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[show abstract]
[hide abstract]
ABSTRACT: We homogeneously analyzed the \chandra\ X-ray observations of 10
gravitational lenses, HE 0047-1756, QJ 0158-4325, SDSS 0246-0805, HE 0435-1223,
SDSS 0924+0219, SDSS 1004+4112, HE 1104-1805, PG 1115+080, Q 1355-2257, and Q
2237+0305, to measure the differential X-ray absorption between images, the
metallicity, and the dust-to-gas ratio of the lens galaxies. We detected
differential absorption in all lenses except SDSS 0924+0219 and HE 1104-1805.
This doubles the sample of dust-to-gas ratio measurements in cosmologically
distant lens galaxies. We successfully measured the gas phase metallicity of
three lenses, Q 2237+0305, SDSS 1004+4112, and B 1152+199 from the X-ray
spectra. Our results suggest a linear correlation between metallicity and
dust-to-gas ratio (i.e., a constant metal-to-dust ratio), consistent with what
is found for nearby galaxies. We obtain an average dust-to-gas ratio
$E(B-V)/N_H=1.17^{+0.41}_{-0.31} \times 10^{-22}\rm mag\,cm^2\,atom^{-1}$ in
the lens galaxies, with an intrinsic scatter of $\rm0.3\,dex$. Combining these
results with data from GRB afterglows and quasar foreground absorbers, we found
a mean dust-to-gas ratio $\mdtg,$ now significantly lower than the average
Galactic value, $1.7\,\times 10^{-22}\,\rm mag\, cm^{2}\, atoms^{-1}.$ This
suggests evolution of dust-to-gas ratios with redshift and lower average
metallicities for the higher redshift galaxies, consistent with current metal
and dust evolution models of interstellar medium. The slow evolution in the
metal-to-dust ratio with redshift implies very rapid dust formation in high
redshift ($z>2$) galaxies.
05/2013;
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Szymon Kozlowski,
Christopher A. Onken, Christopher S. Kochanek,
Andrzej Udalski,
M. K. Szymanski,
M. Kubiak,
G. Pietrzynski,
I. Soszynski,
L. Wyrzykowski,
K. Ulaczyk,
R. Poleski,
P. Pietrukowicz,
J. Skowron,
M. Meixner,
A. Z. Bonanos
[show abstract]
[hide abstract]
ABSTRACT: The Magellanic Quasars Survey (MQS) has now increased the number of quasars
known behind the Magellanic Clouds by almost an order of magnitude. All survey
fields in the Large Magellanic Cloud (LMC) and 70% of those in the Small
Magellanic Cloud (SMC) have been observed. The targets were selected from the
third phase of the Optical Gravitational Lensing Experiment (OGLE-III) based on
their optical variability, mid-IR and/or X-ray properties. We spectroscopically
confirmed 758 (565 LMC and 193 SMC) quasars behind the Clouds, of which 94%
(527 LMC and 186 SMC) are newly identified. The MQS quasars have long-term (12
years and growing for OGLE), high-cadence light curves, enabling unprecedented
variability studies of quasars. The MQS quasars also provide a dense reference
grid for measuring both the internal and bulk proper motions of the Clouds, and
50 quasars are bright enough (I<18 mag) for absorption studies of the
interstellar/galactic (ISM/IGM) medium of the Clouds.
05/2013;
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[show abstract]
[hide abstract]
ABSTRACT: The gravitationally lensed quasar HE 1104-1805 has been observed at a variety
of wavelengths ranging from the mid-infrared to X-ray for nearly 20 years. We
combine flux ratios from the literature, including recent Chandra data, with
new observations from the SMARTS telescope and HST, and use them to investigate
the spatial structure of the central regions using a Bayesian Monte Carlo
analysis of the microlensing variability. The wide wavelength coverage allows
us to constrain not only the accretion disk half-light radius r_1/2, but the
power-law slope \xi\ of the size-wavelength relation r_1/2 ~ \lambda^\xi. With
a logarithmic prior on the source size, the (observed-frame) R-band half-light
radius log(r_1/2/cm) is 16.0+0.3-0.4, and the slope \xi\ is 1.0+0.30-0.56. We
put upper limits on the source size in soft (0.4-1.2 keV) and hard (1.2-8 keV)
X-ray bands, finding 95% upper limits on log (r_1/2/cm) of 15.33 in both bands.
