellar populations and excitation conditions.  Astron. Astrophys. 2010, 522, A7.  https://doi.org/black10.1051/0004-6361/201014746.
  * García-Marín et al. (2006) García-Marín, M.; Colina, L.; Arribas, S.; Alonso-Herrero, A.; Mediavilla, E.  Integral Field Spectroscopy of the Luminous Infrared Galaxy Arp 299 (IC 694 + NGC 3690).  Astrophys. J. 2006, 650, 850–871.  https://doi.org/black10.1086/507411.
  * Riffel et al. (2013) Riffel, R.; Rodríguez-Ardila, A.; Aleman, I.; Brotherton, M.S.; Pastoriza, M.G.; Bonatto, C.; Dors, O.L.  Molecular hydrogen and [Fe II] in active galactic nuclei—III. Low-ionization nuclear emission-line region and star-forming galaxies.  Mon. Not. R. Astron. Soc. 2013, 430, 2002–2017.  https://doi.org/black10.1093/mnras/stt026.
  * Riffel et al. (2021) Riffel, R.A.; Bianchin, M.; Riffel, R.; Storchi-Bergmann, T.; Schönell, A.J.; Dahmer-Hahn, L.G.; Dametto, N.Z.; Diniz, M.R.  Gemini NIFS survey of feeding and feedback in nearby active galaxies—IV. Excitation.  Mon. Not. R. Astron. Soc. 2021, 503, 5161–5178.  https://doi.org/black10.1093/mnras/stab788.
  * Colina et al. (2015) Colina, L.; Piqueras López, J.; Arribas, S.; Riffel, R.; Riffel, R.A.; Rodriguez-Ardila, A.; Pastoriza, M.; Storchi-Bergmann, T.; Alonso-Herrero, A.; Sales, D.  Understanding the two-dimensional ionization structure in luminous infrared galaxies. A near-IR integral field spectroscopy perspective.  Astron. Astrophys. 2015, 578, A48.  https://doi.org/black10.1051/0004-6361/201425567.
  * U et al. (2019) U, V.; Medling, A.M.; Inami, H.; Armus, L.; Díaz-Santos, T.; Charmandaris, V.; Howell, J.; Stierwalt, S.; Privon, G.C.; Linden, S.T.; et al.  Keck OSIRIS AO LIRG Analysis (KOALA): Feedback in the Nuclei of Luminous Infrared Galaxies.  Astrophys. J. 2019, 871, 166.  https://doi.org/black10.3847/1538-4357/aaf1c2.
  * Mattila et al. (2007) Mattila, S.; Väisänen, P.; Farrah, D.; Efstathiou, A.; Meikle, W.P.S.; Dahlen, T.; Fransson, C.; Lira, P.; Lundqvist, P.; Östlin, G.; et al.  Adaptive Optics Discovery of Supernova 2004ip in the Nuclear Regions of the Luminous Infrared Galaxy IRAS 18293-3413.  Astrophys. J. Lett. 2007, 659, L9–L12.  https://doi.org/black10.1086/516821.
  * Väisänen et al. (2017) Väisänen, P.; Reunanen, J.; Kotilainen, J.; Mattila, S.; Johansson, P.H.; Ramphul, R.; Romero-Cañizales, C.; Kuncarayakti, H.  Shutting down or powering up a (U)LIRG? Merger components in distinctly different evolutionary states in IRAS 19115-2124 (the Bird).  Mon. Not. R. Astron. Soc. 2017, 471, 2059–2076.  https://doi.org/black10.1093/mnras/stx1685.
  * Alonso-Herrero et al. (2006) Alonso-Herrero, A.; Rieke, G.H.; Rieke, M.J.; Colina, L.; Pérez-González, P.G.; Ryder, S.D.  Near-Infrared and Star-forming Properties of Local Luminous Infrared Galaxies.  Astrophys. J. 2006, 650, 835–849.  https://doi.org/black10.1086/506958.
  * Larkin et al. (1998) Larkin, J.E.; Armus, L.; Knop, R.A.; Soifer, B.T.; Matthews, K.  A Near-Infrared Spectroscopic Survey of LINER Galaxies.  Astrophys. J. Suppl. Ser. 1998, 114, 59–72.  https://doi.org/black10.1086/313063.
  * Rodríguez-Ardila et al. (2004) Rodríguez-Ardila, A.; Pastoriza, M.G.; Viegas, S.; Sigut, T.A.A.; Pradhan, A.K.  Molecular hydrogen and [Fe II] in Active Galactic Nuclei.  Astron. Astrophys. 2004, 425, 457–474.  https://doi.org/black10.1051/0004-6361:20034285.
  * Rodríguez-Ardila et al. (2005) Rodríguez-Ardila, A.; Riffel, R.; Pastoriza, M.G.  Molecular hydrogen and [FeII] in active galactic nuclei—II. Results for Seyfert 2 galaxies.  Mon. Not. R. Astron. Soc. 2005, 364, 1041–1053.  https://doi.org/black10.1111/j.1365-2966.2005.09638.x.
  * Riffel et al. (2006) Riffel, R.; Rodríguez-Ardila, A.; Pastoriza, M.G.  A 0.8–2.4 $\upmu$m spectral atlas of active galactic nuclei.  Astron. Astrophys. 2006, 457, 61–70.  https://doi.org/black10.1051/0004-6361:20065291.
  * Müller Sánchez et al. (2006) Müller Sánchez, F.; Davies, R.I.; Eisenhauer, F.; Tacconi, L.J.; Genzel, R.; Sternberg, A.  SINFONI adaptive optics integral field spectroscopy of the Circinus Galaxy.  Astron. Astrophys. 2006, 454, 481–492.  https://doi.org/black10.1051/0004-6361:20054387.
  * Müller-Sánchez et al. (2011) Müller-Sánchez, F.; Prieto, M.A.; Hicks, E.K.S.; Vives-Arias, H.; Davies, R.I.; Malkan, M.; Tacconi, L.J.; Genzel, R.  Outflows from Active Galactic Nuclei: Kinematics of the Narrow-line and Coronal-line Regions in Seyfert Galaxies.  Astrophys. J. 2011, 739, 69.  https://doi.org/black10.1088/0004-637X/739/2/69.
  * Piqueras López et al. (2012) Piqueras López, J.; Colina, L.; Arribas, S.; Alonso-Herrero, A.; Bedregal, A.G.  VLT-SINFONI integral field spectroscopy of low-z luminous and ultraluminous infrared galaxies. I. Atlas of the 2D gas structure.  Astron. Astrophys. 2012, 546, A64.  https://doi.org/black10.1051/0004-6361/201219372.
  * Bohn et al. (2021) Bohn, T.; Canalizo, G.; Veilleux, S.; Liu, W.  Near-infrared Coronal Line Observations of Dwarf Galaxies Hosting AGN-driven Outflows.  Astrophys. J. 2021, 911, 70.  https://doi.org/black10.3847/1538-4357/abe70c.
  * Iwasawa et al. (2011) Iwasawa, K.; Mazzarella, J.M.; Surace, J.A.; Sanders, D.B.; Armus, L.; Evans, A.S.; Howell, J.H.; Komossa, S.; Petric, A.; Teng, S.H.; et al.  The location of an active nucleus and a shadow of a tidal tail in the ULIRG Mrk 273.  Astron. Astrophys. 2011, 528, A137.  https://doi.org/black10.1051/0004-6361/201015872.