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"Through the Wormhole with Morgan Freeman" (The Science Channel, 2010)
Dual Supermassive Black Holes
Kiloparsec-scale Spatial Offsets in Double-peaked Narrow-line AGNs. I. Markers for Selection of Compelling Dual AGN Candidates
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
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
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
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.