Research Summary

In the News:

"Song of the Universe" (TEDx talk)
"Supermassive Black Holes Explained" (PhD Comics)
"Through the Wormhole with Morgan Freeman" (The Science Channel)

AGN Feedback:
"Researchers Spot Massive Black Hole In Double Burp" (NPR)
"Huge Black Hole Blasts Out Double Burp" (BBC)
"Giant Black Hole Seen Flickering On and Off after Galaxy Snack" (New Scientist)
"Scientists Observe Rare Supermassive Black Hole Double Belch" (Time)
"How Does a Black Hole Burp? Bleching Supermassive Black Hole Has Big Implications for Physics" (Newsweek)

AGN Feeding:
"Cataclysm Hunters: The Search for Monster Black-Hole Collisions" (
"This Galaxy Has Two Black Holes - And One of Them Is Very Strange" (Washington Post)
"Stripped Black Hole Could Be a Rarely Seen Phenomenon, Study Says" (LA Times)
"Rarely Seen Pair of Black Holes Discovered, One Unlike The Other" (NPR's Here & Now)
"Galaxy Found That Plays Host to Rare Combo of Two Black Holes" (Mashable)
"A Black Hole Dance Party" (U.S. News and World Report)
"Black-Hole Bonanza" (Sky and Telescope)
"Elusive Supermassive-Black-Hole Mergers Finally Found" (Wired)
"Waltzing Quasars Provide Signpost to Merging Galaxies" (Science magazine)

Research Results

Toward a More Complete Optical Census of Active Galactic Nuclei via Spatially Resolved Spectroscopy
See Comerford et al. 2022, ApJ, 927, 23

While emission line flux ratio diagnostics are the most common technique for identifying active galactic nuclei (AGNs) in optical spectra, applying this approach to single-fiber spectra of galaxies can omit entire subpopulations of AGNs. Here, we use spatially resolved spectroscopy from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey to construct a sample of 10 galaxies where Baldwin-Philips-Terlevich line flux ratio diagnostics classify each galaxy's central 3'' spectrum as LINER or star-forming, while more than 10 percent of the spaxels in the galaxy's MaNGA footprint are classified as Seyfert. We obtain Chandra observations of these 10 galaxies with off-nuclear Seyfert regions to determine whether AGNs are actually present in them. Our main result is that 7-10 (depending on strictness of criteria) of the galaxies host one or more X-ray AGNs, even though none of them were classified as AGNs based on their single-fiber optical spectra. We find that these AGNs were not identified in the single-fiber spectra because they are AGNs in the nuclei of companion galaxies, low-luminosity AGNs, dust-obscured AGNs, and/or flickering AGNs. In summary, we find that off-nuclear AGN signatures may increase the number of known AGNs by a factor of two over what conventional single nuclear fiber spectra identify. Our results show that spatially resolved spectroscopy can be leveraged to reveal a more complete census of AGNs that are traditionally missed by single-fiber spectra.


A Catalog of 406 AGNs in MaNGA: A Connection between Radio-mode AGN and Star Formation Quenching
See Comerford et al. 2020, ApJ, 901, 159

Accurate active galactic nucleus (AGN) identifications and spatially resolved host galaxy properties are a powerful combination for studies of the role of AGNs and AGN feedback in the coevolution of galaxies and their central supermassive black holes. Here, we present robust identifications of 406 AGNs in the first 6261 galaxies observed by the integral field spectroscopy survey Mapping Nearby Galaxies at Apache Point Observatory (MaNGA). Instead of using optical line flux ratios, which can be difficult to interpret in light of the effects of shocks and metallicity, we identify the AGNs via mid-infrared WISE colors, Swift/BAT ultra hard X-ray detections, NVSS and FIRST radio observations, and broad emission lines in SDSS spectra. We subdivide the AGNs into radio-quiet and radio-mode AGNs, and examine the correlations of the AGN classes with host galaxy star formation rates and stellar populations. When compared to the radio-quiet AGN host galaxies, we find that the radio-mode AGN host galaxies reside preferentially in elliptical galaxies, lie further beneath the star-forming main sequence (with lower star formation rates at fixed galaxy mass), have older stellar populations, and have more negative stellar age gradients with galactocentric distance (indicating inside-out quenching of star formation). These results establish a connection between radio-mode AGNs and the suppression of star formation.


