113 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
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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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
The Observed Concentration-Mass Relation for Galaxy Clusters
The properties of clusters of galaxies offer key insights into the assembly
process of structure in the universe. Numerical simulations of cosmic structure
formation in a hierarchical, dark matter dominated universe suggest that galaxy
cluster concentrations, which are a measure of a halo's central density,
decrease gradually with virial mass. However, cluster observations have yet to
confirm this correlation. The slopes of the run of measured concentrations with
virial mass are often either steeper or flatter than predicted by simulations.
In this work, we present the most complete sample of observed cluster
concentrations and masses yet assembled, including new measurements for 10
strong lensing clusters, thereby more than doubling the existing number of
strong lensing concentration estimates. We fit a power law to the observed
concentrations as a function of virial mass, and find that the slope is
consistent with the slopes found in simulations, though our normalization
factor is higher. Observed lensing concentrations appear to be systematically
larger than X-ray concentrations, a more pronounced effect than found in
simulations. We also find that at fixed mass, the bulk of observed cluster
concentrations are distributed log-normally, with the exception of a few
anomalously high concentration clusters. We examine the physical processes
likely responsible for the discrepancy between lensing and X-ray
concentrations, and for the anomalously high concentrations in particular. The
forthcoming Millennium simulation results will offer the most comprehensive
comparison set to our findings of an observed concentration-mass power law
relation.Comment: 12 pages, 7 figures, submitted to MNRA
The Nature of Active Galactic Nuclei with Velocity Offset Emission Lines
We obtained Keck/OSIRIS near-IR adaptive optics-assisted integral-field
spectroscopy to probe the morphology and kinematics of the ionized gas in four
velocity-offset active galactic nuclei (AGNs) from the Sloan Digital Sky
Survey. These objects possess optical emission lines that are offset in
velocity from systemic as measured from stellar absorption features. At a
resolution of ~0.18", OSIRIS allows us to distinguish which velocity offset
emission lines are produced by the motion of an AGN in a dual supermassive
black hole system, and which are produced by outflows or other kinematic
structures. In three galaxies, J1018+2941, J1055+1520 and J1346+5228, the
spectral offset of the emission lines is caused by AGN-driven outflows. In the
remaining galaxy, J1117+6140, a counterrotating nuclear disk is observed that
contains the peak of Pa emission 0.2" from the center of the galaxy.
The most plausible explanation for the origin of this spatially and
kinematically offset peak is that it is a region of enhanced Pa
emission located at the intersection zone between the nuclear disk and the bar
of the galaxy. In all four objects, the peak of ionized gas emission is not
spatially coincident with the center of the galaxy as traced by the peak of the
near-IR continuum emission. The peaks of ionized gas emission are spatially
offset from the galaxy centers by 0.1"-0.4" (0.1-0.7 kpc). We find that the
velocity offset originates at the location of this peak of emission, and the
value of the offset can be directly measured in the velocity maps. The
emission-line ratios of these four velocity-offset AGNs can be reproduced only
with a mixture of shocks and AGN photoionization. Shocks provide a natural
explanation for the origin of the spatially and spectrally offset peaks of
ionized gas emission in these galaxies.Comment: 14 pages, 12 figures, accepted for publication in Ap
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
Identification of Outflows and Candidate Dual Active Galactic Nuclei in SDSS Quasars at z=0.8-1.6
We present a sample of 131 quasars from the Sloan Digital Sky Survey at
redshifts 0.8<z<1.6 with double peaks in either of the high-ionization narrow
emission lines [NeV]3426 or [NeIII]3869. These sources were selected with the
intention of identifying high-redshift analogs of the z<0.8 active galactic
nuclei (AGN) with double-peaked [OIII]5007 lines, which might represent AGN
outflows or dual AGN. Lines of high-ionization potential are believed to
originate in the inner, highly photoionized portion of the narrow line region
(NLR), and we exploit this assumption to investigate the possible kinematic
origins of the double-peaked lines. For comparison, we measure the [NeV]3426
and [NeIII]3869 double peaks in low-redshift (z<0.8) [OIII]-selected sources.
We find that [NeV]3426 and [NeIII]3869 show a correlation between
line-splitting and line-width similar to that of [OIII]5007 in other studies;
and the velocity-splittings are correlated with the quasar Eddington ratio.
These results suggest an outflow origin for at least a subset of the
double-peaks, allowing us to study the high-ionization gas kinematics around
quasars. However, we find that a non-neligible fraction of our sample show no
evidence for an ionization stratification. For these sources, the outflow
scenario is less compelling, leaving the dual AGN scenario as a viable
possibility. Finally, we find that our sample shows an anti-correlation between
the velocity-offset ratio and luminosity ratio of the components, which is a
potential dynamical argument for the presence of dual AGN. Therefore, this
study serves as a first attempt at extending the selection of candidate dual
AGN to higher redshifts.Comment: 19 pages, 12 figures, accepted for publication in The Astrophysical
Journa
The Origin of Double-Peaked Narrow Lines in Active Galactic Nuclei III: Feedback from Biconical AGN Outflows
We apply an analytic Markov Chain Monte Carlo model to a sample of 18
AGN-driven biconical outflows that we identified from a sample of active
galaxies with double-peaked narrow emission lines at z < 0.1 in the Sloan
Digital Sky Survey. We find that 8/18 are best described as asymmetric bicones,
8/18 are nested bicones, and 2/18 are symmetric bicones. From the geometry and
kinematics of the models, we find that these moderate-luminosity AGN outflows
are large and energetic. The biconical outflows axes are randomly oriented with
respect to the photometric major axis of the galaxy, implying a randomly
oriented and clumpier torus to collimate the outflow, but the torus also allows
some radiation to escape equatorially. We find that 16/18 (89%) outflows are
energetic enough to drive a two-staged feedback process in their host galaxies.
All of these outflows geometrically intersect the photometric major axis of the
galaxy, and 23% of outflow host galaxies are significantly redder or have
significantly lower specific star formation rates when compared to a matched
sample of active galaxies.Comment: 32 pages, 11 figures, accepted for publication in MNRAS; See Figure 7
for a summary of the finding
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