599 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
Active Galaxies and the Study of Black Hole Demographics
We discuss the critical importance of black hole mass indicators based on
scaling relations in active galaxies. We highlight outstanding uncertainties in
these methods and potential paths to substantial progress in the next decade.Comment: 4 pages, 1 figure, Invited review to appear in PAS
Quasar feedback and the origin of radio emission in radio-quiet quasars
We conduct kinematic analysis of the SDSS spectra of 568 obscured luminous
quasars, with the emphasis on the kinematic structure of the [OIII]5007
emission line. [OIII] emission tends to show blueshifts and blue excess, which
indicates that at least part of the narrow-line gas is undergoing an organized
outflow. The velocity width containing 90% of line power ranges from 370 to
4780 km/sec, suggesting outflow velocities up to 2000 km/sec. The velocity
width of the [OIII] emission is positively correlated with the radio luminosity
among the radio-quiet quasars. We propose that radio emission in radio-quiet
quasars is due to relativistic particles accelerated in the shocks within the
quasar-driven outflows; star formation in quasar hosts is insufficient to
explain the observed radio emission. The median radio luminosity of the sample
of nu L_nu[1.4GHz] = 10^40 erg/sec suggests a median kinetic luminosity of the
quasar-driven wind of L_wind=3x10^44 erg/sec, or about 4% of the estimated
median bolometric luminosity L_bol=8x10^45 erg/sec. Furthermore, the velocity
width of [OIII] is positively correlated with mid-infrared luminosity, which
suggests that outflows are ultimately driven by the radiative output of the
quasar. As the outflow velocity increases, some emission lines characteristic
of shocks in quasi-neutral medium increase as well, which we take as further
evidence of quasar-driven winds propagating into the interstellar medium of the
host galaxy. None of the kinematic components show correlations with the
stellar velocity dispersions of the host galaxies, so there is no evidence that
any of the gas in the narrow-line region of quasars is in dynamical equilibrium
with the host galaxy. Quasar feedback appears to operate above the threshold
luminosity of L_bol=3x10^45 erg/sec.Comment: 23 pages, accepted to MNRA
Dwarf Galaxies with Optical Signatures of Active Massive Black Holes
We present a sample of 151 dwarf galaxies (10^8.5 < M_stellar < 10^9.5 Msun)
that exhibit optical spectroscopic signatures of accreting massive black holes
(BHs), increasing the number of known active galaxies in this stellar mass
range by more than an order of magnitude. Utilizing data from the Sloan Digital
Sky Survey Data Release 8 and stellar masses from the NASA-Sloan Atlas, we have
systematically searched for active BHs in ~25,000 emission-line galaxies with
stellar masses comparable to the Magellanic Clouds and redshifts z<0.055. Using
the narrow-line [OIII]/H-beta versus [NII]/H-alpha diagnostic diagram, we find
photoionization signatures of BH accretion in 136 galaxies, a small fraction of
which also exhibit broad H-alpha emission. For these broad-line AGN candidates,
we estimate BH masses using standard virial techniques and find a range of 10^5
< M_BH < 10^6 Msun and a median of M_BH ~ 2 x 10^5 Msun. We also detect broad
H-alpha in 15 galaxies that have narrow-line ratios consistent with
star-forming galaxies. Follow-up observations are required to determine if
these are true type 1 AGN or if the broad H-alpha is from stellar processes.
The median absolute magnitude of the host galaxies in our active sample is Mg =
-18.1 mag, which is ~1-2 magnitudes fainter than previous samples of AGN hosts
with low-mass BHs. This work constrains the smallest galaxies that can form a
massive BH, with implications for BH feedback in low-mass galaxies and the
origin of the first supermassive BH seeds.Comment: 26 pages, 15 figures, 6 tables. Accepted for publication in The
Astrophysical Journa
A Comprehensive Archival Chandra Search for X-ray Emission from Ultracompact Dwarf Galaxies
We present the first comprehensive archival study of the X-ray properties of
ultracompact dwarf (UCD) galaxies, with the goal of identifying
weakly-accreting central black holes in UCDs. Our study spans 578 UCDs
distributed across thirteen different host systems, including clusters, groups,
fossil groups, and isolated galaxies. Of the 336 spectroscopically-confirmed
UCDs with usable archival Chandra imaging observations, 21 are X-ray-detected.
