559 research outputs found
Combining cluster observables and stacked weak lensing to probe dark energy: Self-calibration of systematic uncertainties
We develop a new method of combining cluster observables (number counts and
cluster-cluster correlation functions) and stacked weak lensing signals of
background galaxy shapes, both of which are available in a wide-field optical
imaging survey. Assuming that the clusters have secure redshift estimates, we
show that the joint experiment enables a self-calibration of important
systematic errors including the source redshift uncertainty and the cluster
mass-observable relation, by adopting a single population of background source
galaxies for the lensing analysis. It allows us to use the relative strengths
of stacked lensing signals at different cluster redshifts for calibrating the
source redshift uncertainty, which in turn leads to accurate measurements of
the mean cluster mass in each bin. In addition, our formulation of stacked
lensing signals in Fourier space simplifies the Fisher matrix calculations, as
well as the marginalization over the cluster off-centering effect, the most
significant uncertainty in stacked lensing. We show that upcoming wide-field
surveys yield stringent constraints on cosmological parameters including dark
energy parameters, without any priors on nuisance parameters that model
systematic uncertainties. Specifically, the stacked lensing information
improves the dark energy FoM by a factor of 4, compared to that from the
cluster observables alone. The primordial non-Gaussianity parameter can also be
constrained with a level of f_NL~10. In this method, the mean source redshift
is well calibrated to an accuracy of 0.1 in redshift, and the mean cluster mass
in each bin to 5-10% accuracies, which demonstrates the success of the
self-calibration of systematic uncertainties from the joint experiment.
(Abridged)Comment: 29 pages, 17 figures, 6 tables, accepted for publication in Phys.
Rev.
X-ray bright active galactic nuclei in massive galaxy clusters III: New insights into the triggering mechanisms of cluster AGN
We present the results of a new analysis of the X-ray selected Active
Galactic Nuclei (AGN) population in the vicinity of 135 of the most massive
galaxy clusters in the redshift range of 0.2 < z < 0.9 observed with Chandra.
With a sample of more than 11,000 X-ray point sources, we are able to measure,
for the first time, evidence for evolution in the cluster AGN population beyond
the expected evolution of field AGN. Our analysis shows that overall number
density of cluster AGN scales with the cluster mass as .
There is no evidence for the overall number density of cluster member X-ray AGN
depending on the cluster redshift in a manner different than field AGN, nor
there is any evidence that the spatial distribution of cluster AGN (given in
units of the cluster overdensity radius r_500) strongly depends on the cluster
mass or redshift. The scaling relation we measure is
consistent with theoretical predictions of the galaxy merger rate in clusters,
which is expected to scale with the cluster velocity dispersion, , as or . This consistency suggests that AGN in
clusters may be predominantly triggered by galaxy mergers, a result that is
further corroborated by visual inspection of Hubble images for 23
spectroscopically confirmed cluster member AGN in our sample. A merger-driven
scenario for the triggering of X-ray AGN is not strongly favored by studies of
field galaxies, however, suggesting that different mechanisms may be primarily
responsible for the triggering of cluster and field X-ray AGN.Comment: 21 Pages, 8 figures, 5 tables. Submitted to MNRAS. Comments are
welcome, and please request Steven Ehlert for higher resolution figure
Weighing neutrinos using high redshift galaxy luminosity functions
Laboratory experiments measuring neutrino oscillations, indicate small mass
differences between different mass eigenstates of neutrinos. The absolute mass
scale is however not determined, with at present the strongest upper limits
coming from astronomical observations rather than terrestrial experiments. The
presence of massive neutrinos suppresses the growth of perturbations below a
characteristic mass scale, thereby leading to a decreased abundance of
collapsed dark matter halos. Here we show that this effect can significantly
alter the predicted luminosity function (LF) of high redshift galaxies. In
particular we demonstrate that a stringent constraint on the neutrino mass can
