924 research outputs found
Two New Gravitationally Lensed Double Quasars from the Sloan Digital Sky Survey
We report the discoveries of the two-image gravitationally lensed quasars,
SDSS J0746+4403 and SDSS J1406+6126, selected from the Sloan Digital Sky Survey
(SDSS). SDSS J0746+4403, which will be included in our lens sample for
statistics and cosmology, has a source redshift of z_s=2.00, an estimated lens
redshift of z_l~0.3, and an image separation of 1.08". SDSS J1406+6126 has a
source redshift of z_s=2.13, a spectroscopically measured lens redshift of
z_l=0.27, and an image separation of 1.98". We find that the two quasar images
of SDSS J1406+6126 have different intervening MgII absorption strengths, which
are suggestive of large variations of absorbers on kpc scales. The positions
and fluxes of both the lensed quasar systems are easily reproduced by simple
mass models with reasonable parameter values. These objects bring to 18 the
number of lensed quasars that have been discovered from the SDSS data.Comment: 25 pages, 6 figures, The Astronomical Journal accepte
Discovery of a Gravitationally Lensed Quasar from the Sloan Digital Sky Survey: SDSS J133222.62+034739.9
We report the discovery of the two-image gravitationally lensed quasar SDSS
J133222.62+034739.9 (SDSS J1332+0347) with an image separation of
Delta_theta=1.14". This system consists of a source quasar at z_s=1.445 and a
lens galaxy at z_l=0.191. The agreement of the luminosity, ellipticity and
position angle of the lens galaxy with those expected from lens model confirms
the lensing hypothesis.Comment: 16 pages, 4 figures, the Astronomical Journal accepte
Exploratory Chandra Observations of the Three Highest Redshift Quasars Known
We report on exploratory Chandra observations of the three highest redshift
quasars known (z = 5.82, 5.99, and 6.28), all found in the Sloan Digital Sky
Survey. These data, combined with a previous XMM-Newton observation of a z =
5.74 quasar, form a complete set of color-selected, z > 5.7 quasars. X-ray
emission is detected from all of the quasars at levels that indicate that the
X-ray to optical flux ratios of z ~ 6 optically selected quasars are similar to
those of lower redshift quasars. The observations demonstrate that it will be
feasible to obtain quality X-ray spectra of z ~ 6 quasars with current and
future X-ray missions.Comment: 15 pages, ApJL, in press; small revisions to address referee Comment
A Simple Likelihood Method for Quasar Target Selection
We present a new method for quasar target selection using photometric fluxes
and a Bayesian probabilistic approach. For our purposes we target quasars using
Sloan Digital Sky Survey (SDSS) photometry to a magnitude limit of g=22. The
efficiency and completeness of this technique is measured using the Baryon
Oscillation Spectroscopic Survey (BOSS) data, taken in 2010. This technique was
used for the uniformly selected (CORE) sample of targets in BOSS year one
spectroscopy to be realized in the 9th SDSS data release. When targeting at a
density of 40 objects per sq-deg (the BOSS quasar targeting density) the
efficiency of this technique in recovering z>2.2 quasars is 40%. The
completeness compared to all quasars identified in BOSS data is 65%. This paper
also describes possible extensions and improvements for this techniqueComment: Updated to accepted version for publication in the Astrophysical
Journal. 10 pages, 10 figures, 3 table
Spectral Energy Distributions and Multiwavelength Selection of Type 1 Quasars
We present an analysis of the mid-infrared (MIR) and optical properties of type 1 (broad-line) quasars detected by the Spitzer Space Telescope. The MIR color-redshift relation is characterized to z ~ 3, with predictions to z = 7. We demonstrate how combining MIR and optical colors can yield even more efficient selection of active galactic nuclei (AGNs) than MIR or optical colors alone. Composite spectral energy distributions (SEDs) are constructed for 259 quasars with both Sloan Digital Sky Survey and Spitzer photometry, supplemented by near-IR, GALEX, VLA, and ROSAT data, where available. We discuss how the spectral diversity of quasars influences the determination of bolometric luminosities and accretion rates; assuming the mean SED can lead to errors as large as 50% for individual quasars when inferring a bolometric luminosity from an optical luminosity. Finally, we show that careful consideration of the shape of the mean quasar SED and its redshift dependence leads to a lower estimate of the fraction of reddened/obscured AGNs missed by optical surveys as compared to estimates derived from a single mean MIR to optical flux ratio
SDSS J115517.35+634622.0: A Newly Discovered Gravitationally Lensed Quasar
We report the discovery of SDSSJ115517.35+634622.0, a previously unknown
gravitationally lensed quasar. The lens system exhibits two images of a quasar, with an image separation of 1{\farcs}832 \pm 0.007 . Near-IR
imaging of the system reveals the presence of the lensing galaxy between the
two quasar images. Based on absorption features seen in the Sloan Digital Sky
Survey (SDSS) spectrum, we determine a lens galaxy redshift of .