An Active Galactic Nucleus Caught in the Act of Turning Off and On
See Comerford, Barrows, Muller-Sanchez, Nevin, Greene, Pooley, Stern, & Harrison, 2017, ApJ, 849,102

We present the discovery of an active galactic nucleus (AGN) that is turning off and then on again in the z=0.06 galaxy SDSS J1354+1327. This episodic nuclear activity is the result of discrete accretion events, which could have been triggered by a past interaction with the companion galaxy that is currently located 12.5 kpc away. We originally targeted SDSS J1354+1327 because its Sloan Digital Sky Survey spectrum has narrow AGN emission lines that exhibit a velocity offset of 69 km/s relative to systemic. To determine the nature of the galaxy and its velocity-offset emission lines, we observed SDSS J1354+1327 with Chandra/ACIS, Hubble Space Telescope/Wide Field Camera 3, Apache Point Observatory optical longslit spectroscopy, and Keck/OSIRIS integral-field spectroscopy.

We find a ~10 kpc cone of photoionized gas south of the galaxy center and a ~1 kpc semi-spherical front of shocked gas, which is responsible for the velocity offset in the emission lines, north of the galaxy center. We interpret these two outflows as the result of two separate AGN accretion events; the first AGN outburst created the southern outflow, and then <10^5 yrs later the second AGN outburst launched the northern shock front. SDSS J1354+1327 is the galaxy with the strongest evidence for an AGN that has turned off and then on again, and it fits into the broader context of AGN flickering that includes observations of AGN light echoes.


Shocks and Spatially Offset Active Galactic Nuclei Produce Velocity Offsets in Emission Lines
See Comerford, Barrows, Greene, & Pooley, 2017, ApJ, 847, 41

While 2% of active galactic nuclei (AGNs) exhibit narrow emission lines with line-of-sight velocities that are significantly offset from the velocity of the host galaxy's stars, the nature of these velocity offsets is unknown. We investigate this question with Chandra/ACIS and Hubble Space Telescope/Wide Field Camera 3 observations of seven velocity-offset AGNs at z<0.12, and all seven galaxies have a central AGN but a peak in emission that is spatially offset by < kpc from the host galaxy's stellar centroid. These spatial offsets are responsible for the observed velocity offsets and are due to shocks, either from AGN outflows (in four galaxies) or gas inflowing along a bar (in three galaxies).

We compare our results to a velocity-offset AGN whose velocity offset originates from a spatially offset AGN in a galaxy merger. The optical line flux ratios of the offset AGN are consistent with pure photoionization, while the optical line flux ratios of our sample are consistent with contributions from photoionization and shocks. We conclude that these optical line flux ratios could be efficient for separating velocity-offset AGNs into subgroups of offset AGNs -- which are important for studies of AGN fueling in galaxy mergers -- and central AGNs with shocks -- where the outflows are biased towards the most energetic outflows that are the strongest drivers of feedback.


Merger-driven Fueling of Active Galactic Nuclei: Six Dual and Offset Active Galactic Nuclei Discovered with Chandra and Hubble Space Telescope Observations
See Comerford, Pooley, Barrows, Greene, Zakamska, Madejski, & Cooper, 2015, ApJ, 806, 219

Dual active galactic nuclei (AGNs) and offset AGNs are kpc-scale separation supermassive black holes pairs created during galaxy mergers, where both or one of the black holes are AGNs, respectively. These dual and offset AGNs are valuable probes of the link between mergers and AGNs but are challenging to identify. Here we present Chandra/ACIS observations of 12 optically-selected dual AGN candidates at z<0.34, where we use the X-rays to identify AGNs. We also present HST/WFC3 observations of 10 of these candidates, which reveal any stellar bulges accompanying the AGNs.

We discover a dual AGN system with a separation of 2.2 kpc, where the two stellar bulges have coincident [O III] and X-ray sources. This system is an extremely minor merger (460:1) that may include a dwarf galaxy hosting an intermediate mass black hole. We also find six single AGNs, and five systems that are either dual or offset AGNs with separations <10 kpc. Four of the six dual AGNs and dual/offset AGNs are in ongoing major mergers, and these AGNs are 10 times more luminous, on average, than the single AGNs in our sample. This hints that major mergers may preferentially trigger higher luminosity AGNs. Further, we find that confirmed dual AGNs have hard X-ray luminosities that are half of those of single AGNs at fixed \oiiiw luminosity, on average. This could be explained by high densities of gas funneled to galaxy centers during mergers, and emphasizes the need for deeper X-ray observations of dual AGN candidates.


Offset Active Galactic Nuclei as Tracers of Galaxy Mergers and Supermassive Black Hole Growth
See Comerford & Greene, 2014, ApJ, 789, 112

Offset active galactic nuclei (AGNs) are AGNs that are in ongoing galaxy mergers, which produce kinematic offsets in the AGNs relative to their host galaxies. Offset AGNs are also close relatives of dual AGNs. We conduct a systematic search for offset AGNs in the Sloan Digital Sky Survey, by selecting AGN emission lines that exhibit statistically significant line-of-sight velocity offsets relative to systemic. From a parent sample of 18314 Type 2 AGNs at z<0.21, we identify 351 offset AGN candidates with velocity offsets of 50 km/s < |v| < 410 km/s. When we account for projection effects in the observed velocities, we estimate that 4% - 8% of AGNs are offset AGNs. We designed our selection criteria to bypass velocity offsets produced by rotating gas disks, AGN outflows, and gravitational recoil of supermassive black holes, but follow-up observations are still required to confirm our candidates as offset AGNs.

