127 research outputs found
The dynamical state of massive galaxy clusters
We study the mass distribution of a sample of 24 X-ray bright Abell clusters
through weak gravitational lensing. This method is independent of the dynamical
state of the galaxy cluster. Hence, by comparing dynamical and lensing mass
estimators, we can access the dynamical state of these clusters. We have found
that clusters with ICM temperatures above 8 keV show strong deviations from the
relaxation, as well as the presence of prominent sub-structures. For the
remaining clusters (the majority of the sample) we have found agreement among
the several mass estimators, which indicates that most of the clusters are in
or close to a state of dynamical equilibrium.Comment: 2 pages, 2 figures. Contributed talk, XIth IAU Latin-American
Regional Meeting (Dec. 2005, Pucon, Chile), to appear in RMxA
Shrinking of Cluster Ellipticals: a Tidal Stripping explanation and Implications for the Intra-Cluster Light
We look for evidence of tidal stripping in elliptical galaxies through the
analysis of homogeneous CCD data corresponding to a sample of 228 elliptical
galaxies belonging to 24 clusters of galaxies at . We
investigate departures from the standard magnitude-isophotal size relation, as
a function of environmental (cluster-centric distance, local galaxy density)
and structural (cluster velocity dispersion, Bautz-Morgan type) properties. We
find that, for any particular galaxy luminosity, the ellipticals in the inner
and denser regions of the clusters are about 5% smaller than those in the outer
regions, which is in good agreement with the finding of Strom & Strom (1978)
based on photographic photometry. The null hypothesis (ie., galaxy sizes are
independent of the cluster-centric distance or density) is rejected at a
significance level of better than 99.7%. Numericals models of Aguilar & White
(1986) predict that tidal stripping can lead to changes in the whole structure
of ellipticals producing shrinkage and brightening of the galaxy, qualitatively
consistent with our measurements and also with the findings of Trujillo et al.
(2002), that more centrally concentrated ellipticals populate denser regions.
Our observational results can be interpreted as evidence for stripping of stars
from ellipticals in the central/denser regions of clusters, contributing to the
intra-cluster light observed in these structures.Comment: AJ Accepted, 15 pages, 9 figure
Weak lensing mass distributions for 24 X-Ray Abell Clusters
We use the weak gravitational lensing effect to study the mass distribution
and dynamical state of a sample of 24 X-ray luminous clusters of galaxies
() observed with the VLT-FORS1 under homogeneous sky conditions
and subarsecond image quality. The galaxy shapes were measured after
deconvolution with a locally determined point-spread-function, while the
two-dimensional mass distributions of the clusters were computed using an
algorithm based on the maximum entropy method. By comparing the mass and light
distributions of the clusters in our sample, we find that their mass centers,
for the majority of the clusters, is consistent with the positions of optical
centers. We find that some clusters present significant mass substructures
which generally have optical counterparts. At least in one cluster (Abell
1451), we detect a mass substructure without an obvious luminous counterpart.
The radial profile of the shear of the clusters was fitted using circular and
elliptical isothermal elliptical distributions, which allowed the finding of a
strong correlation between the orientation of the major-axis of the matter
distribution and the corresponding major-axes of the brightest cluster galaxy
light-profiles. Estimates of how close to dynamical relaxation are these
clusters were obtained through comparison of our weak-lensing mass measurements
with the x-ray and velocity dispersion determinations available in the
literature. We find that clusters with intra-cluster gas colder than 8 keV show
a good agreement between the different mass determinations, but clusters with
gas hotter than 8 keV present discrepant mass values.Comment: 23 pages, 8 figures. Submitted to ApJ A version of this paper with
all the figures of the appendix A can be found at
http://www.astro.iag.usp.br/~eduardo/shear.ps.g
Gemini and Chandra observations of Abell 586, a relaxed strong-lensing cluster
We analyze the mass content of the massive strong-lensing cluster Abell 586
(). We use optical data (imaging and spectroscopy) obtained with the
Gemini Multi-Object Spectrograph (GMOS) mounted on the 8-m Gemini-North
telescope, together with publicly available X-ray data taken with the
\textit{Chandra} space telescope. Employing different techniques -- velocity
distribution of galaxies, weak gravitational lensing, and X-ray spatially
resolved spectroscopy -- we derive mass and velocity dispersion estimates from
each of them. All estimates agree well with each other, within a 68% confidence
level, indicating a velocity dispersion of 1000 -- 1250 \kms. The projected
mass distributions obtained through weak-lensing and X-ray emission are
strikingly similar, having nearly circular geometry. We suggest that Abell 586
is probably a truly relaxed cluster, whose last major merger occurred more than
Gyr agoComment: ApJ accepted, 20 pages, 11 figures; Figure 1 fixe
Cosmic shear requirements on the wavelength dependence of telescope point spread functions
Cosmic shear requires high precision measurement of galaxy shapes in the presence of the observational point spread function (PSF) that smears out the image. The PSF must therefore be known for each galaxy to a high accuracy. However, for several reasons, the PSF is usually wavelength dependent; therefore, the differences between the spectral energy distribution of the observed objects introduce further complexity. In this paper, we investigate the effect of the wavelength dependence of the PSF, focusing on instruments in which the PSF size is dominated by the diffraction limit of the telescope and which use broad-band filters for shape measurement. We first calculate biases on cosmological parameter estimation from cosmic shear when the stellar PSF is used uncorrected. Using realistic galaxy and star spectral energy distributions and populations and a simple three-component circular PSF, we find that the colour dependence must be taken into account for the next generation of telescopes. We then consider two different methods for removing the effect: (i) the use of stars of the same colour as the galaxies and (ii) estimation of the galaxy spectral energy distribution using multiple colours and using a telescope model for the PSF. We find that both of these methods correct the effect to levels below the tolerances required for per cent level measurements of dark energy parameters. Comparison of the two methods favours the template-fitting method because its efficiency is less dependent on galaxy redshift than the broad-band colour method and takes full advantage of deeper photometr
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