106 research outputs found
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
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
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. IV. Association with Galaxy Mergers
Double-peaked narrow emission lines in active galactic nucleus (AGN) spectra
can be produced by AGN outflows, rotation, or dual AGNs, which are AGN pairs in
ongoing galaxy mergers. Consequently, double-peaked narrow AGN emission lines
are useful tracers of the coevolution of galaxies and their supermassive black
holes, as driven by AGN feedback and AGN fueling. We investigate this concept
further with follow-up optical longslit observations of a sample of 95 Sloan
Digital Sky Survey (SDSS) galaxies that have double-peaked narrow AGN emission
lines. Based on a kinematic analysis of the longslit spectra, we confirm
previous work that finds that the majority of double-peaked narrow AGN emission
lines are associated with outflows. We also find that eight of the galaxies
have companion galaxies with line-of-sight velocity separations < 500 km/s and
physical separations <30 kpc. Since we find evidence of AGNs in both galaxies,
all eight of these systems are compelling dual AGN candidates. Galaxies with
double-peaked narrow AGN emission lines occur in such galaxy mergers at least
twice as often as typical active galaxies. Finally, we conclude that at least
3% of SDSS galaxies with double-peaked narrow AGN emission lines are found in
galaxy mergers where both galaxies are resolved in SDSS imaging.Comment: 14 pages, 2 figures, ApJ in pres
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
Kiloparsec-scale Spatial Offsets in Double-peaked Narrow-line Active Galactic Nuclei. I. Markers for Selection of Compelling Dual Active Galactic Nucleus Candidates
Merger-remnant galaxies with kpc-scale separation dual active galactic nuclei
(AGNs) should be widespread as a consequence of galaxy mergers and triggered
gas accretion onto supermassive black holes, yet very few dual AGNs have been
observed. Galaxies with double-peaked narrow AGN emission lines in the Sloan
Digital Sky Survey are plausible dual AGN candidates, 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
have obtained spatial profiles of the AGN emission via follow-up long-slit
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, which suggests that they are produced by kpc-scale dual AGNs or kpc-scale
outflows, jets, or rotating gaseous disks. In addition, 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 find 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.Comment: 12 pages, 8 figures, published in ApJ. Modified from original version
to reflect referee's comment
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