28 research outputs found
Photometric Selection of QSO Candidates From GALEX Sources
We present a catalog of 36,120 QSO candidates from the Galaxy Evolution
Explorer (GALEX) Release Two (GR2) UV catalog and the USNO-A2.0 optical
catalog. The selection criteria are established using known quasars from the
Sloan Digital Sky Survey (SDSS). The SDSS sample is then used to assign
individual probabilities to our GALEX-USNO candidates. The mean probability is
~50%, and would rise to ~65% if better morphological information than that from
USNO were available to eliminate galaxies. The sample is ~40% complete for
i<=19.1. Candidates are cross-identified in 2MASS, FIRST, SDSS, and XMM-Newton
Slewing Survey (XMMSL1), whenever such counterparts exist. The present catalog
covers the 8000 square degrees of GR2 lying above 25 degrees Galactic latitude,
but can be extended to all 24,000 square degress that satisfy this criterion as
new GALEX data become available.Comment: AASTeX v5.2, 31 pages, 9 figures. Accepted for publication in ApJ.
Extended tables available in the online edition of the journa
Evolution of the UV Excess in Early-Type Galaxies
We examine the UV emission from luminous early-type galaxies as a function of
redshift. We perform a stacking analysis using Galaxy Evolution Explorer
(GALEX) images of galaxies in the NOAO Deep Wide Field Survey (NDWFS) Bo\"otes
field and examine the evolution in the UV colors of the average galaxy. Our
sample, selected to have minimal ongoing star formation based on the optical to
mid-IR SEDs of the galaxies, includes 1843 galaxies spanning the redshift range
. We find evidence that the strength of the UV excess
decreases, on average, with redshift, and our measurements also show moderate
disagreement with previous models of the UV excess. Our results show little
evolution in the shape of the UV continuum with redshift, consistent either
with the binary model for the formation of Extreme Horizontal Branch (EHB)
stars or with no evolution in EHB morphology with look-back time. However, the
binary formation model predicts that the strength of the UV excess should also
be relatively constant, in contradiction with our measured results. Finally, we
see no significant influence of a galaxy's environment on the strength of its
UV excess.Comment: 30 pages, 10 figures; accepted by ApJ. Modified from original version
to reflect referee's comment
The Clustering and Halo Masses of Star Forming Galaxies at z<1
We present clustering measurements and halo masses of star forming galaxies
at 0.2 < z < 1.0. After excluding AGN, we construct a sample of 22553 24 {\mu}m
sources selected from 8.42 deg^2 of the Spitzer MIPS AGN and Galaxy Evolution
Survey of Bo\"otes. Mid-infrared imaging allows us to observe galaxies with the
highest star formation rates (SFRs), less biased by dust obscuration afflicting
the optical bands. We find that the galaxies with the highest SFRs have optical
colors which are redder than typical blue cloud galaxies, with many residing
within the green valley. At z > 0.4 our sample is dominated by luminous
infrared galaxies (LIRGs, L_TIR > 10^11 Lsun) and is comprised entirely of
LIRGs and ultra-luminous infrared galaxies (ULIRGs, L_TIR > 10^12 Lsun) at z >
0.6. We observe weak clustering of r_0 = 3-6 Mpc/h for almost all of our star
forming samples. We find that the clustering and halo mass depend on L_TIR at
all redshifts, where galaxies with higher L_TIR (hence higher SFRs) have
stronger clustering. Galaxies with the highest SFRs at each redshift typically
reside within dark matter halos of M_halo ~ 10^12.9 Msun/h. This is consistent
with a transitional halo mass, above which star formation is largely truncated,
although we cannot exclude that ULIRGs reside within higher mass halos. By
modeling the clustering evolution of halos, we connect our star forming galaxy
samples to their local descendants. Most star forming galaxies at z < 1.0 are
the progenitors of L < 2.5L* blue galaxies in the local universe, but star
forming galaxies with the highest SFRs (L_TIR >10^11.7 Lsun) at 0.6<z<1.0 are
the progenitors of early-type galaxies in denser group environments.Comment: 18 pages, 16 figures, 2 tables. Accepted for publication in the
Astrophysical Journa
A Multi-Wavelength Study of Low Redshift Cluster of Galaxies II. Environmental Impact on Galaxy Growth
Galaxy clusters provide powerful laboratories for the study of galaxy
evolution, particularly the origin of correlations of morphology and star
formation rate (SFR) with density. We construct visible to MIR spectral energy
