163 research outputs found
HST WFC3/IR Observations of Active Galactic Nucleus Host Galaxies at z~2: Supermassive Black Holes Grow in Disk Galaxies
We present the rest-frame optical morphologies of active galactic nucleus
(AGN) host galaxies at 1.5<z<3, using near-infrared imaging from the Hubble
Space Telescope Wide Field Camera 3, the first such study of AGN host galaxies
at these redshifts. The AGN are X-ray selected from the Chandra Deep Field
South and have typical luminosities of 1E42 < L_X < 1E44 erg/s. Accreting black
holes in this luminosity and redshift range account for a substantial fraction
of the total space density and black hole mass growth over cosmic time; they
thus represent an important mode of black hole growth in the universe. We find
that the majority (~80%) of the host galaxies of these AGN have low Sersic
indices indicative of disk-dominated light profiles, suggesting that secular
processes govern a significant fraction of the cosmic growth of black holes.
That is, many black holes in the present-day universe grew much of their mass
in disk-dominated galaxies and not in early-type galaxies or major mergers. The
properties of the AGN host galaxies are furthermore indistinguishable from
their parent galaxy population and we find no strong evolution in either
effective radii or morphological mix between z~2 and z~0.05.Comment: 7 pages, 3 figures, 2 tables. Accepted for publication in the
Astrophysical Journal Letter
Chandra Observations of Galaxy Zoo Mergers: Frequency of Binary Active Nuclei in Massive Mergers
We present the results from a Chandra pilot study of 12 massive galaxy
mergers selected from Galaxy Zoo. The sample includes major mergers down to a
host galaxy mass of 10 that already have optical AGN
signatures in at least one of the progenitors. We find that the coincidences of
optically selected active nuclei with mildly obscured ( cm) X-ray nuclei are relatively common (8/12), but the
detections are too faint ( counts per nucleus; erg s cm) to reliably separate starburst and
nuclear activity as the origin of the X-ray emission. Only one merger is found
to have confirmed binary X-ray nuclei, though the X-ray emission from its
southern nucleus could be due solely to star formation. Thus, the occurrences
of binary AGN in these mergers are rare (0-8%), unless most merger-induced
active nuclei are very heavily obscured or Compton thick.Comment: 8 pages, including 5 figures and 1 table. Accepted by Ap
Scattered X-rays in Obscured Active Galactic Nuclei and their Implications for Geometrical Structure and Evolution
We construct a new sample of 32 obscured active galactic nuclei (AGNs)
selected from the Second XMM-Newton Serendipitous Source Catalogue to
investigate their multiwavelength properties in relation to the "scattering
fraction", the ratio of the soft X-ray flux to the absorption-corrected direct
emission. The sample covers a broad range of the scattering fraction
(0.1%-10%). A quarter of the 32 AGNs have a very low scattering fraction
(smaller than 0.5%), which suggests that they are buried in a geometrically
thick torus with a very small opening angle. We investigate correlations
between the scattering fraction and multiwavelength properties. We find that
AGNs with a small scattering fraction tend to have low [O III]lambda5007/X-ray
luminosity ratios. This result agrees with the expectation that the extent of
the narrow-line region is small because of the small opening angle of the
torus. There is no significant correlation between scattering fraction and
far-infrared luminosity. This implies that a scale height of the torus is not
primarily determined by starburst activity. We also compare scattering fraction
with black hole mass or Eddington ratio and find a weak anti-correlation
between the Eddington ratio and scattering fraction. This implies that more
rapidly growing supermassive black holes tend to have thicker tori.Comment: 13 pages, 10 figures, accepted for publication in Ap
Spatially-Resolved Spectroscopy of SDSS J0952+2552: a Confirmed Dual AGN
Most massive galaxies contain supermassive black holes (SMBHs) in their
cores. When galaxies merge, gas is driven to nuclear regions and can accrete
onto the central black hole. Thus one expects to see dual AGN in a fraction of
galaxy mergers. Candidates for galaxies containing dual AGN have been
identified by the presence of double-peaked narrow [O III] emission lines and
by high spatial resolution images of close galaxy pairs. Spatially-resolved
spectroscopy is needed to confirm these galaxy pairs as systems with
spatially-separated double SMBHs. With the Keck 2 Laser Guide Star Adaptive
Optics system and the OSIRIS near-infrared integral field spectrograph, we
obtained spatially-resolved spectra for SDSS J09527.62+255257.2, a radio-quiet
quasar shown by previous imaging to consist of a galaxy and its close (1.0")
companion. We find that the main galaxy is a Type 1 AGN with both broad and
narrow AGN emission lines in its spectrum, while the companion galaxy is a Type
2 AGN with narrow emission lines only. The two AGN are separated by 4.8 kpc,
and their redshifts correspond to those of the double peaks of the [O III]
emission line seen in the SDSS spectrum. Line diagnostics indicate that both
components of the double [O III] emission lines are due to AGN photoionization.