A linear prior yields somewhat larger sizes, particularly in the X-ray bands.
For comparison, the gravitational radius, using a black hole mass estimated
using the H\beta\ line, is log(r_g/cm) = 13.94. We find that the accretion disk
is probably close to face-on, with cos i = 1.0 being four times more likely
than cos i = 0.5. We also find probability distributions for the mean mass of
the stars in the foreground lensing galaxy, the direction of the transverse
peculiar velocity of the lens, and the position angle of the projected
accretion disk's major axis (if not face-on).
04/2013;
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[show abstract]
[hide abstract]
ABSTRACT: We present 11.2 micron observations of the gravitationally lensed, radio-loud
z_s=2.64 quasar MG0414+0534, obtained using the Michelle camera on Gemini
North. We find a flux ratio anomaly of A2/A1= 0.93 +/- 0.02 for the quasar
images A1 and A2. When combined with the 11.7 micron measurements from Minezaki
et al.\ (2009), the A2/A1 flux ratio is nearly 5-sigma from the expected ratio
for a model based on the two visible lens galaxies. The mid-IR flux ratio
anomaly can be explained by a satellite (substructure), 0.3" Northeast of image
A2, as can the detailed VLBI structures of the jet produced by the quasar. When
we combine the mid-IR flux ratios with high-resolution VLBI measurements, we
find a best-fit mass of 10^(7.3+/- 0.2) M_sol inside the Einstein radius for a
satellite substructure modeled as a singular isothermal sphere at the redshift
of the main lens (z_l=0.96). We are unable to set an interesting limit on the
mass to light ratio due to its proximity to the quasar image A2. While the
observations used here were technically difficult, surveys of flux anomalies in
gravitational lenses with the James Webb Space Telescope will be simple, fast,
and should well constrain the abundance of substructure in dark matter haloes.
12/2012;
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Asantha Cooray,
Joseph Smidt,
Francesco De Bernardis,
Yan Gong,
Daniel Stern,
Matthew L N Ashby,
Peter R Eisenhardt,
Christopher C Frazer,
Anthony H Gonzalez, Christopher S Kochanek,
Szymon Kozłowski,
Edward L Wright
[show abstract]
[hide abstract]
ABSTRACT: Unresolved anisotropies of the cosmic near-infrared background radiation are expected to have contributions from the earliest galaxies during the epoch of reionization and from faint, dwarf galaxies at intermediate redshifts. Previous measurements were unable to pinpoint conclusively the dominant origin because they did not sample spatial scales that were sufficiently large to distinguish between these two possibilities. Here we report a measurement of the anisotropy power spectrum from subarcminute to one-degree angular scales, and find the clustering amplitude to be larger than predicted by the models based on the two existing explanations. As the shot-noise level of the power spectrum is consistent with that expected from faint galaxies, a new source population on the sky is not necessary to explain the observations. However, a physical mechanism that increases the clustering amplitude is needed. Motivated by recent results related to the extended stellar light profile in dark-matter haloes, we consider the possibility that the fluctuations originate from intrahalo stars of all galaxies. We find that the measured power spectrum can be explained by an intrahalo light fraction of 0.07 to 0.2 per cent relative to the total luminosity in dark-matter haloes of 10(9) to 10(12) solar masses at redshifts of about 1 to 4.
Nature 10/2012; 490(7421):514-6. · 36.28 Impact Factor
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Asantha Cooray,
Joseph Smidt,
Francesco De Bernardis,
Yan Gong,
Daniel Stern,
Matthew L. N. Ashby,
Peter R. Eisenhardt,
Christopher C. Frazer,
Anthony H. Gonzalez, Christopher S. Kochanek,
Szymon Kozlowski,
Edward L. Wright
[show abstract]
[hide abstract]
ABSTRACT: Unresolved near-infrared background anisotropies are expected to have
contributions from the earliest galaxies during reionization and faint, dwarf
galaxies at intermediate redshifts. Previous measurements were unable to
conclusively pinpoint the dominant origin because they did not sample spatial
scales that were sufficiently large to distinguish between these two
possibilities. Here we report a measurement of the anisotropy power spectrum
from sub-arcminute to one degree angular scales and find the clustering
amplitude to be larger than the model predictions involving the two existing
explanations. As the shot-noise level of the power spectrum is consistent with
that expected from faint galaxies, a new source population on the sky is not
necessary to explain the observations. A physical mechanism that increases the
clustering amplitude, however, is needed. Motivated by recent results related
to the extended stellar light profile in dark matter halos, we consider the
possibility that the fluctuations originate from diffuse intrahalo stars of all
galaxies. We find that the measured power spectrum can be explained by an
intrahalo light fraction of 0.07 to 0.2 % relative to the total luminosity in
dark matter halos of masses log(M/M_Sun) ~ 9 to 12 at redshifts of ~ 1 to 4.
