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

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

Dual Supermassive Black Hole Candidates in the AGN and Galaxy Evolution Survey

Dual supermassive black holes (SMBHs) with kiloparsec scale separations in merger-remnant galaxies are informative tracers of galaxy evolution, but the avenue for identifying them in large numbers for such studies is not yet clear. One promising approach is to target spectroscopic signatures of systems where both SMBHs are fueled as dual active galactic nuclei (AGNs), or where one SMBH is fueled as an offset AGN. 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.Comment: 10 pages, 8 figures, accepted for publication in Ap

Extended X-ray Emission From a Quasar-Driven Superbubble

We present observations of extended, 20-kpc scale soft X-ray gas around a luminous obscured quasar hosted by an ultra-luminous infrared galaxy caught in the midst of a major merger. The extended X-ray emission is well fit as a thermal gas with a temperature of kT ~ 280 eV and a luminosity of L_X ~ 10^42 erg/s and is spatially coincident with a known ionized gas outflow. Based on the X-ray luminosity, a factor of ~10 fainter than the [OIII] emission, we conclude that the X-ray emission is either dominated by photoionization, or by shocked emission from cloud surfaces in a hot quasar-driven wind.Comment: Accepted for publication in ApJ, 6 pages, 2 figure

The Stellar Halos of Massive Elliptical Galaxies II: Detailed Abundance Ratios at Large Radius

We study the radial dependence in stellar populations of 33 nearby early-type galaxies with central stellar velocity dispersions sigma* > 150 km/s. We measure stellar population properties in composite spectra, and use ratios of these composites to highlight the largest spectral changes as a function of radius. Based on stellar population modeling, the typical star at 2 R_e is old (~10 Gyr), relatively metal poor ([Fe/H] -0.5), and alpha-enhanced ([Mg/Fe]~0.3). The stars were made rapidly at z~1.5-2 in shallow potential wells. Declining radial gradients in [C/Fe], which follow [Fe/H], also arise from rapid star formation timescales due to declining carbon yields from low-metallicity massive stars. In contrast, [N/Fe] remains high at large radius. Stars at large radius have different abundance ratio patterns from stars in the center of any present-day galaxy, but are similar to Milky Way thick disk stars. Our observations are thus consistent with a picture in which the stellar outskirts are built up through minor mergers with disky galaxies whose star formation is truncated early (z~1.5-2).Comment: ApJ in press, 12 pages, 6 figure

Spatially Offset Active Galactic Nuclei. II. Triggering in Galaxy Mergers

Galaxy mergers are likely to play a role in triggering active galactic nuclei (AGNs), but the conditions under which this process occurs are poorly understood. In Paper I, we constructed a sample of spatially offset X-ray AGNs that represent galaxy mergers hosting a single AGN. In this paper, we use our offset AGN sample to constrain the parameters that affect AGN observability in galaxy mergers. We also construct dual-AGN samples with similar selection properties for comparison. We find that the offset AGN fraction shows no evidence for a dependence on AGN luminosity, while the dual-AGN fractions show stronger evidence for a positive dependence, suggesting that the merger events forming dual AGNs are more efficient at instigating accretion onto supermassive black holes than those forming offset AGNs. We also find that the offset and dual-AGN fractions both have a negative dependence on nuclear separation and are similar in value at small physical scales. This dependence may become stronger when restricted to high AGN luminosities, although a larger sample is needed for confirmation. These results indicate that the probability of AGN triggering increases at later merger stages. This study is the first to systematically probe down to nuclear separations of (~0.8 kpc) and is consistent with predictions from simulations that AGN observability peaks in this regime. We also find that the offset AGNs are not preferentially obscured compared to the parent AGN sample, suggesting that our selection may be targeting galaxy mergers with relatively dust-free nuclear regions

An Active Galactic Nucleus Caught in the Act of Turning Off and On

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 that 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−1 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 \u3c105 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

Monopoles, Dyons and Black Holes in the Four-Dimensional Einstein-Yang-Mills Theory

A continuum of monopole, dyon and black hole solutions exist in the Einstein-Yang-Mills theory in asymptotically anti-de Sitter space. Their structure is studied in detail. The solutions are classified by non-Abelian electric and magnetic charges and the ADM mass. The stability of the solutions which have no node in non-Abelian magnetic fields is established. There exist critical spacetime solutions which terminate at a finite radius, and have universal behavior. The moduli space of the solutions exhibits a fractal structure as the cosmological constant approaches zero.Comment: 36 Pages, 16 Figures. Minor typos corrected and one figure modifie