203 research outputs found
Moderate-Luminosity Growing Black Holes From 1.25 < z < 2.7: Varied Accretion In Disk-Dominated Hosts
We compute black hole masses and bolometric luminosities for 57 active
galactic nuclei (AGN) in the redshift range 1.25 < z < 2.67, selected from the
GOODS-South deep multi-wavelength survey field via their X-ray emission. We
determine host galaxy morphological parameters by separating the galaxies from
their central point sources in deep HST images, and host stellar masses and
colors by multi-wavelength SED fitting. 90% of GOODS AGN at these redshifts
have detected rest-frame optical nuclear point sources; bolometric luminosities
range from 2e43 - 2e46 erg/s. The black holes are growing at a range of
accretion rates, with at least 50% of the sample having L/L_Edd < 0.1. 70% of
host galaxies have stellar masses M* > 1e10 M_sun, with a range of colors
suggesting a complex star formation history. We find no evolution of AGN
bolometric luminosity within the sample, and no correlation between AGN
bolometric luminosity and host stellar mass, color or morphology. Fully half
the sample of host galaxies is disk-dominated, with another 25% having strong
disk components. Fewer than 15% of the systems appear to be at some stage of a
major merger. These moderate-luminosity AGN hosts are therefore inconsistent
with a dynamical history dominated by mergers strong enough to destroy disks,
indicating minor mergers or secular processes dominate the co-evolution of
galaxies and their central black holes at z ~ 2.Comment: 11 pages, 6 figures, accepted to ApJ. Sersic indices, AGN/galaxy
luminosity ratios, stellar masses etc. provided in Table
Recommended from our members
Patterns and Determinants of Economic Reform in Transition Economies: 1990-1998
The paper begins by presenting a framework for evaluating transition. The framework identifies categories of influences or "determinants of transition" and how they interact to produce short-term impacts, intermediate outcomes, and long-term socio-economic performance. Among the determinants are the so-called "initial conditions" of transition. The initial conditions describe the situation a country finds itself at the start of the process and are a mixture of geographic fixed characteristics, hard-to-change institutional and economic conditions, and relatively easy-to-change policy conditions. The paper then uses the initial conditions to create a country cluster typology, which is used throughout the rest of the paper. Focusing on the "cluster" as the central unit of analysis underscores our belief that this method greatly simplifies the analysis while at the same time illuminating common features that would otherwise be obscured by country-specific details. This approach also recognizes that the inter-cluster differences are so profound as to make it senseless to compare, say capital market developments in Poland with the Kyrgyz Republic; their initial conditions are just too different. While it is difficult to draw lessons between countries in different clusters, the opposite is true within clusters. By specifically controlling for common initial conditions among countries within a cluster we find ourselves with a powerful assessment tool to evaluate the effectiveness of the alternative policies that countries have taken within the cluster. In brief, a cluster based analysis is a more productive approach upon which to formulate donor programs
The FIRST-2MASS Red Quasar Survey
Combining radio observations with optical and infrared color selection --
demonstrated in our pilot study to be an efficient selection algorithm for
finding red quasars -- we have obtained optical and infrared spectroscopy for
120 objects in a complete sample of 156 candidates from a sky area of 2716
square degrees. Consistent with our initial results, we find our selection
criteria -- J-K>1.7, R-K>4.0 -- yield a ~50% success rate for discovering
quasars substantially redder than those found in optical surveys. Comparison
with UVX- and optical color-selected samples shows that >~ 10% of the quasars
are missed in a magnitude-limited survey. Simultaneous two-frequency radio
observations for part of the sample indicate that a synchrotron continuum
component is ruled out as a significant contributor to reddening the quasars'
spectra. We go on to estimate extinctions for our objects assuming their red
colors are caused by dust. Continuum fits and Balmer decrements suggest E(B-V)
values ranging from near zero to 2.5 magnitudes. Correcting the K-band
magnitudes for these extinctions, we find that for K <= 14.0, red quasars make
up between 25% and 60% of the underlying quasar population; owing to the
incompleteness of the 2MASS survey at fainter K-band magnitudes, we can only
set a lower limit to the radio-detected red quasar population of >20-30%.Comment: 80 pages (single-column, preprint format) 20 figures, Accepted for
publicated in Ap
Extreme Variability in a Broad Absorption Line Quasar
CRTS J084133.15+200525.8 is an optically bright quasar at z=2.345 that has
shown extreme spectral variability over the past decade. Photometrically, the