Nature 10/2012; 490:514. · 36.28 Impact Factor
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Roberto J. Assef,
Daniel Stern, Christopher S. Kochanek,
Andrew W. Blain,
Mark Brodwin,
Michael J. I. Brown,
Emilio Donoso,
Peter R. M. Eisenhardt,
Buell T. Jannuzi,
Thomas H. Jarrett,
S. Adam Stanford,
Chao-Wei Tsai,
Jingwen Wu,
Lin Yan
[show abstract]
[hide abstract]
ABSTRACT: Stern et al.(2012) presented a study of WISE selection of AGN in the 2 deg^2
COSMOS field, finding that a simple criterion W1-W2>=0.8 provides a highly
reliable and complete AGN sample for W2<15.05, where the W1 and W2 passbands
are centered at 3.4 and 4.6 microns, respectively. Here we extend this study
using the larger 9 deg^2 NOAO Deep Wide-Field Survey Bootes field which also
has considerably deeper WISE observations than the COSMOS field, and find that
this simple color-cut significantly loses reliability at fainter fluxes. We
define a modified selection criterion combining the W1-W2 color and the W2
magnitude to provide highly reliable or highly complete AGN samples for fainter
WISE sources. In particular, we define a color-magnitude cut that finds 130+/-4
deg^-2 AGN candidates for W2<17.11 with 90% reliability. Using the extensive UV
through mid-IR broad-band photometry available in this field, we study the
spectral energy distributions of WISE AGN candidates. As expected, the WISE AGN
selection is biased towards objects where the AGN dominates the bolometric
luminosity output, and that it can identify highly obscured AGN. We study the
distribution of reddening in the AGN sample and discuss a formalism to account
for sample incompleteness based on the step-wise maximum-likelihood method of
Efstathiou et al.(1988). The resulting dust obscuration distributions depend
strongly on AGN luminosity, consistent with the trend expected for a Simpson
(2005) receding torus. At L_AGN~3x10^44 erg/s, 29+/-7% of AGN are observed as
Type 1, while at ~4x10^45 erg/s the fraction is 64+/-13%. The distribution of
obscuration values suggests that dust in the torus is present as both a diffuse
medium and in optically thick clouds.
09/2012;
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John J. Ruan,
Scott F. Anderson,
Chelsea L. MacLeod,
Andrew C. Becker,
T. H. Burnett,
James R. A. Davenport,
Zeljko Ivezic, Christopher S. Kochanek,
Richard M. Plotkin,
Branimir Sesar,
J. Scott Stuart
[show abstract]
[hide abstract]
ABSTRACT: We investigate the use of optical photometric variability to select and
identify blazars in large-scale time-domain surveys, in part to aid in the
identification of blazar counterparts to the ~30% of gamma-ray sources in the
Fermi 2FGL catalog still lacking reliable associations. Using data from the
optical LINEAR asteroid survey, we characterize the optical variability of
blazars by fitting a damped random walk model to individual light curves with
two main model parameters, the characteristic timescales of variability (tau),
and driving amplitudes on short timescales (sigma). Imposing cuts on minimum
tau and sigma allows for blazar selection with high efficiency E and
completeness C. To test the efficacy of this approach, we apply this method to
optically variable LINEAR objects that fall within the several-arcminute error
ellipses of gamma-ray sources in the Fermi 2FGL catalog. Despite the extreme
stellar contamination at the shallow depth of the LINEAR survey, we are able to
recover previously-associated optical counterparts to Fermi AGN with E > 88%
and C = 88% in Fermi 95% confidence error ellipses having semimajor axis r <
8'. We find that the suggested radio counterpart to Fermi source 2FGL
J1649.6+5238 has optical variability consistent with other gamma-ray blazars,
and is likely to be the gamma-ray source. Our results suggest that the
variability of the non-thermal jet emission in blazars is stochastic in nature,
with unique variability properties due to the effects of relativistic beaming.
