141 research outputs found
Subaru Weak Lensing Study of Seven Merging Clusters: Distributions of Mass and Baryons
We present and compare projected distributions of mass, galaxies, and the
intracluster medium (ICM) for a sample of merging clusters of galaxies based on
the joint weak-lensing, optical photometric, and X-ray analysis. Our sample
comprises seven nearby Abell clusters, for which we have conducted systematic,
deep imaging observations with Suprime-Cam on Subaru telescope. Our seven
target clusters, representing various merging stages and conditions, allow us
to investigate in details the physical interplay between dark matter, ICM, and
galaxies associated with cluster formation and evolution. A1750 and A1758 are
binary systems consisting of two cluster-sized components, A520, A754, A1758N,
A1758S, and A1914 are on-going cluster mergers, and A2034 and A2142 are
cold-front clusters. In the binary clusters, the projected mass, optical light,
and X-ray distributions are overall similar and regular without significant
substructures. On-going and cold-front merging clusters, on the other hand,
reveal highly irregular mass distributions. Overall the mass distribution
appears to be similar to the galaxy luminosity distribution, whereas their
distributions are quite different from the ICM distribution in a various ways.
We also measured for individual targets the global cluster parameters such as
the cluster mass,the mass-to-light ratio, and the ICM temperature. A comparison
of the ICM and virial temperatures of merging clusters from X-ray and
weak-lensing analyses, respectively, shows that the ICM temperature of on-going
and cold-front clusters is significantly higher than the cluster virial
temperature by a factor of . This temperature excess in the ICM could
be explained by the effects of merger boosts.Comment: "High-resolution pictures available at
http://www.astr.tohoku.ac.jp/~okabe/files/7merger_color.pdf". The published
version is available on-line free of charge by the end of 2008 at
http://pasj.asj.or.jp/v60/n2/600223/600223.pd
LoCuSS: Weak-lensing mass calibration of galaxy clusters
We present weak-lensing mass measurements of 50 X-ray luminous galaxy
clusters at , based on uniform high quality observations with
Suprime-Cam mounted on the 8.2-m Subaru telescope. We pay close attention to
possible systematic biases, aiming to control them at the per cent
level. The dominant source of systematic bias in weak-lensing measurements of
the mass of individual galaxy clusters is contamination of background galaxy
catalogues by faint cluster and foreground galaxies. We extend our conservative
method for selecting background galaxies with colours redder than the
red sequence of cluster members to use a colour-cut that depends on
cluster-centric radius. This allows us to define background galaxy samples that
suffer per cent contamination, and comprise galaxies per square
arcminute. Thanks to the purity of our background galaxy catalogue, the largest
systematic that we identify in our analysis is a shape measurement bias of
per cent, that we measure using simulations that probe weak shears upto
. Our individual cluster mass and concentration measurements are in
excellent agreement with predictions of the mass-concentration relation.
Equally, our stacked shear profile is in excellent agreement with the Navarro
Frenk and White profile. Our new LoCuSS mass measurements are consistent with
the CCCP and CLASH surveys, and in tension with the Weighing the Giants at
significance. Overall, the consensus at that is
emerging from these complementary surveys represents important progress for
cluster mass calibration, and augurs well for cluster cosmology.Comment: 30 pages, 14 figures, 7 tables : accepted for the publication in
MNRAS : mass table update
Suzaku observations of subhalos in the Coma cluster
We observed three massive subhalos in the Coma cluster with {\it Suzaku}.
These subhalos, labeled "ID 1", "ID 2", and "ID 32", were detected with a
weak-lensing survey using the Subaru/Suprime-Cam (Okabe et al. 2014a), and are
located at the projected distances of 1.4 , 1.2 , and 1.6
from the center of the Coma cluster, respectively. The subhalo "ID 1"
has a compact X-ray excess emission close to the center of the weak-lensing
mass contour, and the gas mass to weak-lensing mass ratio is about 0.001. The
temperature of the emission is about 3 keV, which is slightly lower than that
of the surrounding intracluster medium (ICM) and that expected for the
temperature vs. mass relation of clusters of galaxies. The subhalo "ID 32"
shows an excess emission whose peak is shifted toward the opposite direction
from the center of the Coma cluster. The gas mass to weak-lensing mass ratio is
also about 0.001, which is significantly smaller than regular galaxy groups.
The temperature of the excess is about 0.5 keV and significantly lower than
that of the surrounding ICM and far from the temperature vs. mass relation of
clusters. However, there is no significant excess X-ray emission in the "ID 2"
subhalo. Assuming an infall velocity of about 2000 , at the
border of the excess X-ray emission, the ram pressures for "ID 1" and "ID 32"
are comparable to the gravitational restoring force per area. We also studied
the effect of the Kelvin-Helmholtz instability to strip the gas. Although we
found X-ray clumps associated with the weak-lensing subhalos, their X-ray
luminosities are much lower than the total ICM luminosity in the cluster
outskirts.Comment: 19 pages, 8 figures, ApJ in pres
A New Interpretation of the Mass-Temperature Relation and Mass Calibration of Galaxy Clusters Based on the Fundamental Plane
Observations and numerical simulations have shown that the relation between
the mass scaled with the critical density of the universe and the X-ray
temperature of galaxy clusters is approximately represented by (e.g. ). This relation is often interpreted as
evidence that clusters are in virial equilibrium. However, the recently
discovered fundamental plane (FP) of clusters indicates that the temperature of
clusters primarily depends on a combination of the characteristic mass
and radius of the Navarro-Frenk-White profile rather than .
Moreover, the angle of the FP revealed that clusters are not in virial
equilibrium because of continuous mass accretion from the surrounding matter.