be obtained using the well measured galaxy LF and our semi-analytic structure
formation models. Combining the constraints from the Wilkinson Microwave
Anisotropy Probe 7 year (WMAP7) data with the LF data at z = 4, we get a limit
on the sum of the masses of 3 degenerate neutrinos \Sigma m_\nu < 0.52 eV at
the 95 % CL. The additional constraints using the prior on Hubble constant
strengthens this limit to \Sigma m_\nu < 0.29 eV at the 95 % CL. This neutrino
mass limit is a factor of order 4 improvement compared to the constraint based
on the WMAP7 data alone, and as stringent as known limits based on other
astronomical observations. As different astronomical measurements may suffer
from different set of biases, the method presented here provides a
complementary probe of \Sigma m_\nu . We suggest that repeating this exercise
on well measured luminosity functions over different redshift ranges can
provide independent and tighter constraints on \Sigma m_\nu .Comment: 14 pages, 7 figures, submitted to PR
Cosmology and astrophysics from relaxed galaxy clusters - IV: Robustly calibrating hydrostatic masses with weak lensing
This is the fourth in a series of papers studying the astrophysics and
cosmology of massive, dynamically relaxed galaxy clusters. Here, we use
measurements of weak gravitational lensing from the Weighing the Giants project
to calibrate Chandra X-ray measurements of total mass that rely on the
assumption of hydrostatic equilibrium. This comparison of X-ray and lensing
masses provides a measurement of the combined bias of X-ray hydrostatic masses
due to both astrophysical and instrumental sources. Assuming a fixed cosmology,
and within a characteristic radius (r_2500) determined from the X-ray data, we
measure a lensing to X-ray mass ratio of 0.96 +/- 9% (stat) +/- 9% (sys). We
find no significant trends of this ratio with mass, redshift or the
morphological indicators used to select the sample. In accordance with
predictions from hydro simulations for the most massive, relaxed clusters, our
results disfavor strong, tens-of-percent departures from hydrostatic
equilibrium at these radii. In addition, we find a mean concentration of the
sample measured from lensing data of c_200 = . Anticipated
short-term improvements in lensing systematics, and a modest expansion of the
relaxed lensing sample, can easily increase the measurement precision by
30--50%, leading to similar improvements in cosmological constraints that
employ X-ray hydrostatic mass estimates, such as on Omega_m from the cluster
gas mass fraction.Comment: 13 pages. Submitted to MNRAS. Comments welcom
Thermodynamic Profiles of Galaxy Clusters from a Joint X-ray/SZ Analysis
We jointly analyze Bolocam Sunyaev-Zeldovich (SZ) effect and Chandra X-ray
data for a set of 45 clusters to derive gas density and temperature profiles
without using spectroscopic information. The sample spans the mass and redshift
range
and . We define cool-core (CC) and non-cool core (NCC)
subsamples based on the central X-ray luminosity, and 17/45 clusters are
classified as CC. In general, the profiles derived from our analysis are found
to be in good agreement with previous analyses, and profile constraints beyond
are obtained for 34/45 clusters. In approximately 30% of the CC
clusters our analysis shows a central temperature drop with a statistical
significance of ; this modest detection fraction is due mainly to a
combination of coarse angular resolution and modest S/N in the SZ data. Most
clusters are consistent with an isothermal profile at the largest radii near
, although 9/45 show a significant temperature decrease with
increasing radius. The sample mean density profile is in good agreement with
previous studies, and shows a minimum intrinsic scatter of approximately 10%
near . The sample mean temperature profile is consistent
with isothermal, and has an intrinsic scatter of approximately 50% independent
of radius. This scatter is significantly higher compared to earlier X-ray-only
studies, which find intrinsic scatters near 10%, likely due to a combination of
unaccounted for non-idealities in the SZ noise, projection effects, and sample
selection.Comment: 42 pages, 52 figure
X-ray Bright Active Galactic Nuclei in Massive Galaxy Clusters II: The Fraction of Galaxies Hosting Active Nuclei
We present a measurement of the fraction of cluster galaxies hosting X-ray
bright Active Galactic Nuclei (AGN) as a function of clustercentric distance
scaled in units of . Our analysis employs high quality Chandra X-ray