source had a visual magnitude of V~17.3 between 2002 and 2008. Then, over the
following five years, the source slowly brightened by approximately one
magnitude, to V~16.2. Only ~1 in 10,000 quasars show such extreme variability,
as quantified by the extreme parameters derived for this quasar assuming a
damped random walk model. A combination of archival and newly acquired spectra
reveal the source to be an iron low-ionization broad absorption line (FeLoBAL)
quasar with extreme changes in its absorption spectrum. Some absorption
features completely disappear over the 9 years of optical spectra, while other
features remain essentially unchanged. We report the first definitive redshift
for this source, based on the detection of broad H-alpha in a Keck/MOSFIRE
spectrum. Absorption systems separated by several 1000 km/s in velocity show
coordinated weakening in the depths of their troughs as the continuum flux
increases. We interpret the broad absorption line variability to be due to
changes in photoionization, rather than due to motion of material along our
line of sight. This source highlights one sort of rare transition object that
astronomy will now be finding through dedicated time-domain surveys.Comment: 6 pages, 4 figures; accepted for publication in Ap
A Near-Infrared Spectral Template for Quasars
We present a near-infrared quasar composite spectrum spanning the wavelength
range 0.58 - 3.5 um. The spectrum has been constructed from observations of 27
quasars obtained at the NASA IRTF telescope and satisfying the criteria Ks <
14.5 and Mi < -23; the redshift range is 0.118 < z < 0.418. The signal-to-noise
is moderate, reaching a maximum of 150 between 1.6 and 1.9 um. While a
power-law fit to the continuum of the composite spectrum requires two breaks, a
single power-law slope of alpha=-0.92 plus a 1260 K blackbody provides an
excellent description of the spectrum from H-alpha to 3.5 um, strongly
suggesting the presence of significant quantities of hot dust in this
blue-selected quasar sample. We measure intensities and line widths for ten
lines, finding that the Paschen line ratios rule out Case B recombination. We
compute K-corrections for the J, H, K, and Spitzer 3.6 um bands which will be
useful in analyzing observations of quasars up to z=10.Comment: 39 pages, 11 figures, accepted for publication in ApJ Spectra will be
made available in the online version of the journal, once published. Until
then, if you wish to obtain the composite spectrum send an email request to
eilatg[at]astro.columbia.ed
The z=5 Quasar Luminosity Function from SDSS Stripe 82
We present a measurement of the Type I quasar luminosity function at z=5
using a large sample of spectroscopically confirmed quasars selected from
optical imaging data. We measure the bright end (M_1450<-26) with Sloan Digital
Sky Survey (SDSS) data covering ~6000 deg^2, then extend to lower luminosities
(M_1450<-24) with newly discovered, faint z~5 quasars selected from 235 deg^2
of deep, coadded imaging in the SDSS Stripe 82 region (the celestial equator in
the Southern Galactic Cap). The faint sample includes 14 quasars with spectra
obtained as ancillary science targets in the SDSS-III Baryon Oscillation
Spectroscopic Survey (BOSS), and 59 quasars observed at the MMT and Magellan
telescopes. We construct a well-defined sample of 4.7<z<5.1 quasars that is
highly complete, with 73 spectroscopic identifications out of 92 candidates.
Our color selection method is also highly efficient: of the 73 spectra
obtained, 71 are high redshift quasars. These observations reach below the
break in the luminosity function (M_1450* ~ -27). The bright end slope is steep
(beta <~ -4), with a constraint of beta < -3.1 at 95% confidence. The break
luminosity appears to evolve strongly at high redshift, providing an
explanation for the flattening of the bright end slope reported previously. We
find a factor of ~2 greater decrease in the number density of luminous quasars
(M_1450<-26) from z=5 to z=6 than from z=4 to z=5, suggesting a more rapid
decline in quasar activity at high redshift than found in previous surveys. Our
model for the quasar luminosity function predicts that quasars generate ~30% of
the ionizing photons required to keep the universe ionized at z=5.Comment: 29 pages, 22 figures, ApJ accepted (updated to published version
The Reddest Quasars
In a survey of quasar candidates selected by matching the FIRST and 2MASS
catalogs, we have found two extraordinarily red quasars. FIRST J013435.7-093102
is a 1 Jy source at z=2.216 and has B-K > 10, while FIRST J073820.1+275045 is a
2.5 mJy source at z=1.985 with B-K = 8.4. FIRST J073820.1+275045 has strong
absorption lines of MgII and CIV in the rest frame of the quasar and is highly
polarized in the rest frame ultraviolet, strongly favoring the interpretation
that its red spectral energy distribution is caused by dust reddening local to
the quasar. FIRST J073820.1+275045 is thus one of the few low radio-luminosity,
highly dust-reddened quasars known. The available observational evidence for
FIRST J013435.7-093102 leads us to conclude that it too is reddened by dust. We