By considering both the FP and the mass dependence of the cluster concentration
parameter, we show that this paradox can be solved and the relation actually reflects the central structure of clusters. We also
find that the intrinsic scatter in the halo concentration-mass relation can
largely account for the spread of clusters on the FP. We also show that X-ray
data alone form the FP and the angle and the position are consistent with those
of the FP constructed from gravitational lensing data. We demonstrate that a
possible shift between the two FPs can be used to calibrate cluster masses
obtained via X-ray observations.Comment: Published on ApJ. Matched to published versio
Magnetic field generation by the Weibel instability at temperature gradients in collisionless plasmas
The Weibel instability could be responsible for the generation of magnetic
fields in various objects such as gamma-ray bursts, jets from active galactic
nuclei, and clusters of galaxies. Using numerical simulations, the development
of the Weibel instability at a temperature gradient is studied. It is found
that current sheets are first generated at the gradient, and then they are
rounded off and turn into current filaments. During this process, return
currents are generated around the filaments and they prevent filaments from
merger. The magnetic fields around the filaments persist at least until
t~8000/\omega_p, where \omega_p is the plasma frequency, and it is very likely
that they survive for a much longer time.Comment: Physics of Plasmas in pres
Stacked phase-space density of galaxies around massive clusters: Comparison of dynamical and lensing masses
We present a measurement of average histograms of line-of-sight velocities
over pairs of galaxies and galaxy clusters. Since the histogram can be measured
at different galaxy-cluster separations, this observable is commonly referred
to as the stacked phase-space density. We formulate the stacked phase-space
density based on a halo-model approach so that the model can be applied to real
samples of galaxies and clusters. We examine our model by using an actual
sample of massive clusters with known weak-lensing masses and spectroscopic
observations of galaxies around the clusters. A likelihood analysis with our
model enables us to infer the spherical-symmetric velocity dispersion of
observed galaxies in massive clusters. We find the velocity dispersion of
galaxies surrounding clusters with their lensing masses of to be at the 68\%
confidence level. Our constraint confirms that the relation between the galaxy
velocity dispersion and the host cluster mass in our sample is consistent with
the prediction in dark-matter-only N-body simulations under General Relativity.
Assuming that the Poisson equation in clusters can be altered by an effective
gravitational constant of , our measurement of the velocity
dispersion can place a tight constraint of at length scales of a few Mpc about Giga years ago,
where is the Newton's constant.Comment: 22 pages, 8 figures, 3 tables. Accepted for publication in MNRA
Subaru Weak-Lensing Survey of Dark Matter Subhalos in the Coma Cluster : Subhalo Mass Function and Statistical Properties
We present a 4 deg^2 weak gravitational lensing survey of subhalos in the
very nearby Coma cluster using the Subaru/Suprime-Cam. The large apparent size
of cluster subhalos allows us to measure the mass of 32 subhalos detected in a
model-independent manner, down to the order of 10^-3 of the virial mass of the
cluster. Weak-lensing mass measurements of these shear-selected subhalos enable
us to investigate subhalo properties and the correlation between subhalo masses
and galaxy luminosities for the first time. The mean distortion profiles
stacked over subhalos show a sharply truncated feature which is well-fitted by
a Navarro-Frenk-White (NFW) mass model with the truncation radius, as expected
due to tidal destruction by the main cluster. We also found that subhalo
masses, truncation radii, and mass-to-light ratios decrease toward the cluster
center. The subhalo mass function, dn/dln M_sub, in the range of 2 orders of
magnitude in mass, is well described by a single power law or a Schechter
function. Best-fit power indices of 1.09_-0.32^+0.42 for the former model and
0.99_-0.23^+0.34 for the latter, are in remarkable agreement with slopes of
~0.9-1.0 predicted by the cold dark matter paradigm. The tangential distortion
signals in the radial range of 0.02-2Mpc/h from the cluster center show a
complex structure which is well described by a composition of three mass
components of subhalos, the NFW mass distribution as a smooth component of the
main cluster, and a lensing model from a large scale structure behind the
cluster. Although the lensing signals are 1 order of magnitude lower than those
for clusters at z~0.2, the total signal-to-noise ratio, S/N=13.3, is comparable
to, or higher, because the enormous number of background source galaxies
compensates for the low lensing efficiency of the low lensing efficiency of the
nearby cluster.Comment: 30 pages, 18 figures, 9 tables, ApJ in press. Full resolution version
is available at http://www.asiaa.sinica.edu.tw/~okabe/files/coma_survey.pd
LoCuSS: Exploring the selection of faint blue background galaxies for cluster weak-lensing
Cosmological constraints from galaxy clusters rely on accurate measurements
of the mass and internal structure of clusters. An important source of
systematic uncertainty in cluster mass and structure measurements is the secure
selection of background galaxies that are gravitationally lensed by clusters.
This issue has been shown to be particular severe for faint blue galaxies. We
therefore explore the selection of faint blue background galaxies, by reference
to photometric redshift catalogs derived from the COSMOS survey and our own
observations of massive galaxy clusters at z~0.2. We show that methods relying
on photometric redshifts of galaxies in/behind clusters based on observations
through five filters, and on deep 30-band COSMOS photometric redshifts are both
inadequate to identify safely faint blue background galaxies. This is due to
the small number of filters used by the former, and absence of massive galaxy
clusters at redshifts of interest in the latter. We therefore develop a
pragmatic method to combine both sets of photometric redshifts to select a
population of blue galaxies based purely on photometric analysis. This sample
yields stacked weak-lensing results consistent with our previously published
results based on red galaxies. We also show that the stacked clustercentric
number density profile of these faint blue galaxies is consistent with
expectations from consideration of the lens magnification signal of the
clusters. Indeed, the observed number density of blue background galaxies
changes by ~10-30 per cent across the radial range over which other surveys
assume it to be flat.Comment: submitted to MNRA
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