and Subaru optical imaging for 42 massive X-ray selected galaxy cluster fields
spanning the redshift range of . In total, our study involves
176 AGN with bright () optical counterparts above a keV flux
limit of . When excluding
central dominant galaxies from the calculation, we measure a cluster-galaxy AGN
fraction in the central regions of the clusters that is times lower
that the field value. This fraction increases with clustercentric distance
before becoming consistent with the field at . Our data
exhibit similar radial trends to those observed for star formation and
optically selected AGN in cluster member galaxies, both of which are also
suppressed near cluster centers to a comparable extent. These results strongly
support the idea that X-ray AGN activity and strong star formation are linked
through their common dependence on available reservoirs of cold gas.Comment: 9 Pages, 4 Figures, accepted for publication in MNRAS, please contact
Steven Ehlert ([email protected]) with any querie
CO and C_2 Absorption Toward W40 IRS 1a
The H II region W40 harbors a small group of young, hot stars behind roughly
9 magnitudes of visual extinction. We have detected gaseous carbon monoxide
(CO) and diatomic carbon (C_2) in absorption toward the star W40 IRS 1a. The
2-0 R0, R1, and R2 lines of 12CO at 2.3 micron were measured using the CSHELL
on the NASA IR Telescope Facility (with upper limits placed on R3, R4, and R5)
yielding an N_CO of (1.1 +/- 0.2) x 10^18 cm^-2. Excitation analysis indicates
T_kin > 7 K. The Phillips system of C_2 transitions near 8775 Ang. was measured
using the Kitt Peak 4-m telescope and echelle spectrometer. Radiative pumping
models indicate a total C_2 column density of (7.0 +/- 0.4) x 10^14 cm^-2, two
excitation temperatures (39 and 126 K), and a total gas density of n ~ 250
cm^-3. The CO ice band at 4.7 micron was not detected, placing an upper limit
on the CO depletion of delta < 1 %. We postulate that the sightline has
multiple translucent components and is associated with the W40 molecular cloud.
Our data for W40 IRS 1a, coupled with other sightlines, shows that the ratio of
CO/C_2 increases from diffuse through translucent environs. Finally, we show
that the hydrogen to dust ratio seems to remain constant from diffuse to dense
environments, while the CO to dust ratio apparently does not.Comment: To appear in The Astrophysical Journal 17 pages total, 5 figures Also
available at http://casa.colorado.edu/~shuping/research/w40/w40.htm
Cosmology and Astrophysics from Relaxed Galaxy Clusters II: Cosmological Constraints
We present cosmological constraints from measurements of the gas mass
fraction, , for massive, dynamically relaxed galaxy clusters. Our data
set consists of Chandra observations of 40 such clusters, identified in a
comprehensive search of the Chandra archive, as well as high-quality weak
gravitational lensing data for a subset of these clusters. Incorporating a
robust gravitational lensing calibration of the X-ray mass estimates, and
restricting our measurements to the most self-similar and accurately measured
regions of clusters, significantly reduces systematic uncertainties compared to
previous work. Our data for the first time constrain the intrinsic scatter in
, % in a spherical shell at radii 0.8-1.2 ,
consistent with the expected variation in gas depletion and non-thermal
pressure for relaxed clusters. From the lowest-redshift data in our sample we
obtain a constraint on a combination of the Hubble parameter and cosmic baryon
fraction, , that is insensitive to the
nature of dark energy. Combined with standard priors on and ,
this provides a tight constraint on the cosmic matter density,
, which is similarly insensitive to dark energy. Using
the entire cluster sample, extending to , we obtain consistent results for
and interesting constraints on dark energy:
for non-flat CDM models, and
for flat constant- models. Our results are both competitive
and consistent with those from recent CMB, SNIa and BAO data. We present
constraints on models of evolving dark energy from the combination of
data with these external data sets, and comment on the possibilities for
improved constraints using current and next-generation X-ray
observatories and lensing data. (Abridged)Comment: 25 pages, 14 figures, 8 tables. Accepted by MNRAS. Code and data can
be downloaded from http://www.slac.stanford.edu/~amantz/work/fgas14/ . v2:
minor fix to table 1, updated bibliograph
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