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Princeton University Dept. of Astrophysical Sciences
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QuasarsMost of my recent research has been focused on the Sloan Digital Sky Survey (SDSS), in particular SDSS quasars. This work began in the summer of 1995 when I obtained photometric data for a set of known quasars in five bands with a filter system similar to the SDSS system. These data were taken for the purpose of testing the ability of the SDSS system to separate quasar colors from the colors of the stellar locus. We demonstrated that SDSS photometry is an effective means of separating quasars from the stellar locus. The filters, in particular the u and g filters, allow for high efficiency quasar candidate selection based on color (for objects with z<2.5). Quasars with z>3.1 can be distinguished using one or more color-color plots. [Richards et al. 1997, PASP, 109, 39] Continuing this project, H. Newberg, M. Richmond, X. Fan and I obtained more imaging data of known quasars using the USNO 1.0m telescope in Flagstaff, AZ. The resulting paper presented the first catalog of objects measured with SDSS filters. Color plots of the data show that stars, galaxies, and quasars are fairly well separated by color; the stellar locus populates a ribbon-like subset of color-color-color space. The stars were compared with synthetic photometry from Kurucz models; the agreement was found to be consistent with the errors in the data. The stellar locus was found to move in color space by about a tenth of a magnitude from r=14 to r=19.5. The shift is consistent with a shift in the metallicity from about [Fe/H] = -1 to [Fe/H] = -2. [Newberg et al. 1999, ApJS, 123, 377] During 2000/2001, my efforts were focused on developing the SDSS
Quasar Target Selection Algorithm, which was completed in Summer
2001. [Richards
et al. 2002b, AJ, 123, 2945] I had the responsibility for the
final development and testing of the quasar selection code. To aid in
this process, I led an effort to study the colors of quasars in the
SDSS photometric system. [Richards
et al. 2001, AJ, 121, 2308] We studied the colors of 2625 quasars
from 0 With the plethora of SDSS data coming in, I have either begun or
become involved in a number of other studies. The first of these
projects being a catalog of the first 3814 bona-fide quasars
discovered by the SDSS, 3000 of which are new quasars. [Schneider,
Richards, Fan et al. 2002, AJ, 123, 567] In January of 2003, we
will release the next SDSS quasar catalog, which will contain over
20,000 quasars.
In collaboration with Daniel Vanden Berk (Fermilab), I have been
studying the properties of quasars through the use of quasar composite
spectra. We created an SDSS composite quasar spectrum that is also of
use in the development of the quasar target selection algorithm for
the SDSS. [Vanden
Berk, Richards, Bauer et al. 2001, AJ, 122, 549] In addition to
this general quasar composite, we have created a number of different
composite quasar spectra in order to study how the spectra of quasars
change with changing quasar properties. We are currently
investigating the redshift dependence, luminosity dependence, radio
dependence, and color dependence of quasars using composite spectra.
We are particularly excited about an investigation of the well-known
velocity shifts of high-ionization quasar emission lines as compared
to low-ionization emission lines. There are very clear correlations
between these emission line shifts and other properties of quasars
that are related to Boroson \& Green (1992) type eigenvector analysis;
these properties will help to shed light on the physics of quasars.
