3,303 research outputs found
Offset Active Galactic Nuclei as Tracers of Galaxy Mergers and Supermassive Black Hole Growth
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(L_bol) [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.Comment: 14 pages, 14 figures, accepted for publication in Ap
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Chandra Observations of A 1.9 Kpc Separation Double X-Ray Source in A Candidate Dual Active Galactic Nucleus Galaxy At Z=0.16
We report Chandra observations of a double X-ray source in the z = 0.1569 galaxy SDSS J171544.05+600835.7. The galaxy was initially identified as a dual active galactic nucleus (AGN) candidate based on the double-peaked [O III] lambda 5007 emission lines, with a line-of-sight velocity separation of 350 km s(-1), in its Sloan Digital Sky Survey spectrum. We used the Kast Spectrograph at Lick Observatory to obtain two long-slit spectra of the galaxy at two different position angles, which reveal that the two Type 2 AGN emission components have not only a velocity offset, but also a projected spatial offset of 1.9 h(70)(-1) kpc on the sky. Chandra/ACIS observations of two X-ray sources with the same spatial offset and orientation as the optical emission suggest that the galaxy most likely contains Compton-thick dual AGNs, although the observations could also be explained by AGN jets. Deeper X-ray observations that reveal Fe K lines, if present, would distinguish between the two scenarios. The observations of a double X-ray source in SDSS J171544.05+600835.7 are 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.W.J. McDonald Postdoctoral FellowshipCollege of Natural SciencesDepartment of Astronomy at the University of Texas at AustinMcDonald ObservatoryU.S. Department of Energy DE-AC02-76SF00515Astronom
Spatially Offset Active Galactic Nuclei III: Discovery of Late-Stage Galaxy Mergers with The Hubble Space Telescope
Galaxy pairs with separations of only a few kpc represent important stages in
the merger-driven growth of supermassive black holes (SMBHs). However, such
mergers are difficult to identify observationally due to the correspondingly
small angular scales. In Paper I we presented a method of finding candidate
kpc-scale galaxy mergers that is leveraged on the selection of X-ray sources
spatially offset from the centers of host galaxies. In this paper we analyze
new Hubble Space Telescope (HST) WFC3 imaging for six of these sources to
search for signatures of galaxy mergers. The HST imaging reveals that four of
the six systems are on-going galaxy mergers with separations of 1.2-6.6 kpc
(offset AGN). The nature of the remaining two spatially offset X-ray sources is
ambiguous and may be associated with super-Eddington accretion in X-ray
binaries. The ability of this sample to probe small galaxy separations and
minor mergers makes it uniquely suited for testing the role of galaxy mergers
for AGN triggering. We find that galaxy mergers with only one AGN are
predominantly minor mergers with mass ratios similar to the overall population
of galaxy mergers. By comparison, galaxy mergers with two AGN are biased toward
major mergers and larger nuclear gas masses. Finally, we find that the level of
SMBH accretion increases toward smaller mass ratios (major mergers). This
result suggests the mass ratio effects not only the frequency of AGN triggering
but also the rate of SMBH growth in mergers.Comment: 15 pages, 7 figures, accepted for publication in The Astrophysical
Journa
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