We find that the fraction of AGNs that are offset candidates increases with AGN bolometric luminosity, from 0.7% to 6% over the luminosity range 43 < log(Lbol) [erg/s] < 46. If these candidates are shown to be bona fide offset AGNs, then this would be direct observational evidence that galaxy mergers preferentially trigger high-luminosity AGNs. Finally, we find that the fraction of AGNs that are offset AGN candidates increases from 1.9% at z=0.1 to 32% at z=0.7, in step with the growth in the galaxy merger fraction over the same redshift range.


Dual Supermassive Black Hole Candidates in the AGN and Galaxy Evolution Survey
See Comerford et al. 2013, ApJ, 777, 64

Dual supermassive black holes (SMBHs) with kpc-scale separations are a natural consequence of galaxy mergers. Galaxy mergers may trigger inflows of gas that fuel one or both of the SMBHs as active galactic nuclei (AGNs), in systems known as offset AGNs and dual AGNs, respectively. Dual AGNs may produce double-peaked narrow AGN emission lines, while offset AGNs may produce single-peaked narrow AGN emission lines with line-of-sight velocity offsets relative to the host galaxy. We search for such dual and offset systems among 173 Type 2 AGNs at z<0.37 in the AGN and Galaxy Evolution Survey (AGES), and we find two double-peaked AGNs and five offset AGN candidates. When we compare these results to a similar search of the DEEP2 Galaxy Redshift Survey and match the two samples in color, absolute magnitude, and minimum velocity offset, we find that the fraction of AGNs that are dual SMBH candidates increases from z=0.25 to z=0.7 by a factor of ~6 (from 2/70 to 16/91, or 2.9% to 18%). This may be associated with the rise in the galaxy merger fraction over the same cosmic time.

As further evidence for a link with galaxy mergers, the AGES offset and dual AGN candidates are tentatively ~3 times more likely than the overall AGN population to reside in a host galaxy that has a companion galaxy (from 16/173 to 2/7, or 9% to 29%). Follow-up observations of the seven offset and dual AGN candidates in AGES will definitively distinguish velocity offsets produced by dual SMBHs from those produced by narrow-line region kinematics, and will help sharpen our observational approach to detecting dual SMBHs.


Kiloparsec-scale Spatial Offsets in Double-peaked Narrow-line AGNs. I. Markers for Selection of Compelling Dual AGN Candidates
See Comerford et al. 2012, ApJ, 753, 42

Galaxies with double-peaked narrow AGN emission lines in the Sloan Digital Sky Survey are plausible candidates for kpc-scale separation dual AGN, but their double-peaked profiles could also be the result of gas kinematics or AGN-driven outflows and jets on small or large scales. To help distinguish between these scenarios, we obtained spatial profiles of the AGN emission via follow-up longslit spectroscopy of 81 double-peaked narrow-line AGNs in SDSS at 0.03 < z < 0.36 using Lick, Palomar, and MMT Observatories.

We find that all 81 systems exhibit double AGN emission components with kpc projected spatial separations on the sky (0.2 kpc < x < 5.5 kpc; median x=1.1 kpc), which suggests that they are produced by kpc-scale dual AGNs or kpc-scale outflows, jets, or rotating gaseous disks. Further, we find that the subsample (58%) of the objects with spatially-compact emission components may be preferentially produced by dual AGNs, while the subsample (42%) with spatially-extended emission components may be preferentially produced by AGN outflows. We also show that for 32% of the sample the two AGN emission components are preferentially aligned with the host galaxy major axis, as expected for dual AGNs orbiting in the host galaxy potential. Our results both narrow the list of possible physical mechanisms producing the double AGN components, and suggest several observational criteria for selecting the most promising dual AGN candidates from the full sample of double-peaked narrow-line AGNs. Using these criteria, we determine the 17 most compelling dual AGN candidates in our sample.


Chandra Observations of a 1.9 kpc Separation Double X-ray Source in a Candidate Dual AGN Galaxy at z=0.16
See Comerford et al. 2011, ApJ, 737, L19

The z=0.1569 galaxy SDSS J171544.05+600835.7 was initially identified as a candidate for dual active galactic nuclei (AGN) by the double-peaked AGN emission lines in its Sloan Digital Sky Survey spectrum. These double peaks have a line-of-sight velocity separation of 350 km/s, and our follow-up longslit spectra with the Kast Spectrograph at Lick Observatory show that the two AGN emission components also have a projected spatial offset of 1.9 kpc on the sky.