These results confirm that J0952+2552 contains two spatially-separated AGN. As
one of the few confirmed dual AGN at an intermediate separation of < 10 kpc,
this system offers a unique opportunity to study galaxy mergers and their
effect on black hole growth.Comment: 6 pages, 4 figures, 1 table, submitted to ApJL. See
http://people.ucsc.edu/~rmcgurk/McGurk_dualagn.pdf for a high-resolution
version of the pape
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The XMM-Newton Wide field survey in the COSMOS field: redshift evolution of AGN bias and subdominant role of mergers in triggering moderate luminosity AGN at redshift up to 2.2
We present a study of the redshift evolution of the projected correlation
function of 593 X-ray selected AGN with I_AB<23 and spectroscopic redshifts
z<4, extracted from the 0.5-2 keV X-ray mosaic of the 2.13 deg^2 XMM-COSMOS
survey. We introduce a method to estimate the average bias of the AGN sample
and the mass of AGN hosting halos, solving the sample variance using the halo
model and taking into account the growth of the structure over time. We find
evidence of a redshift evolution of the bias factor for the total population of
XMM-COSMOS AGN from b(z=0.92)=2.30 +/- 0.11 to b(z=1.94)=4.37 +/- 0.27 with an
average mass of the hosting DM halos logM [h^-1 M_sun] ~ 13.12 +/- 0.12 that
remains constant at all z < 2. Splitting our sample into broad optical lines
AGN (BL), AGN without broad optical lines (NL) and X-ray unobscured and
obscured AGN, we observe an increase of the bias with redshift in the range
z=0.7-2.25 and z=0.6-1.5 which corresponds to a constant halo mass logM [h^-1
M_sun] ~ 13.28 +/- 0.07 and logM [h^-1 M_sun] ~ 13.00 +/- 0.06 for BL /X-ray
unobscured AGN and NL/X-ray obscured AGN, respectively. The theoretical models
which assume a quasar phase triggered by major mergers can not reproduce the
high bias factors and DM halo masses found for X-ray selected BL AGN with L_BOL
~ 2e45 erg s^-1. Our work extends up to z ~ 2.2 the z <= 1 statement that, for
moderate luminosity X-ray selected BL AGN, the contribution from major mergers
is outnumbered by other processes, possibly secular such as tidal disruptions
or disk instabilities.Comment: 16 emulateapj pages, 18 figures and 3 tables. Accepted for the
publication in The Astrophysical Journa
A population of luminous accreting black holes with hidden mergers
Major galaxy mergers are thought to play an important part in fuelling the
growth of supermassive black holes. However, observational support for this
hypothesis is mixed, with some studies showing a correlation between merging
galaxies and luminous quasars and others showing no such association. Recent
observations have shown that a black hole is likely to become heavily obscured
behind merger-driven gas and dust, even in the early stages of the merger, when
the galaxies are well separated (5 to 40 kiloparsecs). Merger simulations
further suggest that such obscuration and black-hole accretion peaks in the
final merger stage, when the two galactic nuclei are closely separated (less
than 3 kiloparsecs). Resolving this final stage requires a combination of
high-spatial-resolution infrared imaging and high-sensitivity hard-X-ray
observations to detect highly obscured sources. However, large numbers of
obscured luminous accreting supermassive black holes have been recently
detected nearby (distances below 250 megaparsecs) in X-ray observations. Here
we report high-resolution infrared observations of hard-X-ray-selected black
holes and the discovery of obscured nuclear mergers, the parent populations of
supermassive-black-hole mergers. We find that obscured luminous black holes
(bolometric luminosity higher than 2x10^44 ergs per second) show a significant
(P<0.001) excess of late-stage nuclear mergers (17.6 per cent) compared to a
sample of inactive galaxies with matching stellar masses and star formation
rates (1.1 per cent), in agreement with theoretical predictions. Using
hydrodynamic simulations, we confirm that the excess of nuclear mergers is
indeed strongest for gas-rich major-merger hosts of obscured luminous black
holes in this final stage.Comment: To appear in the 8 November 2018 issue of Nature. This is the
authors' version of the wor
The Demographics of Broad Line Quasars in the Mass-Luminosity Plane II. Black Hole Mass and Eddington Ratio Functions
We employ a flexible Bayesian technique to estimate the black hole mass and
Eddington ratio functions for Type 1 (i.e., broad line) quasars from a
uniformly-selected data set of ~58,000 quasars from the SDSS DR7. We find that
the SDSS becomes significantly incomplete at M_{BH} < 3 x 10^8 M_{Sun} or L /
L_{Edd} < 0.07, and that the number densities of Type 1 quasars continue to
increase down to these limits. Both the mass and Eddington ratio functions show
evidence of downsizing, with the most massive and highest Eddington ratio black
holes experiencing Type 1 quasar phases first, although the Eddington ratio
number densities are flat at z < 2. We estimate the maximum Eddington ratio of
