46 research outputs found
Torus skin outflow in a near-Eddington quasar revealed by spectropolarimetry
Even when the direct view toward the active nucleus is obscured, nuclear
emission propagating along other directions can scatter off surrounding
material, become polarized and reach the observer. Spectropolarimetry can thus
be an important tool in investigating the circumnuclear geometry and kinematics
of quasars on scales that cannot yet be probed via direct observations. Here we
discuss an intriguing class of quasars where the polarization position angle
swings by large amounts (90 deg) within an emission line. We investigate a
kinematic model in which the scattering dust or electrons are in an
axisymmetric outflow. We propagate Stokes parameters in a variety of geometries
of emitter, scatterer and observer. We use these models to predict polarization
fraction, line profiles and polarization position angles and compare them to
observations. We demonstrate that the swinging polarization angle can be a
result of the geometry of the outflow and the orientation of the observer.
Polarization properties of a near-Eddington extremely red quasar SDSS J1652 can
be successfully explained by a model in which the quasar is surrounded by a
geometrically thick disk, whose `skin' is outflowing at 1000 km/s and acts as
the scatterer on scales of a few tens of pc. The line of sight to the observer
in this source is within or close to the skin of the torus, in agreement with
multi-wavelength data. Spectropolarimetric data and models presented here
strongly support the thick-disk geometry of circumnuclear material suggested by
recent numerical simulations of high-rate accretion flows onto black holes.Comment: 17 pages, accepted to MNRA
SEARCHING FOR QUASAR FEEDBACK IN OBSCURED AND RED QUASARS AT THE PEAK OF GALAXY FORMATION
Though small on the scale of their host galaxy, supermassive black holes likely play a crucial role in galaxy evolution. A galaxy-scale effect driven by supermassive black holes (or ``quasar feedback”) is necessary to stop the production of too many massive galaxies in semi-analytical simulations and to drive observational correlations between host galaxy and black hole properties. Nevertheless, such a mechanism is not yet well supported by observational evidence, especially at the peak of galaxy formation. This thesis represents an effort to ground every aspect of quasar feedback at the peak of galaxy formation with strong observational evidence. I first define a sample of obscured and actively accreting supermassive black holes (``quasars”) at the peak of galaxy formation using the Sloan Digital Sky Survey. Then, using targeted observations of small sub-samples of these objects across a variety of wavelengths, I show that in a sample of the most luminous quasars, we see emerging evidence for quasar feedback. I show that luminous quasars are capable of launching winds by studying their scattering geometry and kinematics using spectropolarimetry. I advance the theory that sensitive, high resolution radio observations allow us to search for the presence of such winds on galaxy scales. Finally, I show that these winds go on to clear their host galaxies of the molecular gas necessary for star formation. All together, this body of work provides a robust observational window into important mechanisms of galaxy growth and quenching at the peak of galaxy formation and evolution
Sensitive Radio Survey of Obscured Quasar Candidates
We study the radio properties of moderately obscured quasars over a range of
redshifts to understand the role of radio activity in accretion using the
Jansky Very Large Array (JVLA) at 6.0GHz and 1.4GHz. Our z~2.5 sample consists
of optically-selected obscured quasar candidates, all of which are radio-quiet,
with typical radio luminosities of [1.4 GHz] < erg
s. Only a single source is individually detected in our deep (rms~10
Jy) exposures. This population would not be identified by radio-based
selection methods used for distinguishing dusty star-forming galaxies and
obscured active nuclei. In our pilot A-array study of z~0.5 radio-quiet
quasars, we spatially resolve four of five objects on scales ~ 5 kpc and find
they have steep spectral indices. Therefore, radio emission in these sources
could be due to jet-driven or radiatively driven bubbles interacting with
interstellar material on the scale of the host galaxy. Finally, we also study
the population of ~ 200 faint (~40 Jy - 40 mJy) radio sources observed
over ~ 120 arcmin of our data. 60% of these detections are matched in the
SDSS and/or WISE and are, in roughly equal shares, active nuclei at a broad
range of redshifts, passive galaxies with no other signs of nuclear activity
and IR-bright but optically faint sources. Spectroscopically or photometrically
confirmed star-forming galaxies constitute only a small minority of the
matches. Such sensitive radio surveys allow us to address important questions
of AGN evolution and evaluate the AGN contribution to the radio-quiet sky.Comment: 18 pages, submitted to MNRA
Winds as the origin of radio emission in radio-quiet extremely red quasars
Most active galactic nuclei (AGNs) are radio-quiet, and the origin of their
radio emission is not well-understood. One hypothesis is that this radio
emission is a by-product of quasar-driven winds. In this paper, we present the
radio properties of 108 extremely red quasars (ERQs) at . ERQs are among
the most luminous quasars ( erg/s) in the Universe,
with signatures of extreme ( km/s) outflows in their
[OIII]5007 \AA\ emission, making them the best subjects to seek the
connection between radio and outflow activity. All ERQs but one are unresolved