After correcting for beaming, we estimate that the characteristic timescale of
blazar variability is ~3 years in the rest-frame of the jet, in contrast with
the ~320 day disk flux timescale observed in quasars. The variability-based
selection method presented will be useful for blazar identification in
time-domain optical surveys, and is also a probe of jet physics.
09/2012;
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Masamune Oguri,
Tim Schrabback,
Eric Jullo,
Naomi Ota, Christopher S. Kochanek,
Xinyu Dai,
Eran O. Ofek,
Gordon T. Richards,
Roger D. Blandford,
Emilio E. Falco,
Janine Fohlmeister
[show abstract]
[hide abstract]
ABSTRACT: We present Hubble Space Telescope (HST) ACS and WFC3 observations of SDSS
J1029+2623, a three-image quasar lens system produced by a foreground cluster
at z=0.584. Our strong lensing analysis reveals 6 additional multiply imaged
galaxies. We confirm the complex nature of the mass distribution of the lensing
cluster, with a bimodal distribution which deviates from the Chandra X-ray SB
distribution. The Einstein radius is estimated to be \theta_E=15.2" \pm 0.5"
for the quasar redshift of z=2.197. We derive a radial mass distribution from
the combination of strong lensing, HST/ACS weak lensing, and Subaru/Suprime-cam
weak lensing analysis results, finding a best-fit virial mass of
M_vir=(1.5+0.40-0.35) \times 10^14 h^-1 M_sun and a concentration parameter of
c_vir=25.7+14.1-7.5. The lensing mass estimate at the outer radius is smaller
than the X-ray mass estimate by a factor of ~2. We ascribe this large mass
discrepancy to shock heating of the intracluster gas during a merger, which is
also suggested by the complex mass and gas distributions and the high value of
the concentration parameter. In the HST image, we also identify a probable
galaxy, GX, in the vicinity of the faintest quasar image C. The inclusion of GX
explains the anomalous flux ratios between the quasar images. The morphology of
the highly elongated quasar host galaxy is also well reproduced. The best-fit
model suggests large total magnifications of 30 for the quasar and 35 for the
quasar host galaxy, and has an AB time delay consistent with the measured
value. (Abridged)
09/2012;
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[show abstract]
[hide abstract]
ABSTRACT: We present 279 epochs of optical monitoring data spanning 5.4 years from 2007
January to 2012 June for the largest image separation (22.6 arcsec)
gravitationally lensed quasar, SDSS J1029+2623. We find that image A leads the
images B and C by dt_AB = (744+-10) days (90% confidence); the uncertainty
includes both statistical uncertainties and systematic differences due to the
choice of models. With only a ~1% fractional error, the interpretation of the
delay is limited primarily by cosmic variance due to fluctuations in the mean
line-of-sight density. We cannot separate the fainter image C from image B, but
since image C trails image B by only 2-3 days in all models, the estimate of
the time delay between image A and B is little affected by combining the fluxes
of images B and C. There is weak evidence for a low level of microlensing,
perhaps created by the small galaxy responsible for the flux ratio anomaly in
this system. Interpreting the delay depends on better constraining the shape of
the gravitational potential using the lensed host galaxy, other lensed arcs and
the structure of the X-ray emission.
07/2012;
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Christopher W. Morgan,
Laura J. Hainline,
Bin Chen,
Malte Tewes, Christopher S. Kochanek,
Xinyu Dai,
Szymon Kozlowski,
Jeffrey A. Blackburne,
Ana M. Mosquera,
George Chartas,
Frederic Courbin,
Georges Meylan
[show abstract]
[hide abstract]
ABSTRACT: We present four new seasons of optical monitoring data and six epochs of
X-ray photometry for the doubly-imaged lensed quasar Q J0158-4325. The
high-amplitude, short-period microlensing variability for which this system is
known has historically precluded a time delay measurement by conventional
methods. We attempt to circumvent this limitation by application of a Monte
Carlo microlensing analysis technique, but we are only able to prove that the
delay must have the expected sign (image A leads image B). Despite our failure
to robustly measure the time delay, we successfully model the microlensing at
optical and X-ray wavelengths to find a half light radius for soft X-ray
emission log(r_{1/2,X,soft}/cm) = 14.3^{+0.4}_{-0.5}, an upper limit on the
half-light radius for hard X-ray emission log(r_{1/2,X,hard}/cm) <= 14.6 and a
refined estimate of the inclination-corrected scale radius of the optical
R-band (rest frame 3100 Angstrom) continuum emission region of log(r_s/cm) =
15.6+-0.3.
