10 research outputs found
The Star-Forming Dwarf Galaxy Populations of two z ~ 0.4 Clusters: MS1512.4+3647 and Abell 851
We present the results of a deep narrow-band [OII] 3727 \AA emission-line
search for faint ( 27), star-forming galaxies in the field of the
MS1512.4+3647 cluster. We find no evidence for an over-density of emission-line
sources relative to the field at 0.4 (Hogg et al. 1998), and therefore
conclude that the MS1512.4+3647 sample is dominated by field [OII]
emission-line galaxies which lie along the 180 Mpc line of sight
immediately in front and behind the cluster. This is surprising, given that the
previously surveyed cluster Abell 851 has 3-4 times the field
emission-line galaxy density (Martin et al. 2000). We find that the
MS1512.4+3647 sample is deficient in galaxies with intermediate colors (1.0 2.0) and implied star-formation exponential decay timescales
100 Myr - 1 Gyr that dominate the Abell 851 emission-line galaxy population.
Instead, the majority of [OII] emission-line galaxies surrounding the
MS1512.4+3647 cluster are blue () and forming stars in bursts
with 100 Myr. In both samples, galaxies with the shortest
star-formation timescales are preferentially among the faintest star-forming
objects. Their i luminosities are consistent with young stellar populations
\sim 10^8 - 10^9 \Msun, although an additional factor of ten in stellar mass
could be hiding in underlying old stellar populations. We discuss the
implications for the star-formation histories of dwarf galaxies in the field
and rich clusters.Comment: 26 pages, including 5 tables and 13 figures; accepted for publication
in the Astrophysical Journa
Gravitational Lens Statistics for Generalized NFW Profiles: Parameter Degeneracy and Implications for Self-Interacting Cold Dark Matter
Strong lensing is a powerful probe of the distribution of matter in the cores
of clusters of galaxies. Recent studies suggest that the cold dark matter model
predicts cores that are denser than those observed in galaxies, groups and
clusters. One possible resolution of the discrepancy is that the dark matter
has strong interactions (SIDM), which leads to lower central densities. A
generalized form of the Navarro, Frenk and White profile (Zhao profile) may be
used to describe these halos. In this paper we examine gravitational lensing
statistics for this class of model. The optical depth to multiple imaging is a
very sensitive function of the profile parameters in the range of interest for
SIDM halos around clusters of galaxies. Less concentrated profiles, which
result from larger self-interaction cross-sections, can produce many fewer
lensed pairs. Lensing statistics provide a powerful test for SIDM. More
realistic and observationally oriented calculations remain to be done, however
larger self-interaction cross-sections may well be ruled out by the very
existence of strong lenses on galaxy cluster scales. The inclusion of centrally
dominant cluster galaxies should boost the cross-section to multiple imaging.
However our preliminary calculations suggest that the additional multiple
imaging rate is small with respect to the differences in multiple imaging rate
for different halo profiles. In future statistical studies, it will be
important to properly account for the scatter among halo profiles since the
optical depth to multiple imaging is dominated by the most concentrated members
of a cluster population.Comment: 58 pages, 14 figures. To be published in ApJ. Revised version
includes discussion of magnification bias and the effect of a centrally
dominant galax
Probing the Core Structure of Dark Halos with Tangential and Radial Arc Statistics
We study the arc statistics of gravitational lensing generated by dark matter
halos in order to probe their density profile. We characterize the halo profile
by two parameters, the inner slope of the central cusp , and the median
amplitude of the concentration parameter, , for a halo of mass
at , and compute the numbers of tangential and
radial arcs produced by gravitational lensing of galaxy clusters. We find that
the number of arcs divided by the number of halos is a good statistic which is
sensitive to both and with very weak dependence on the
cosmological parameters. If the arc samples with well-defined selection
criteria for the clusters become available, one can strongly constrain both
and . While our tentative comparison with the existing
observational data indicates that the inner density profile of dark halos is
indeed as steep as predicted by recent simulations (), the
homogeneous samples of tangential and radial arcs are required for more
quantitative discussions.Comment: 23 pages, 8 figures, accepted for publication in Ap
A combined analysis of cluster mass estimates from strong lensing, X-ray measurement and the universal density profile
(Abridged)We present a combined analysis of mass estimates in the central
cores of galaxy clusters from the strong lensing, the X-ray measurements and
the universal density profile (NFW). Special attention is paid to the questions
(1)whether the previously claimed mass discrepancy between the strong lensing
and X-ray measurements is associated with the presence of cooling/non-cooling
flows, (2)whether the cusped NFW density model can provide a consistent cluster
mass with the strong lensing result and (3)whether a non-zero cosmological
constant can be of any help to reducing the strong lensing - X-ray mass ratios.
We analyse a sample of 26 arc-like images among 21 clusters. The X-ray and NFW
cluster masses are obtained by assuming that the intracluster gas is isothermal
and in hydrostatic equilibrium with the underlying gravitational potential of
the clusters. A statistical comparison of these three mass estimates reveals
that the mass discrepancies for all the events are well within a factor of 2.