show that FIRST J013435.7-093102 is gravitationally lensed, increasing the
number of known lensed, extremely dust-reddened quasars to at least three,
including MG0414-0534 and PKS1830-211. We discuss the implications of whether
these objects are reddened by dust in the host or lensing galaxies. If reddened
by their local environment, then we estimate that between 10 and 20% of the
radio-loud quasar population is reddened by dust in the host galaxy. The
discovery of FIRST J073820.1+275045 and objects now emerging from X-ray surveys
suggests the existence of an analogous radio-quiet red quasar population. Such
objects will be entirely missed by standard radio or optical quasar surveys. If
dust in the lensing galaxies is primarily responsible for the extreme redness
of the lensed quasars, then an untold number of gravitationally lensed quasars
are being overlooked.Comment: AASTEX 24 pp., 7 figs; accepted by ApJ. See also the preprint
astro-ph/0107435 by Winn et al., who independently discovered that
J013435.7-093102 is gravitationally lense
Heavily Obscured Quasar Host Galaxies at z~2 are Disks, Not Major Mergers
We explore the nature of heavily obscured quasar host galaxies at z~2 using
deep Hubble Space Telescope WFC3/IR imaging of 28 Dust Obscured Galaxies (DOGs)
to investigate the role of major mergers in driving black hole growth. The high
levels of obscuration of the quasars selected for this study act as a natural
coronagraph, blocking the quasar light and allowing a clear view of the
underlying host galaxy. The sample of heavily obscured quasars represents a
significant fraction of the cosmic mass accretion on supermassive black holes
as the quasars have inferred bolometric luminosities around the break of the
quasar luminosity function. We find that only a small fraction (4%, at most
11-25%) of the quasar host galaxies are major mergers. Fits to their surface
brightness profiles indicate that 90% of the host galaxies are either disk
dominated, or have a significant disk. This disk-like host morphology, and the
corresponding weakness of bulges, is evidence against major mergers and
suggests that secular processes are the predominant driver of massive black
hole growth. Finally, we suggest that the co-incidence of mergers and AGN
activity is luminosity dependent, with only the most luminous quasars being
triggered mostly by major mergers.Comment: 5 pages, 4 figures, 1 table. To appear as a Letter in MNRA
A Candidate Dual QSO at Cosmic Noon
We report the discovery of a candidate dual QSO at z=1.889, a redshift that
is in the era known as "cosmic noon" where most of the Universe's black hole
and stellar mass growth occurred. The source was identified in Hubble Space
Telescope WFC3/IR images of a dust-reddened QSO that showed two
closely-separated point sources at a projected distance of 0.26", or 2.2 kpc.
This red QSO was targeted for imaging to explore whether red QSOs are hosted by
merging galaxies. We subsequently obtained a spatially-resolved STIS spectrum
of the system, covering the visible spectral range, and verifying the presence
of two distinct QSO components. We also obtained high-resolution radio
continuum observations with the VLBA at 1.4 GHz (21-cm L band) and found two
sources coincident with the optical positions. The sources have similar black
hole masses, bolometric luminosities, and radio loudness parameters. However,
their colors and reddenings differ significantly. The redder QSO has a higher
Eddington ratio, consistent with previous findings. We consider the possibility
of gravitational lensing and and find that it would require extreme and
unlikely conditions. If confirmed as a bona-fide dual QSO, this system would
link dust-reddening to galaxy and supermassive black hole mergers, opening up a
new population in which to search for samples of dual AGN.Comment: Accepted for publication in ApJ Letter
Performance measurement with multiple interrelated variables and threshold target levels:evidence from retail firms in the US
In this study, we developed a DEA-based performance measurement methodology that is consistent with performance assessment frameworks such as the Balanced Scorecard. The methodology developed in this paper takes into account the direct or inverse relationships that may exist among the dimensions of performance to construct appropriate production frontiers. The production frontiers we obtained are deemed appropriate as they consist solely of firms with desirable levels for all dimensions of performance. These levels should be at least equal to the critical values set by decision makers. The properties and advantages of our methodology against competing methodologies are presented through an application to a real-world case study from retail firms operating in the US. A comparative analysis between the new methodology and existing methodologies explains the failure of the existing approaches to define appropriate production frontiers when directly or inversely related dimensions of performance are present and to express the interrelationships between the dimensions of performance
- …