3,169 research outputs found
Constraints on the Asymptotic Baryon Fractions of Galaxy Clusters at Large Radii
While X-ray measurements have so far revealed an increase in the
volume-averaged baryon fractions of galaxy clusters with cluster radii
, should asymptotically reach a universal value ,
provided that clusters are representative of the Universe. In the framework of
hydrostatic equilibrium for intracluster gas, we have derived the necessary
conditions for : The X-ray surface brightness profile
described by the model and the temperature profile approximated by the
polytropic model should satisfy and
for , respectively, which sets
a stringent limit to the polytropic index: . In particular, a
mildly increasing temperature with radius is required if the observationally
fitted parameter is in the range . It is likely that a
reliable determination of the universal baryon fraction can be achieved in the
small clusters because the disagreement between the exact and
asymptotic baryon fractions for clusters with breaks down at rather
large radii (\ga30r_c) where hydrostatic equilibrium has probably become
inapplicable. We further explore how to obtain the asymptotic value
of baryon fraction from the X-ray measurement made primarily over
the finite central region of a cluster. We demonstrate our method using a
sample of 19 strong lensing clusters, which enables us to place a useful
constraint on : .
An optimal estimate of based on three cooling flow clusters with
or .Comment: 6 pages + 4 figures; accepted for publication in MNRA
Constraints on the warm dark matter model from gravitational lensing
Formation of sub-galactic halos is suppressed in warm dark matter (WDM) model
due to thermal motion of WDM particles. This may provide a natural resolution
to some puzzles in standard cold dark matter (CDM) theory such as the cusped
density profiles of virialized dark halos and the overabundance of low mass
satellites. One of the observational tests of the WDM model is to measure the
gravitationally lensed images of distant quasars below sub-arcsecond scales. In
this Letter, we report a comparison of the lensing probabilities of multiple
images between CDM and WDM models using a singular isothermal sphere model for
the mass density profiles of dark halos and the Press-Schechter mass function
for their distribution and cosmic evolution. It is shown that the differential
probability of multiple images with small angular separations down to 10
milliarcseconds should allow one to set useful constraints on the WDM particle
mass. We discuss briefly the feasibility and uncertainties of this method in
future radio surveys (e.g. VLBI) for gravitational lensing.Comment: 3 pages, 1 figure, accepted for publication in ApJ Let
A comparison of different cluster mass estimates: consistency or discrepancy ?
Rich and massive clusters of galaxies at intermediate redshift are capable of
magnifying and distorting the images of background galaxies. A comparison of
different mass estimators among these clusters can provide useful information
about the distribution and composition of cluster matter and their dynamical
evolution. Using a hitherto largest sample of lensing clusters drawn from
literature, we compare the gravitating masses of clusters derived from the
strong/weak gravitational lensing phenomena, from the X-ray measurements based
on the assumption of hydrostatic equilibrium, and from the conventional
isothermal sphere model for the dark matter profile characterized by the
velocity dispersion and core radius of galaxy distributions in clusters. While
there is an excellent agreement between the weak lensing, X-ray and isothermal
sphere model determined cluster masses, these methods are likely to
underestimate the gravitating masses enclosed within the central cores of
clusters by a factor of 2--4 as compared with the strong lensing results. Such
a mass discrepancy has probably arisen from the inappropriate applications of
the weak lensing technique and the hydrostatic equilibrium hypothesis to the
central regions of clusters as well as an unreasonably large core radius for
both luminous and dark matter profiles. Nevertheless, it is pointed out that
these cluster mass estimators may be safely applied on scales greater than the
core sizes. Namely, the overall clusters of galaxies at intermediate redshift
can still be regarded as the dynamically relaxed systems, in which the velocity
dispersion of galaxies and the temperature of X-ray emitting gas are good
indicators of the underlying gravitational potentials of clusters.Comment: 16 pages with 7 PS figures, MNRAS in pres
Correlation Between the Halo Concentration (c) and the Virial Mass (Mvir) Determined from X-ray Clusters
Numerical simulations of structure formation have suggested that there exists
a good correlation between the halo concentration c (or the characteristic
density delta_c) and the virial mass Mvir for any virialized dark halo
described by the Navarro, Frenk & White (1995) density profile. In this Letter,
we present an observational determination of the c-Mvir (or delta_c-Mvir)
relation in the mass range of 10^14< Mvir <10^16 (solar mass) using a sample of
63 X-ray luminous clusters. The best-fit power law relation, which is roughly
independent of the values of Omega_M and Lambda, is c propto Mvir^(-0.5) or
delta_c propto Mvir^(-1.2), indicating n=-0.7 for a scale-free power spectrum
of the primordial density fluctuations. We discuss the possible reasons for the
conflict with the predictions by typical CDM models such as SCDM, LCDM and
OCDM.Comment: 13 pages, 1 figure, two tables. Accepted for publication in ApJ
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