[Richards
et al. 2002, AJ, 124, 1]
In collaboration with a number of colleagues, I have also been
studying Broad Absorption Line (BAL) quasars. With PSU undergraduate,
Tim Reichard, I have been studying the continuum properties of BALs
from the SDSS in order to determine if they show signs of being
reddened. In the process we have compiled a catalog of 224 BAL
quasars in collaboration with a group at JHU and Pat Hall. [Reichard,
Richards, et al. 2002, in preparation] Also, with Pat Hall (Princeton)
and others, I have been studying the most unusual examples of SDSS BAL
quasars that show extreme cases of absorption. [Hall
et al. 2002, ApJS, 141, 267]
The discovery and study of very high redshift quasars in the SDSS is
another project that I am working on. In a series of papers, we have
presented the discovery of over 90 new z>4 quasars. [e.g., Anderson
et al. 2001, AJ 122, 503] Using these quasars, we have studied the
high-redshift quasar luminosity function. [Fan
et al. 2001, AJ, 121, 54] Included in these high-z quasars are 26
of the 30 more distant quasars, the three highest of which have
z=5.8,6.2,6.3. [Fan
et al. 2002, AJ, 122, 2833] The most distant of these objects has
been used as a probe of the re-ionization epoch in the early Universe
through the study of the Gunn-Peterson (1965) trough. [Becker
et al. 2002, AJ, 122, 2850]
As a graduate student at The University of Chicago, my research
focused on absorption lines in quasars, particularly those that are
intrinsic to quasars. Don York's group at Chicago has compiled a
catalog of all known quasar absorption line systems. Using this
revised catalog we presented the results of a study of QSO Absorption
Line Systems (QSOALS) with respect to intrinsic quasar properties. We
searched the literature for 6 and 20 cm radio flux densities and
studied 20 cm contour plots from the Very Large Array (VLA) in order
to compare the absorption properties with radio luminosity, radio
spectral index and radio morphology.
BWe found that the distribution of narrow, CIV absorption systems
with relative velocities exceeding 5000 km/s (with respect to the rest
frame of the quasar) is dependent on the intrinsic properties of
quasars. The properties of CIV absorption lines are different towards
steep-spectrum radio quasars than towards flat-spectrum radio quasars.
These observations are apparently inconsistent with the hypothesis
that these systems are entirely due to intervening galaxies; it seems
that the contamination of the intervening systems by those that are
intrinsic to the environment of the quasar is larger than expected.
There must exist a population of narrow CIV absorbers that are
intrinsic to quasars, but that are ejected at very high velocities
(v>5000km/s) -- in contrast to the standard picture in which all
narrow CIV absorption lines with large relative velocities are caused
by intervening galaxies. [Richards
et al. 1999, ApJ, 513, 576]
I continued this work using a radio-selected sample of quasars from
the Faint Images of the Radio Sky at Twenty centimeters (FIRST) Bright
Quasar Survey (FBQS; Gregg et al. 1998, White et al. 2000, Becker et
al. 2001), which is another project to which I have contributed.
Using moderate resolution spectroscopy of z~2.5 quasars, I
investigated the possibility that some fraction of narrow CIV
absorption systems are likely to be material ejected from quasars. I
showed that, for flat-spectrum quasars, there is indeed an excess of
narrow CIV absorbers as compared to the average for those absorption
systems that are supposed to be caused by intervening galaxies. No
such excess was found for MgII absorbers. These results are
consistent with a picture in which MgII absorption is predominantly
caused by intervening galaxies, whereas higher ionization absorption
is caused by both intervening galaxies and a population of intrinsic,
narrow absorbers. [Richards
2001, ApJS, 133, 53] In a related paper, using high-resolution
images from the VLA in the A configuration, we studied the
distribution of quasar absorption lines as a function of quasar
orientation measures such as the radio spectral index and the
core-to-lobe ratio. [Richards
et al. 2001, ApJ, 547, 635] Further evidence for intrinsic, but
narrow absorption comes from an investigation of a FIRST quasar that
has more CIV absorption systems than have ever been seen before in a
quasar spectrum. [Richards
et al. 2002, ApJ, 567, L13]
Once the SDSS started to produce data, a previous interest in
gravitational lensing was rekindled, and I began to look for
gravitational lenses in the SDSS commissioning data. I, along with a
group of SDSS scientists (particularly Ed Turner, Bart Pindor, Josh
Frieman, and Dave Johnston) have been taking spectra of lens
candidates found in the SDSS imaging data. It is expected that the
SDSS photometric sample will contain of order 1000 gravitationally
lensed quasars: substantially more lenses than are currently known.
To date, the group has turned up four gravitationally lensed quasars.
We are actively following up on the SDSS imaging and spectroscopic
data to look for additional lensed quasars; this includes an HST
snapshot survey of all of the z>4 SDSS quasars in collaboration with
Michael Strauss.
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Gordon Richards (gtr[at]astro.princeton.edu) Last Modified: 16 October 2002 |