We confirm the result of Allen that the larger mass discrepancy is only
detected in the intermediate cooling, especially non-cooling flow clusters,
thus attributing the mass discrepancy to the local dynamical activities in the
central regions. We show that the NFW profile yields a consistent cluster mass
with the conventional X-ray measurement, and any difference between these two
models must occur at even smaller radii (e.g. within the arc-like images) or at
large radii. Finally, a non-zero cosmological constant is able to moderately
reduce the mass ratios of m_lens to m_xray.Comment: 10 pages, 3 figures, to appear in MNRAS July issu
Arc Statistics in Triaxial Dark Matter Halos: Testing the Collisionless Cold Dark Matter Paradigm
Statistics of lensed arcs in clusters of galaxies serve as a powerful probe
of both the non-sphericity and the inner slope of dark matter halos. We develop
a semi-analytic method to compute the number of arcs in triaxial dark matter
halos. This combines the lensing cross section from the Monte Carlo ray-tracing
simulations, and the probability distribution function (PDF) of the axis ratios
evaluated from cosmological N-body simulations. This approach enables one to
incorporate both asymmetries in the projected mass density and elongations
along the line-of-sight analytically, for the first time in cosmological lensed
arc statistics. As expected, triaxial dark matter halos significantly increase
the number of arcs relative to spherical models; the difference amounts to more
than one order of magnitude while the value of enhancement depends on the
specific properties of density profiles. Then we compare our theoretical
predictions with the observed number of arcs from 38 X-ray selected clusters.
In contrast to the previous claims, our triaxial dark matter halos with inner
density profile \rho \propto r^{-1.5} in a Lambda-dominated cold dark matter
(CDM) universe reproduces well the observation. Since both the central mass
concentration and triaxial axis ratios (minor to major axis ratio ~0.5)
required to account for the observed data are consistent with cosmological
N-body simulations, our result may be interpreted to lend strong support for
the collisionless CDM paradigm at the mass scale of clusters.Comment: 19 pages, 15 figures, 3 tables, error in eq. (47) corrected,
conclusion unchange
Deep Optical Imaging of A Compact Group of Galaxies, Seyfert's Sextet
In order to investigate the dynamical status of Seyfert's Sextet (SS), we
have obtained a deep optical () image of this group. Our image shows that
a faint envelope, down to a surface brightness (AB) mag arcsec, surrounds the member galaxies. This envelope is
irregular in shape. It is likely that this shape is attributed either to
recent-past or to on-going galaxy interactions in SS. If the member galaxies
have experienced a number of mutual interactions over a long timescale, the
shape of the envelope should be rounder. Therefore, the irregular-shaped
morphology suggests that SS is in an early phase of dynamical interaction among
the member galaxies. It is interesting to note that the soft X-ray image
obtained with ROSAT (Pildis et al. 1995) is significantly similar in
morphology. We discuss the possible future evolution of SS briefly.Comment: 7 pages text (emulateapj LaTeX), 8 figures (3 EPS files, 1 PostScript
file, and 4 jpeg files) figures, The Astronomical Journal, 120, No. 5 inpres
Measuring Cosmological Parameters from the Evolution of Cluster X-ray Temperatures
We have determined the cluster X-ray temperature function from two flux- and
redshift-limited samples of clusters. The first sample is comprised of 25
clusters with average redshift 0.05. The local temperature function derived
from it supercedes the one we previously published. Fourteen clusters with
average redshift 0.38 comprise the second sample. We perform maximum likelihood
fits of cluster evolution models to these data in order to constrain
cosmological parameters. For an open model with zero cosmological constant we
find that the density parameter is Omega_0 = 0.49+/-0.12, the rms mass density
fluctuation averaged over 8 h-1 Mpc spheres is sigma_8 = 0.72+/-0.10, and the
effective index of the mass density fluctuation spectrum on cluster scales is n
= -(1.72+/-0.34) where all errors are symmetrized at 68% confidence. The
corresponding results for the case where a cosmological constant produces a
flat universe are : 0.44+/-0.12, 0.77+/-0.15, and -(1.68+/-0.38). These results
agree with those determined from a variety of different independent methods,
including supernovae, galaxy-galaxy correlations, fluctuations in the microwave
background, gravitational lens statistics, and cluster peculiar velocities.Comment: 29 pages, 15 postscript figures, accepted for publication in volume
533 of the Astrophysical Journal (April 20, 2000
Gravitational Lensing
Gravitational lensing has developed into one of the most powerful tools for
the analysis of the dark universe. This review summarises the theory of
gravitational lensing, its main current applications and representative results
achieved so far. It has two parts. In the first, starting from the equation of
geodesic deviation, the equations of thin and extended gravitational lensing
are derived. In the second, gravitational lensing by stars and planets,
galaxies, galaxy clusters and large-scale structures is discussed and
summarised.Comment: Invited review article to appear in Classical and Quantum Gravity, 85
pages, 15 figure