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 $0.015<z<0.080$. 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 ($0.05<z<0.31$) 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

The Compact Group of Galaxies HCG 31 is in an early phase of merging

We have obtained high spectral resolution (R = 45900) Fabry-Perot velocity maps of the Hickson Compact Group HCG 31 in order to revisit the important problem of the merger nature of the central object A+C and to derive the internal kinematics of the candidate tidal dwarf galaxies in this group. Our main findings are: (1) double kinematic components are present throughout the main body of A+C, which strongly suggests that this complex is an ongoing merger (2) regions $A2$ and E, to the east and south of complex A+C, present rotation patterns with velocity amplitudes of $\sim 25 km s^{-1}$ and they counterrotate with respect to A+C, (3) region F, which was previously thought to be the best example of a tidal dwarf galaxy in HCG 31, presents no rotation and negligible internal velocity dispersion, as is also the case for region $A1$. HCG 31 presents an undergoing merger in its center (A+C) and it is likely that it has suffered additional perturbations due to interactions with the nearby galaxies B, G and Q.Comment: 5 pages + figures - Accepted to ApJ Lette