The Astrophysical Journal 05/2012; · 6.02 Impact Factor
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Naohisa Inada,
Masamune Oguri,
Min-Su Shin,
Issha Kayo,
Michael A. Strauss,
Tomoki Morokuma,
Cristian E. Rusu,
Masataka Fukugita, Christopher S. Kochanek,
Gordon T. Richards,
Donald P. Schneider,
Donald G. York,
Neta A. Bahcall,
Joshua A. Frieman,
Patrick B. Hall,
and Richard L. White
[show abstract]
[hide abstract]
ABSTRACT: We present the final statistical sample of lensed quasars from the Sloan Digital Sky Survey (SDSS) Quasar Lens Search (SQLS). The well-defined statistical lens sample consists of 26 lensed quasars brighter than i = 19.1 and in the redshift range of 0.6 < z < 2.2 selected from 50,826 spectroscopically confirmed quasars in the SDSS Data Release 7 (DR7), where we restrict the image separation range to 1'' < θ < 20'' and the i-band magnitude differences in two images to be smaller than 1.25 mag. The SDSS DR7 quasar catalog also contains 36 additional lenses identified with various techniques. In addition to these lensed quasars, we have identified 81 pairs of quasars from follow-up spectroscopy, 26 of which are physically associated binary quasars. The statistical lens sample covers a wide range of image separations, redshifts, and magnitudes, and therefore is suitable for systematic studies of cosmological parameters and surveys of the structure and evolution of galaxies and quasars.
The Astronomical Journal 04/2012; 143(5):119. · 4.03 Impact Factor
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Masamune Oguri,
Naohisa Inada,
Michael A. Strauss, Christopher S. Kochanek,
Issha Kayo,
Min-Su Shin,
Tomoki Morokuma,
Gordon T. Richards,
Cristian E. Rusu,
Joshua A. Frieman,
Masataka Fukugita,
Donald P. Schneider,
Donald G. York,
Neta A. Bahcall,
and Richard L. White
[show abstract]
[hide abstract]
ABSTRACT: We present a statistical analysis of the final lens sample from the Sloan Digital Sky Survey Quasar Lens Search (SQLS). The number distribution of a complete subsample of 19 lensed quasars selected from 50,836 source quasars is compared with theoretical expectations, with particular attention given to the selection function. Assuming that the velocity function of galaxies does not evolve with redshift, the SQLS sample constrains the cosmological constant to ΩΛ = 0.79+0.06 –0.07(stat.)+0.06 –0.06(syst.) for a flat universe. The dark energy equation of state is found to be consistent with w = –1 when the SQLS is combined with constraints from baryon acoustic oscillation (BAO) measurements or results from the Wilkinson Microwave Anisotropy Probe (WMAP). We also obtain simultaneous constraints on cosmological parameters and redshift evolution of the galaxy velocity function, finding no evidence for redshift evolution at z 1 in any combinations of constraints. For instance, number density evolution quantified as ν n ≡ dln */dln (1 + z) and the velocity dispersion evolution νσ ≡ dln σ*/dln (1 + z) are constrained to ν n = 1.06+1.36 –1.39(stat.)+0.33 –0.64(syst.) and νσ = –0.05+0.19 –0.16(stat.)+0.03 –0.03(syst.), respectively, when the SQLS result is combined with BAO and WMAP for flat models with a cosmological constant. We find that a significant amount of dark energy is preferred even after fully marginalizing over the galaxy evolution parameters. Thus, the statistics of lensed quasars robustly confirm the accelerated cosmic expansion.