in the radio on kpc scales, and the median radio luminosity of ERQs
is erg/s, in the radio-quiet regime, but
one to two orders of magnitude higher than that of other quasar samples. The
radio spectra are steep, with a mean spectral index . In addition, ERQs neatly follow the extrapolation of the low-redshift
correlation between radio luminosity and the velocity dispersion of
[OIII]-emitting ionized gas. Uncollimated winds, with a power of one per cent
of the bolometric luminosity, can account for all these observations. Such
winds would interact with and shock the gas around the quasar and in the host
galaxy, resulting in acceleration of relativistic particles and the consequent
synchrotron emission observed in the radio. Our observations support the
picture in which ERQs are signposts of extremely powerful episodes of quasar
feedback, and quasar-driven winds as a contributor of the radio emission in the
intermediate regime of radio luminosity erg/s.Comment: accepted by MNRA
Near Infrared Spectra and Intrinsic Luminosities of Candidate Type II Quasars at 2 < z < 3.4
We present JHK near-infrared (NIR) spectroscopy of 25 candidate Type II
quasars selected from the Sloan Digital Sky Survey, using Triplespec on the
Apache Point Observatory 3.5m telescope, FIRE at the Magellan/Baade 6.5m
telescope, and GNIRS on Gemini. At redshifts of 2 < z < 3.4, our NIR spectra
probe the rest-frame optical region of these targets, which were initially
selected to have strong lines of CIV and Ly alpha, with FWHM<2000 km/s from the
SDSS pipeline. We use the [OIII]5007 line shape as a model for the narrow line
region emission, and find that \halpha\ consistently requires a broad component
with FWHMs ranging from 1000 to 7500 km/s. Interestingly, the CIV lines also
require broad bases, but with considerably narrower widths of 1000 to 4500
km/s. Estimating the extinction using the Balmer decrement and also the
relationship in lower-z quasars between rest equivalent width and luminosity in
the [OIII] line, we find typical A_V values of 0-2 mag, which naturally explain
the attenuated CIV lines relative to Halpha. We propose that our targets are
moderately obscured quasars. We also describe one unusual object with three
distinct velocity peaks in its [OIII] spectrum.Comment: Accepted for publication in ApJ, 18 pages, 14 figure
Candidate Type II Quasars at 2 < z < 4.3 in the Sloan Digital Sky Survey III
At low redshifts, dust-obscured quasars often have strong yet narrow
permitted lines in the rest-frame optical and ultraviolet, excited by the
central active nucleus, earning the designation Type II quasars. We present a
sample of 145 candidate Type II quasars at redshifts between 2 and 4.3,
encompassing the epoch at which quasar activity peaked in the universe. These
objects, selected from the quasar sample of the Baryon Oscillation
Spectroscopic Survey of the Sloan Digital Sky Survey III, are characterized by
weak continuum in the rest-frame ultraviolet (typical continuum magnitude of i
\approx 22) and strong lines of CIV and Ly \alpha, with Full Width at Half
Maximum less than 2000 kms-1. The continuum magnitudes correspond to an
absolute magnitude of -23 or brighter at redshift 3, too bright to be due
exclusively to the host galaxies of these objects. Roughly one third of the
objects are detected in the shorter-wavelength bands of the WISE survey; the
spectral energy distributions (SEDs) of these objects appear to be intermediate
between classic Type I and Type II quasars seen at lower redshift. Five objects
are detected at rest frame 6\mu m by Spitzer, implying bolometric luminosities
of several times 10^46 erg s-1. We have obtained polarization measurements for
two objects; they are roughly 3% polarized. We suggest that these objects are
luminous quasars, with modest dust extinction (A_V ~ 0.5 mag), whose
ultraviolet continuum also includes a substantial scattering contribution.
Alternatively, the line of sight to the central engines of these objects may be
partially obscured by optically thick material.Comment: 26 pages, 13 figures, 10 tables, 4 machine readable tables. Accepted
for publication in MNRA
Host galaxies of high-redshift extremely red and obscured quasars
We present Hubble Space Telescope 1.4-1.6 micron images of the hosts of ten
extremely red quasars (ERQs) and six type 2 quasar candidates at z=2-3. ERQs,
whose bolometric luminosities range between 10^47 and 10^48 erg/sec, show
spectroscopic signs of powerful ionized winds, whereas type 2 quasar candidates
are less luminous and show only mild outflows. After performing careful
subtraction of the quasar light, we clearly detect almost all host galaxies.
The median rest-frame B-band luminosity of the ERQ hosts in our sample is
10^11.2 L_Sun, or 4 L* at this redshift. Two of the ten hosts of ERQs are in
ongoing mergers. The hosts of the type 2 quasar candidates are 0.6 dex less
luminous, with 2/6 in likely ongoing mergers. Intriguingly, despite some signs
of interaction and presence of low-mass companions, our objects do not show
nearly as much major merger activity as do high-redshift radio-loud galaxies
and quasars. In the absence of an overt connection to major ongoing gas-rich
merger activity, our observations are consistent with a model in which the
near-Eddington accretion and strong feedback of ERQs are associated with
relatively late stages of mergers resulting in early-type remnants. These
results are in some tension with theoretical expectations of galaxy formation
models, in which rapid black hole growth occurs within a short time of a major
merger. Type 2 quasar candidates are less luminous, so they may instead be
powered by internal galactic processes.Comment: Accepted to MNRAS; 23 pages, including 9 figures and 2 table