The Two-Component Virial Theorem and the Physical Properties of Stellar Systems

Motivated by present indirect evidences that galaxies are surrounded by dark matter halos, we investigate whether their physical properties can be described by a formulation of the virial theorem which explicitly takes into account the gravitational potential term representing the interaction of the dark halo with the barionic or luminous component. Our analysis shows that the application of such a ``two-component virial theorem'' not only accounts for the scaling relations displayed, in particular, by elliptical galaxies, but also for the observed properties of all virialized stellar systems, ranging from globular clusters to galaxy clusters.Comment: 13 pages, 2 figures, LaTeX, corrected few typos. This version matches the published versio

The structure and dynamics of the AC114 galaxy cluster revisited

We present a dynamical analysis of the galaxy cluster AC114 based on a catalogue of 524 velocities. Of these, 169 (32%) are newly obtained at ESO (Chile) with the VLT and the VIMOS spectrograph. Data on individual galaxies are presented and the accuracy of the measured velocities is discussed. Dynamical properties of the cluster are derived. We obtain an improved mean redshift value z= 0.31665 +/- 0.0008 and velocity dispersion \sigma= 1893+73-82 \kms. A large velocity dispersion within the core radius and the shape of the infall pattern suggests that this part of the cluster is in a radial phase of relaxation with a very elongated radial filament spanning 12000 \kms. A radial foreground structure is detected within the central 0.5/h Mpc radius, recognizable as a redshift group at the same central redshift value. We analyze the color distribution for this archetype Butcher-Oemler galaxy cluster and identify the separate red and blue galaxy sequences. The latter subset contains 44% of confirmed members of the cluster, reaching magnitudes as faint as R_{f}= 21.1 (1.0 magnitude fainter than previous studies). We derive a mass M_{200}= (4.3 \pm 0.7) x 10^15 Msun/h. In a subsequent paper we will utilize the spectral data presented here to explore the mass-metallicity relation for this intermediate redshift cluster.Comment: 17 pages, 11 figures, 4 tables, accepted for publication in MNRA

"Fundamental-Plane"-Like Relations From Collisionless Stellar Dynamics: A Comparison of Mergers and Collapses

We present a new set of dissipationless N-body simulations aiming to better understand the pure dynamical aspects of the ``Fundamental Plane'' (FP) of elliptical galaxies. We have extended our previous hierarchical merger scheme by considering the Hernquist profile for the initial galaxy model. Two-component Hernquist galaxy models were also used to study the effect of massive dark halos on the end-products characteristics. We have also performed new collapse simulations including initial spin. We found that the one-component Hernquist mergers give results similar to those found for the one-component King models, namely both were able to build-up small scatter FP-like correlations with slopes consistent with what is found for the near-infrared FP of nearby galaxies. The two-component models also reproduce a FP-like correlation, but with a significantly steeper slope. This is in agreement with what has been found for elliptical galaxies at higher redshift (0.1 $<$ z $<$ 0.6). We discuss some structural properties of the simulated galaxies and their ability to build-up FP-like correlations. We confirm that collapses generally do not follow a FP-like correlation regardless of the initial spin. We suggest that the evolution of gradients in the gravitational field of the merging galaxies may be the main ingredient dictating the final non-homology property of the end products.Comment: 15 pages, 11 figures, uses mn2e.cls. Accepted for publication in MNRA

The Nature of the Activity in Hickson Compact Groups of Galaxies

We present the results of the spectral classification of the 82 brightest galaxies in a sample of 17 compact groups. We verify that the AGNs are preferentially located in the most early-type and luminous galaxies of the groups, as is usually observed in the field. But these AGNs also appear to be systematically concentrated towards the central parts of the groups. Our observations suggest a correlation between activity types, morphologies and densities of galaxies in the compact groups. This is consistent with a scenario in which galaxies of compact groups evolve by interacting with their environment and are currently in a quiet phase of their activityComment: 22 pages, LaTeX (aasms4.sty) + 7 figures; accepted for publication in the Astrophysical Journa

Evidence of Substructure in the Cluster of Galaxies A3558

We investigate the dynamical properties of the cluster of galaxies A3558 (Shapley 8). Studying a region of one square degree ($\sim$ 3 Mpc$^2$) centered on the cluster cD galaxy, we have obtained a statistically complete photometric catalog with positions and magnitudes of 1421 galaxies (down to a limiting magnitude of $B \sim 21$). This catalog has been matched to the recent velocity data obtained by Mazure et al. (1997) and from the literature, yielding a radial velocity catalog containing 322 galaxies. Our analysis shows that the position/velocity space distribution of galaxies shows significant substructure. A central bimodal core detected previously in preliminary studies is confirmed by using the Adaptive Kernel Technique and Wavelet Analysis. We show that this central bimodal subtructure is nevertheless composed of a projected feature, kinematically unrelated to the cluster, plus a group of galaxies probably in its initial merging phase into a relaxed core. The cD velocity offset with respect to the average cluster redshift, reported earlier by several authors, is completely eliminated as a result of our dynamical analysis. The untangling of the relaxed core component also allows a better, more reliable determination of the central velocity dispersion, which in turn eliminates the ``$\beta$-problem'' for A3558. The cluster also shows a ``preferential'' distribution of subclumps coinciding with the direction of the major axis position angle of the cD galaxy and of the central X-ray emission ellipsoidal distribution, in agreement with an anisotropic merger scenario.Comment: 35 pages in latex, 17 figures in Postscript, accepted for publication in the Astrophysical Journa

The Velocity Field in the Neighbourhood of Clusters of Galaxies: a Study Based on Photometric Redshifts

International audienceWe present a preliminary study of the velocity field in the neighbourhood of the Abell 1942 cluster of galaxies (z &ap; 0.22), for which both photometric and spectroscopic redshifts are available. Besides characterizing the dynamical stage of this particular cluster, the aim of this work is to search for efficient tools capable to provide reliable representations of the infall velocity fields for samples of clusters of galaxies, based solely on photometric redshifts