The Astronomical Journal 04/2012; 143(5):120. · 4.03 Impact Factor
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Richard J. Cool,
Daniel J. Eisenstein, Christopher S. Kochanek,
Michael J. I. Brown,
Nelson Caldwell,
Arjun Dey,
William R. Forman,
Ryan C. Hickox,
Buell T. Jannuzi,
Christine Jones,
John Moustakas,
and Stephen S. Murray
[show abstract]
[hide abstract]
ABSTRACT: We present the galaxy optical luminosity function for the redshift range 0.05 < z < 0.75 from the AGN and Galaxy Evolution Survey, a spectroscopic survey of 7.6 deg2 in the Boötes field of the NOAO Deep Wide-Field Survey. Our statistical sample is composed of 12,473 galaxies with known redshifts down to I = 20.4 (AB). Our results at low redshift are consistent with those from Sloan Digital Sky Survey; at higher redshift, we find strong evidence for evolution in the luminosity function, including differential evolution between blue and red galaxies. We find that the luminosity density evolves as (1 + z)(0.54 ± 0.64) for red galaxies and (1 + z)(1.64 ± 0.39) for blue galaxies.
The Astrophysical Journal 02/2012; 748(1):10. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We present \emph{Chandra} monitoring data for six gravitationally lensed
quasars: QJ 0158$-$4325, HE 0435$-$1223, HE 1104$-$1805, SDSS 0924+0219, SDSS
1004+4112, and Q 2237+0305. We detect X-ray microlensing variability in all six
lenses with high confidence. We detect energy dependent microlensing in HE
0435$-$1223, SDSS 1004+4112, SDSS 0924+0219 and Q 2237+0305. We present a
detailed spectral analysis for each lens, and find that simple power-law models
plus Gaussian emission lines give good fits to the spectra. We detect intrinsic
spectral variability in two epochs of Q 2237+0305. We detect differential
absorption between images in four lenses. We also detect the \feka\ emission
line in all six lenses, and the Ni XXVII K$\alpha$ line in two images of Q
2237+0305. The rest frame equivalent widths of the \feka\ lines are measured to
be 0.4--1.2 keV, significantly higher than those measured in typical active
galactic nuclei of similar X-ray luminosities. This suggests that the \feka\
emission region is more compact or centrally concentrated than the continuum
emission region.
02/2012;
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[show abstract]
[hide abstract]
ABSTRACT: We present results from Chandra observations of the cluster lens SDSS
J1029+2623 at z_l=0.58, which is a gravitationally lensed quasar with the
largest known image separation. We clearly detect X-ray emission both from the
lensing cluster and the three lensed quasar images. The cluster has an X-ray
temperature of kT = 8.1 (+2.0, -1.2) keV and bolometric luminosity of L_X =
9.6e44 erg s^-1. Its surface brightness is centered near one of the brightest
cluster galaxies, and it is elongated East-West. We identify a subpeak
North-West of the main peak, which is suggestive of an ongoing merger. Even so,
the X-ray mass inferred from the hydrostatic equilibrium assumption appears to
be consistent with the lensing mass from the Einstein radius of the system. We
find significant absorption in the soft X-ray spectrum of the faintest quasar
image, which can be caused by an intervening material at either the lens or
source redshift. The X-ray flux ratios between the quasar images (after
correcting for absorption) are in reasonable agreement with those at optical
and radio wavelengths, and all the flux ratios are inconsistent with those
predicted by simple mass models. This implies that microlensing effect is not
significant for this system and dark matter substructure is mainly responsible
for the anomalous flux ratios.
02/2012;
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Richard J. Cool,
Daniel J. Eisenstein, Christopher S. Kochanek,
Michael J. I. Brown,
Nelson Caldwell,
Arjun Dey,
William R. Forman,
Ryan C. Hickox,
Buell T. Jannuzi,
Christine Jones,
John Moustakas,
Stephen S. Murray
[show abstract]
[hide abstract]
ABSTRACT: We present the galaxy optical luminosity function for the redshift range
0.05<z<0.75 from the AGN and Galaxy Evolution Survey (AGES), a spectroscopic
survey of 7.6 sq. deg. in the Bootes field of the NOAO Deep Wide-Field Survey.
Our statistical sample is comprised of 12,473 galaxies with known redshifts
down to I=20.4 (AB). Our results at low redshift are consistent with those from
SDSS; at higher redshift, we find strong evidence for evolution in the
luminosity function, including differential evolution between blue and red
galaxies. We find that the luminosity density evolves as (1+z)^(0.54+/-0.64)
for red galaxies and (1+z)^(1.64+/-0.39) for blue galaxies.