Chandra/ACIS observations support the presence of dual AGN, by revealing two X-ray sources with the same spatial offset and orientation as the optical emission. Based on the optical and X-ray observations, we conclude that SDSS J171544.05+600835.7 most likely hosts Compton-thick dual AGN. This object is a proof of concept for a new, systematic detection method that selects promising dual AGN candidates from ground-based spectroscopy that exhibits both velocity and spatial offsets in the AGN emission features.


1.75 h-1 kpc Separation Dual Active Galactic Nuclei at z=0.36 in the COSMOS Field
See Comerford et al. 2009, ApJ, 702, L82 

While visually inspecting postage stamp images of COSMOS galaxies, we serendipitously identified the z=0.36 galaxy COSMOS J100043.15+020637.2 as a candidate for dual active galactic nuclei (AGN).  The Hubble Space Telescope Advanced Camera for Surveys (HST/ACS) image of the galaxy shows a tidal tail indicating the galaxy recently underwent a merger, as well as two bright nuclei, separated by 1.75 h-1 kpc, near the galaxy's center.  The two bright nuclei are candidate dual AGN, and although existing observations from XMM-Newton, Very Large Array, and Spitzer confirm the galaxy is active, they cannot spatially resolve whether the galaxy hosts one AGN or two.

To confirm whether the two nuclei correspond to dual AGN we conducted follow-up Keck/DEIMOS slit spectroscopy of the galaxy, which revealed two spatially-separated emission components coinciding with the two nuclei.  The line flux ratios confirmed that each emission component was indeed an AGN.  Our finding indicates that COSMOS J100043.15+020637.2 is a merger-remnant galaxy with dual AGN that have a 150 km/s line-of-sight velocity separation and a 1.75 h-1 kpc projected spatial separation.  We have demonstrated that dual AGN candidates can be selected as active galaxies, as identified by multiwavelength observations, with bright double sources in HST/ACS imaging, and confirmed through follow-up optical spectroscopy.


Inspiralling Supermassive Black Holes: A New Signpost for Galaxy Mergers
See Comerford et al. 2009, ApJ, 698, 956

Galaxies grow hierarchically through mergers and nearly every galaxy hosts a central supermassive black hole (SMBH), which implies that a merger between two galaxies can bring two SMBHs to the merger-remnant galaxy. Dynamical friction causes the SMBHs to inspiral toward the center of the remnant, spending ~100 Myr at separations >1 kpc. Hierarchical structure formation thus implies that some galaxies should harbor two SMBHs near their centers as the result of a recent merger, and if sufficient gas is available for accretion one or both of the SMBHs may power AGN.

We pioneered a new method of observationally identifying galaxy mergers used this technique to determine both the fraction of AGN hosted by red galaxy mergers and the red galaxy merger rate. We accomplished this by searching the DEEP2 Galaxy Redshift Survey dataset for galaxies that host two SMBHs as the result of a recent merger, where both SMBHs are moving relative to the host galaxy and have not yet coalesced and settled at the bottom of the galaxy's potential well. If one or both SMBHs are powering AGN, such cases are identifiable by AGN-fueled [O III] emission lines that have a different redshift than the redshift of the galaxy's stellar absorption lines. A redshift difference corresponds to a velocity offset. We call objects ``offset AGN'' when one SMBH powers an AGN and the galaxy spectrum exhibits one set of offset [O III] emission lines, and ``dual AGN'' are objects where both SMBHs power AGN and the galaxy spectrum exhibits two sets of offset [O III] emission lines. Within the set of DEEP2 red galaxies at 0.3 < z < 0.8, we discovered 30 offset AGN and 2 dual AGN with statistically significant velocity offsets, ranging from ~50 km/s to ~300 km/s. After careful consideration of physical effects such as AGN outflows that could cause such offsets, we found that the most plausible explanation was an AGN inspiralling within the host galaxy as the result of a merger.

Using our sample of offset and dual AGN, we measured both the fraction of AGN found in merger-remnant galaxies and the galaxy merger rate. Of the DEEP2 red galaxies hosting AGN, we found that half of the AGN exhibit velocity offsets due to a recent merger. This striking result, that at least half of red galaxies hosting AGN are also merger remnants, suggests a strong link between AGN activity and red galaxy mergers. If late-type galaxy mergers trigger gas inflows that fuel AGN, as suggested by simulations, then it may be that red galaxy mergers also have sufficient gas to fuel AGN in a similar way. We also used our sample of offset and dual AGN to measure a galaxy merger rate of ~3 mergers/Gyr for DEEP2 red galaxies at 0.3 < z < 0.8. This merger rate includes both minor and major mergers.