RASS-SDSS Galaxy Cluster Survey. VII. On the Cluster Mass to Light ratio and the Halo Occupation Distribution

We explore the mass-to-light ratio in galaxy clusters and its relation to the cluster mass. We study the relations among the optical luminosity ($L_{op}$), the cluster mass ($M_{200}$) and the number of cluster galaxies within $r_{200}$ ($N_{gal}$) in a sample of 217 galaxy clusters with confirmed 3D overdensity. We correct for projection effects, by determining the galaxy surface number density profile in our cluster sample. This is best fitted by a cored King profile in low and intermediate mass systems. The core radius decreases with cluster mass, and, for the highest mass clusters, the profile is better represented by a generalized King profile or a cuspy Navarro, Frenk & White profile. We find a very tight proportionality between $L_{op}$ and $N_{gal}$, which, in turn, links the cluster mass-to-light ratio to the Halo Occupation Distribution $N_{gal}$ vs. $M_{200}$. After correcting for projection effects, the slope of the $L_{op}-M_{200}$ and $N_{gal}-M_{200}$ relations is found to be $0.92\pm0.03$, close, but still significantly less than unity. We show that the non-linearity of these relations cannot be explained by variations of the galaxy luminosity distributions and of the galaxy M/L with the cluster mass. We suggest that the nonlinear relation between number of galaxies and cluster mass reflects an underlying nonlinear relation between number of subhaloes and halo mass.Comment: 15 pages, 15 figures, accepted for publication in A&

The Mass Function of Nearby Galaxy Clusters

We present the distribution of virial masses for nearby galaxy clusters, as obtained from a data-set of 75 clusters, each having at least 20 galaxy members with measured redshifts within 1 Abell radius. After having accounted for problems of incompleteness of the data-set, we fitted a power-law to the cluster mass distribution.Comment: 10 pages (2 figures not included, available upon request), LATEX, Ref.SISSA 54/93/

The ESO Nearby Abell Cluster Survey. XIII. The orbits of the different types of galaxies in rich clusters

We study the orbits of the various types of galaxies observed in the ESO Nearby Abell Cluster Survey. Galaxies within and outside substructures are considered separately. We use the mass profile we determined from the distribution and kinematics of the early-type galaxies (i.e. ellipticals, excluding the brightest ones, and S0s) outside substructures; the latter were assumed to be on isotropic orbits, which is supported by the shape of their velocity distribution. The projected distribution and kinematics of the galaxies of other types are used to search for equilibrium solutions in the gravitational potential derived from the early-type galaxies, using the method described by Binney and Mamon as implemented by Solanes and Salvador-Sole'. For the brightest ellipticals we are not able to construct equilibrium solutions. This is most likely the result of the formation history and the special location of these galaxies at the centres of their clusters. The data for the early spirals allow equilibrium solutions and are consistent with isotropic orbits, although there is an apparent radial anisotropy at about 0.45 r200. For the late spirals an equilibrium solution with isotropic orbits is rejected by the data. The orbits are nearly isotropic within about 0.7 r200, but then become increasingly radial outwards. Finally, the data for the galaxies in substructures indicate that isotropic solutions are not acceptable, and tangential orbits are indicated. We briefly discuss the possible implications of these velocity-anisotropy profiles for current ideas of the evolution and transformation of galaxies in clusters. (Abridged)Comment: A&A, accepted. 13 pages, 10 figure

Spitzer Observations of Galaxy Clusters

We present preliminary results of a project to study three rich nearby clusters of galaxies with the Spitzer space telescope. The Spitzer observations in the four IRAC and three MIPS bands cover a region up to three virial radii, approximately, and have been recently completed. On the basis of the first Spitzer images, we followed up spectroscopically the far-infrared sources with the multi-fiber spectrograph HYDRA on the WIYN telescope. 70% of the sources brighter than 0.3 mJy at 24 μm and r’ < 20.5 have been observed for a total of 1078 spectra. For 87% of them we were able to measure redshifts obtaining 50 to 100 members for the different clusters. This first study shows that the far-IR sources in these clusters are predominantly powered by star formation and clustered in regions far from the center. In the case of A1763, they seem to be situated along a filament supporting the idea of infalling galaxies experiencing bursts of star formation during their first contact with the hot intra-cluster medium

The AGN fraction - velocity dispersion relation in clusters of galaxies

Some previous investigations have found that the fraction (f_AGN) of active galactic nuclei (AGNs) is lower in clusters than in the field. This can result from the suppression of galaxy-galaxy mergers in high-velocity dispersion (sigma_v) clusters, if the formation and/or fueling of AGNs is directly related to the merging process. We investigate the existence of a relation between f_AGN and sigma_v in galaxy clusters in order to shed light on the formation and evolution processes of AGNs and cluster galaxies. Using data from the Sloan Digital Sky Survey we determine f_AGN and sigma_v for the clusters in two samples, extracted from the catalogs of Popesso et al. (2006a) and Miller et al. (2005), and excluding clusters with significant evidence for substructures. We find a significant f_AGN-sigma_v anti-correlation. Clusters with sigma_v lower and, respectively, higher than 500 km/s have AGN fractions of $0.21 \pm 0.01$ and $0.15 \pm 0.01$, on average. The f_AGN-sigma_v relation can be described by a model that assumes f_AGN is proportional to the galaxies merging rate, plus a constant. Since f_AGN increases with decreasing sigma_v, AGNs are likely to have played a significant role in heating the intra-cluster medium and driving galaxy evolution in cluster precursors and groups.Comment: 4 pages, 3 figures, A&A Letter accepted for publicatio