01/2012;
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Chelsea L. MacLeod,
Zeljko Ivezic,
Branimir Sesar,
Wim de Vries, Christopher S. Kochanek,
Brandon C. Kelly,
Andrew C. Becker,
Robert H. Lupton,
Patrick B. Hall,
Gordon T. Richards,
Scott F. Anderson,
Donald P. Schneider
[show abstract]
[hide abstract]
ABSTRACT: We provide a quantitative description and statistical interpretation of the
optical continuum variability of quasars. The Sloan Digital Sky Survey (SDSS)
has obtained repeated imaging in five UV-to-IR photometric bands for 33,881
spectroscopically confirmed quasars. About 10,000 quasars have an average of 60
observations in each band obtained over a decade along Stripe 82 (S82), whereas
the remaining ~25,000 have 2-3 observations due to scan overlaps. The observed
time lags span the range from a day to almost 10 years, and constrain quasar
variability at rest-frame time lags of up to 4 years, and at rest-frame
wavelengths from 1000A to 6000A. We publicly release a user-friendly catalog of
quasars from the SDSS Data Release 7 that have been observed at least twice in
SDSS or once in both SDSS and the Palomar Observatory Sky Survey, and we use it
to analyze the ensemble properties of quasar variability. Based on a damped
random walk (DRW) model defined by a characteristic time scale and an
asymptotic variability amplitude that scale with the luminosity, black hole
mass, and rest wavelength for individual quasars calibrated in S82, we can
fully explain the ensemble variability statistics of the non-S82 quasars such
as the exponential distribution of large magnitude changes. All available data
are consistent with the DRW model as a viable description of the optical
continuum variability of quasars on time scales of ~5-2000 days in the rest
frame. We use these models to predict the incidence of quasar contamination in
transient surveys such as those from PTF and LSST.
12/2011;
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[show abstract]
[hide abstract]
ABSTRACT: Microlensing has proven an effective probe of the structure of the innermost
regions of quasars, and an important test of accretion disk models. We present
light curves of the lensed quasar HE 0435-1223 in the R band and in the
ultraviolet, and consider them together with X-ray light curves in two energy
bands that are presented in a companion paper. Using a Bayesian Monte Carlo
method, we constrain the size of the accretion disk in the rest-frame near- and
far-UV, and constrain for the first time the size of the X-ray emission regions
in two X-ray energy bands. The R-band scale size of the accretion disk is about
10^15.53 cm (~46 r_g), slightly smaller than previous estimates, but larger
than is predicted by the standard thin disk model. In the UV, the source size
is weakly constrained, with a strong prior dependence. The UV to R-band size
ratio is consistent with the thin disk model prediction, with large error bars.
In soft and hard X-rays, the source size is smaller than ~10^15 cm (~13 r_g) at
90% confidence. We do not find evidence of structure in the X-ray emission
region, as the most likely value for the ratio of the hard X-ray size to the
soft X-ray size is unity. The simulations do not support the idea that quasar
flux variability is due to coherent changes in accretion disk area. Finally, we
find that the most likely value for the mean mass of stars in the lens galaxy
is ~0.3 M_sun, consistent with other studies.
11/2011;
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[show abstract]
[hide abstract]
ABSTRACT: We present our long term Chandra X-ray monitoring data for the
gravitationally lensed quasar Q2237+0305 with 20 epochs spanning 10 years. We
easily detect microlensing variability between the images in the full (0.2--8
keV), soft (0.2--2 keV), and hard (2--8 keV) bands at very high confidence. We
also detect, for the first time, chromatic microlensing differences between the
soft and hard X-ray bands. The hard X-ray band is more strongly microlensed
than the soft band, suggesting that the corona above the accretion disk thought
to generate the X-rays has a non-uniform electron distribution, in which the
hotter and more energetic electrons occupy more compact regions surrounding the
black holes. Both the hard and soft X-ray bands are more strongly microlensed
than the optical (restframe UV) emission, indicating that the X-ray emission is
more compact than the optical, confirming the microlensing results from other
lenses.
The Astrophysical Journal 06/2011; · 6.02 Impact Factor