Imposing a completeness limit of erg s, the global
X-ray detection fraction for the UCD population is . Of the 21
X-ray-detected UCDs, seven show evidence of long-term X-ray time variability on
the order of months to years. X-ray-detected UCDs tend to be more compact than
non-X-ray-detected UCDs, and we find tentative evidence that the X-ray
detection fraction increases with surface luminosity density and global stellar
velocity dispersion. The X-ray emission of UCDs is fully consistent with
arising from a population of low-mass X-ray binaries (LMXBs). In fact, there
are fewer X-ray sources than expected using a naive extrapolation from globular
clusters. Invoking the fundamental plane of black hole activity for SUCD1 near
the Sombrero galaxy, for which archival Jansky Very Large Array imaging at 5
GHz is publicly available, we set an upper limit on the mass of a hypothetical
central black hole in that UCD to be . While the
majority of our sources are likely LMXBs, we cannot rule out central black
holes in some UCDs based on X-rays alone, and so we address the utility of
follow-up radio observations to find weakly-accreting central black holes.Comment: 20 pages, 6 figures, re-submitted to ApJ after minor revision
Testing the presence of multiple photometric components in nearby early-type galaxies using SDSS
We investigate two-dimensional image decomposition of nearby, morphologically
selected early-type galaxies (ETGs). We are motivated by recent observational
evidence of significant size growth of quiescent galaxies and theoretical
development advocating a two-phase formation scenario for ETGs. We find that a
significant fraction of nearby ETGs show changes in isophotal shape that
require multi-component models. The characteristic sizes of the inner and outer
component are and kpc. The inner component lies on the
mass-size relation of ETGs at , while the outer component
tends to be more elliptical and hints at a stochastic buildup process. We find
real physical differences between the single- and double-component ETGs, with
the double-component galaxies being younger and more metal-rich. The fraction
of double component ETGs increases with increasing and decreases in
denser environments. We hypothesize that double-component systems were able to
accrete gas and small galaxies until later times, boosting their central
densities, building up their outer parts, and lowering their typical central
ages. In contrast, the oldest galaxies, perhaps due to residing in richer
environments, have no remaining hints of their last accretion episode.Comment: resubmitted to ApJ after referee's repor
X-ray Properties of Intermediate-mass Black Holes in Active Galaxies. III. Spectral Energy Distribution and Possible Evidence for Intrinsically X-ray-weak AGNs
We present a systematic X-ray study, the third in a series, of 49 active
galactic nuclei with intermediate-mass black holes (IMBH; ~10^5-10^6 M_sun)
using Chandra observations. We detect 42 out of 49 targets with a 0.5-2 keV
X-ray luminosity 10^41-10^43 erg/s. We perform spectral fitting for the 10
objects with enough counts (>200), and they are all well fit by a simple
power-law model modified by Galactic absorption, with no sign of significant
intrinsic absorption. While we cannot fit the X-ray spectral slope directly for
the rest of the sample, we estimate it from the hardness ratio and find a range
of photon indices consistent with those seen in more luminous and massive
objects. The X-ray-to-optical spectral slope (alphaox) of our IMBH sample is
systematically flatter than in active galaxies with more massive black holes,
consistent with the well-known correlation between alphaox and UV luminosity.
Thanks to the wide dynamic range of our sample, we find evidence that alphaox
increases with decreasing M_BH as expected from accretion disk models, where
the UV emission systematically decreases as M_BH decreases and the disk
temperature increases. We also find a long tail toward low alphaox values.
While some of these sources may be obscured, given the high L_bol/L_Eddington
values in the sample, we argue that some may be intrinsically X-ray-weak,
perhaps owing to a rare state that radiates very little coronal emission.Comment: 13 pages (double columns), 2 tables, 9 figures, ApJ accepte
Similarity of ionized gas nebulae around unobscured and obscured quasars
Quasar feedback is suspected to play a key role in the evolution of massive
galaxies, by removing or reheating gas in quasar host galaxies and thus
limiting the amount of star formation. In this paper we continue our
investigation of quasar-driven winds on galaxy-wide scales. We conduct Gemini
Integral Field Unit spectroscopy of a sample of luminous unobscured (type 1)
quasars, to determine the morphology and kinematics of ionized gas around these
objects, predominantly via observations of the [O III]5007 emission line. We
find that ionized gas nebulae extend out to ~13 kpc from the quasar, that they
are smooth and round, and that their kinematics are inconsistent with gas in
dynamical equilibrium with the host galaxy. The observed morphological and
kinematic properties are strikingly similar to those of ionized gas around
obscured (type 2) quasars with matched [O III] luminosity, with marginal
evidence that nebulae around unobscured quasars are slightly more compact.
Therefore in samples of obscured and unobscured quasars carefully matched in [O
III] luminosity we find support for the standard geometry-based unification
model of active galactic nuclei, in that the intrinsic properties of quasars,
of their hosts and of their ionized gas appear to be very similar. Given the
apparent ubiquity of extended ionized regions, we are forced to conclude that
either the quasar is at least partially illuminating pre-existing gas or that
both samples of quasars are seen during advanced stages of quasar feedback. In
the latter case, we may be biased by our [O III]-based selection against
quasars in the early "blow-out" phase, for example due to dust obscuration.Comment: 17 pages, 10 figures, 2 tables. Published in MNRAS, 201
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