The lens is rather unusual in that one of the quasar images is only
0{\farcs}22\pm0{\farcs}07 () from the center of the
lens galaxy and photometric modeling indicates that this image is significantly
brighter than predicted by a SIS model. This system was discovered in the
course of an ongoing search for strongly lensed quasars in the dataset from the
SDSS.Comment: 18 pages, 6 figures. Accepted for publication in A
Dust Reddening in SDSS Quasars
We explore the form of extragalactic reddening toward quasars using a sample
of 9566 quasars with redshifts 0<z<2.2, and accurate optical colors from the
Sloan Digital Sky Survey (SDSS). We confirm that dust reddening is the primary
explanation for the red ``tail'' of the color distribution of SDSS quasars. Our
fitting to 5-band photometry normalized by the modal quasar color as a function
of redshift shows that this ``tail'' is well described by SMC-like reddening
but not by LMC-like, Galactic, or Gaskell et al. (2004) reddening. Extension to
longer wavelengths using a subset of 1886 SDSS-2MASS matches confirms these
results at high significance. We carry out Monte-Carlo simulations that match
the observed distribution of quasar spectral energy distributions using a
Lorentzian dust reddening distribution; 2% of quasars selected by the main SDSS
targeting algorithm (i.e., which are not extincted out of the sample) have
E_{B-V} > 0.1; less than 1% have E_{B-V} > 0.2, where the extinction is
relative to quasars with modal colors. Reddening is uncorrelated with the
presence of intervening narrow-line absorption systems, but reddened quasars
are much more likely to show narrow absorption at the redshift of the quasar
than are unreddened quasars. Thus the reddening towards quasars is dominated by
SMC-like dust at the quasar redshift.Comment: 29 pages including 8 figures. AJ, September 2004 issu
Quasar Absorption Lines as a Function of Quasar Orientation Measures
We present high resolution radio data at 3.5 and 20 cm from the VLA in the A
configuration for 144 quasars with known CIV absorption line properties. Using
these measurements, we compare and contrast a number of quasar orientation
indicators. These quantities are used to study the velocity distribution of CIV
absorption lines as a function of quasar orientation measures. That there is an
excess of narrow, high-velocity CIV absorbers in flat-spectrum quasars as
compared to steep-spectrum quasars is confirmed. Orientation indicators that
are not based upon spectral indices (e.g. R_V and the core-to-lobe ratio) do
not show the same effect. These seemingly contradictory results may be
reconciled if the absorber distribution is not strictly a function of
orientation, but rather a function of intrinsic radio properties that may or
may not be good orientation indicators.Comment: 34 pages, 7 figures, 3 tables, ApJ in pres
Photometric redshifts and quasar probabilities from a single, data-driven generative model
We describe a technique for simultaneously classifying and estimating the
redshift of quasars. It can separate quasars from stars in arbitrary redshift
ranges, estimate full posterior distribution functions for the redshift, and
naturally incorporate flux uncertainties, missing data, and multi-wavelength
photometry. We build models of quasars in flux-redshift space by applying the
extreme deconvolution technique to estimate the underlying density. By
integrating this density over redshift one can obtain quasar flux-densities in
different redshift ranges. This approach allows for efficient, consistent, and
fast classification and photometric redshift estimation. This is achieved by
combining the speed obtained by choosing simple analytical forms as the basis
of our density model with the flexibility of non-parametric models through the
use of many simple components with many parameters. We show that this technique
is competitive with the best photometric quasar classification
techniques---which are limited to fixed, broad redshift ranges and high
signal-to-noise ratio data---and with the best photometric redshift techniques
when applied to broadband optical data. We demonstrate that the inclusion of UV
and NIR data significantly improves photometric quasar--star separation and
essentially resolves all of the redshift degeneracies for quasars inherent to
the ugriz filter system, even when included data have a low signal-to-noise
ratio. For quasars spectroscopically confirmed by the SDSS 84 and 97 percent of
the objects with GALEX UV and UKIDSS NIR data have photometric redshifts within
0.1 and 0.3, respectively, of the spectroscopic redshift; this amounts to about
a factor of three improvement over ugriz-only photometric redshifts. Our code
to calculate quasar probabilities and redshift probability distributions is
publicly available
A Snapshot Survey for Gravitational Lenses Among z>=4.0 Quasars: I. The z>5.7 Sample
Over the last few years, the Sloan Digital Sky Survey (SDSS) has discovered
several hundred quasars with redshift between 4.0 and 6.4. Including the
effects of magnification bias, one expects a priori that an appreciable
fraction of these objects are gravitationally lensed. We have used the Advanced
Camera for Surveys on the Hubble Space Telescope to carry out a snapshot
imaging survey of high-redshift SDSS quasars to search for gravitationally
split lenses. This paper, the first in a series reporting the results of the
survey, describes snapshot observations of four quasars at z = 5.74, 5.82, 5.99
and 6.30, respectively. We find that none of these objects has a lensed
companion within 5 magnitudes with a separation larger than 0.3 arcseconds;
within 2.5 magnitudes, we can rule out companions within 0.1 arcseconds. Based
on the non-detection of strong lensing in these four systems, we constrain the
z~6 luminosity function to a slope of beta>-4.63 (3 sigma), assuming a break in
the quasar luminosity function at M_{1450}^*=-24.0. We discuss the implications
of this constraint on the ionizing background due to quasars in the early
universe. Given that these quasars are not highly magnified, estimates of the
masses of their central engines by the Eddington argument must be taken
seriously, possibly challenging models of black hole formation.Comment: 23 pages, 8 figures, 2 tables, submitted to A
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