Type 1 quasars in the observable Universe to be L / L_{Edd} ~ 3. Consistent
with our results in Paper I, we do not find statistical evidence for a
so-called "sub-Eddington boundary" in the mass-luminosity plane of broad line
quasars, and demonstrate that such an apparent boundary in the observed
distribution can be caused by selection effect and errors in virial BH mass
estimates. Based on the typical Eddington ratio in a given mass bin, we
estimate typical growth times for the black holes in Type 1 quasars and find
that they are typically comparable to or longer than the age of the universe,
implying an earlier phase of accelerated (i.e., with higher Eddington ratios)
and possibly obscured growth. The large masses probed by our sample imply that
most of our black holes reside in what are locally early type galaxies, and we
interpret our results within the context of models of self-regulated black hole
growth.Comment: Submitted to ApJ, 25 pages (emulateapj), 15 figures; revised to match
accepted version with primary changes to the introduction and discussion,
replaced Fig 1
Double-peaked Narrow-Line Signatures of Dual Supermassive Black Holes in Galaxy Merger Simulations
We present a first attempt to model the narrow-line (NL) region of active
galactic nuclei (AGN) in hydrodynamic simulations of galaxy mergers, using a
novel physical prescription. This model is used to determine the origin of
double-peaked NL (dNL) AGN in merging galaxies and their connection to
supermassive black hole (SMBH) pairs, motivated by recent observations of such
objects. We find that dNL AGN induced by the relative motion of SMBH pairs are
a generic but short-lived feature of gaseous major mergers. dNL AGN should
often be observed in late-stage mergers, during the kpc-scale phase of SMBH
inspiral or soon after the SMBH merger. However, even within the kpc-scale
phase, only a minority of dNL AGN are directly induced by SMBH motion; their
lifetimes are typically a few Myr. Most double peaks arise from gas kinematics
near the SMBH, although prior to the SMBH merger up to 80% of all dNL profiles
may be influenced by SMBH motion via altered peak ratios or velocity offsets.
The total lifetimes of dNL AGN depend strongly on viewing angle and on
properties of the merging galaxies. Also, in a typical merger, at least 10-40%
of the double peaks induced by SMBH motion have small projected separations,
0.1-1 kpc, such that dual peaks of stellar surface brightness are not easily
resolved. Diffuse tidal features can indicate late-stage galaxy mergers,
although they do not distinguish SMBH pairs from merged SMBHs. We show that dNL
profiles with peak velocity splittings > 500 km s^-1 or with measurable overall
velocity shifts are often associated with SMBH pairs. Our results support the
notion that selection of dNL AGN is a promising method for identifying dual
SMBH candidates, but demonstrate the critical importance of high-resolution,
multi-wavelength follow-up observations, and the use of multiple lines of
evidence, for confirming the dual nature of candidate SMBH pairs. (Abridged)Comment: 24 pages, 9 figures. Moderate revisions; accepted to MNRA
Host Galaxy Properties of the Swift BAT Ultra Hard X-ray Selected AGN
We have assembled the largest sample of ultra hard X-ray selected (14-195
keV) AGN with host galaxy optical data to date, with 185 nearby (z<0.05),
moderate luminosity AGN from the Swift BAT sample. The BAT AGN host galaxies
have intermediate optical colors (u-r and g-r) that are bluer than a comparison
sample of inactive galaxies and optically selected AGN from the Sloan Digital
Sky Survey (SDSS) which are chosen to have the same stellar mass. Based on
morphological classifications from the RC3 and the Galaxy Zoo, the bluer colors
of BAT AGN are mainly due to a higher fraction of mergers and massive spirals
than in the comparison samples. BAT AGN in massive galaxies (log M_*>10.5) have
a 5 to 10 times higher rate of spiral morphologies than in SDSS AGN or inactive
galaxies. We also see enhanced far-IR emission in BAT AGN suggestive of higher
levels of star formation compared to the comparison samples. BAT AGN are
preferentially found in the most massive host galaxies with high concentration
indexes indicative of large bulge-to-disk ratios and large supermassive black
holes. The narrow-line BAT AGN have similar intrinsic luminosities as the SDSS
NL Seyferts based on measurements of [O III]. There is also a correlation
between the stellar mass and X-ray emission. The BAT AGN in mergers have bluer
colors and greater ultra hard X-ray emission compared to the BAT sample as
whole. In agreement with the Unified Model of AGN, and the relatively unbiased
nature of the BAT sources, the host galaxy colors and morphologies are
independent of measures of obscuration such as X-ray column density or Seyfert
type. The high fraction of massive spiral galaxies and galaxy mergers in BAT
AGN suggest that host galaxy morphology is related to the activation and
fueling of local AGN.Comment: 30 pages, 30 figures, accepted for publication in Ap
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