distributions (SEDs) of cluster galaxies and use them to measure stellar masses
and SFRs in eight low redshift clusters, which we examine as a function of
environment. A partial correlation analysis indicates that SFR depends strongly
on R/R200 (>99.9% confidence) and is independent of projected local density at
fixed radius. SFR also shows no residual dependence on stellar mass. We
therefore conclude that interactions with the intra-cluster medium drive the
evolution of SFRs in cluster galaxies. A merged sample of galaxies from the
five most complete clusters shows \propto(R/R200)^(1.3+/-0.7) for galaxies
with R/R200<0.4. A decline in the fraction of SFGs toward the cluster center
contributes most of this effect, but it is accompanied by a reduction in SFRs
among star-forming galaxies (SFGs) near the cluster center. The increase in the
fraction of SFGs toward larger R/R200 and the isolation of SFGs with reduced
SFRs near the cluster center are consistent with ram pressure stripping as the
mechanism to truncate star formation in galaxy clusters. We conclude that
stripping drives the properties of SFGs over the range of radii we examine. We
also find that galaxies near the cluster center are more massive than galaxies
farther out in the cluster at ~3.5\sigma, which suggests that cluster galaxies
experience dynamical relaxation during the course of their evolution.Comment: 22 ApJ pages, 11 figures. Submitted to Ap
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
The Color Variability of Quasars
We quantify quasar color-variability using an unprecedented variability
database - ugriz photometry of 9093 quasars from SDSS Stripe 82, observed over
8 years at ~60 epochs each. We confirm previous reports that quasars become
bluer when brightening. We find a redshift dependence of this blueing in a
given set of bands (e.g. g and r), but show that it is the result of the flux
contribution from less-variable or delayed emission lines in the different SDSS
bands at different redshifts. After correcting for this effect, quasar
color-variability is remarkably uniform, and independent not only of redshift,
but also of quasar luminosity and black hole mass. The color variations of
individual quasars, as they vary in brightness on year timescales, are much
more pronounced than the ranges in color seen in samples of quasars across many
orders of magnitude in luminosity. This indicates distinct physical mechanisms
behind quasar variability and the observed range of quasar luminosities at a
given black hole mass - quasar variations cannot be explained by changes in the
mean accretion rate. We do find some dependence of the color variability on the
characteristics of the flux variations themselves, with fast, low-amplitude,
brightness variations producing more color variability. The observed behavior
could arise if quasar variability results from flares or ephemeral hot spots in
an accretion disc.Comment: Accepted for publication in ApJ - in press, 17 pages, 14 figures -
v2: abstract typo corrected & reference clean-u
Type II-P Supernovae from the SDSS-II Supernova Survey and the Standardized Candle Method
We apply the Standardized Candle Method (SCM) for Type II Plateau supernovae
(SNe II-P), which relates the velocity of the ejecta of a SN to its luminosity
during the plateau, to 15 SNe II-P discovered over the three season run of the
Sloan Digital Sky Survey - II Supernova Survey. The redshifts of these SNe -
0.027 < z < 0.144 - cover a range hitherto sparsely sampled in the literature;
in particular, our SNe II-P sample contains nearly as many SNe in the Hubble
flow (z > 0.01) as all of the current literature on the SCM combined. We find
that the SDSS SNe have a very small intrinsic I-band dispersion (0.22 mag),
which can be attributed to selection effects. When the SCM is applied to the
combined SDSS-plus-literature set of SNe II-P, the dispersion increases to 0.29
mag, larger than the scatter for either set of SNe separately. We show that the
standardization cannot be further improved by eliminating SNe with positive
plateau decline rates, as proposed in Poznanski et al. (2009). We thoroughly
examine all potential systematic effects and conclude that for the SCM to be
useful for cosmology, the methods currently used to determine the Fe II
velocity at day 50 must be improved, and spectral templates able to encompass
the intrinsic variations of Type II-P SNe will be needed.Comment: Accepted for publication by ApJ; data used in this paper can be
downloaded from http://sdssdp47.fnal.gov/sdsssn/photometry/SNIIp.tgz;
citation errors correcte