Multiple merging events in Abell 521

We present a detailed spatial and dynamical analysis of the central $\sim$~2.2~\h~Mpc region of the galaxy cluster Abell~521 (z=0.247), based on 238 spectra obtained at the 3.6~m Telescope of ESO and at the CFHT. From the analysis of the 125 galaxies confirmed members of the cluster, we derive a mean velocity of $74019 ^{+112}_{-125}$ km/s and detect a complex velocity distribution with high velocity dispersion, $1325 ^{+145}_{-100}$ km/s), but clear departure from a single gaussian component. The general structure of the cluster follows a NW/SE direction, crossed by a perpendicular high density ``ridge'' of galaxies in the core region. The northern region of the cluster is characterized by a lower velocity dispersion as compared to the whole cluster value; it hosts the BCG and a dynamically bound complex of galaxies, and it is associated to a group detected in X-ray (Arnaud et al 2000). This region could be in pre-merger stage onto the main cluster nearly in the plane of the sky. These results, taken together with the fact that most of the clumps detected on the isodensity maps, as well as the early type galaxies and the brightest ones are aligned, suggest that this NW/SE direction is the preferred one for the formation of this cluster. The central high dense region shows a lower velocity location ($73625 ^{+344}_{-350}$ km/s) and significantly higher scale ($1780 ^{+234}_{-142}$ km/s) as compared to the whole cluster values. This is due to the presence of a low-velocity group of galaxies with a high fraction of emission line objects. This can be explained in a scenario in which a merging of subclusters has recently occurred along the direction of the ``ridge'' with a significant component along the line of sight.Comment: 21 pages, 32 figures, uses aa.cls style, Latex. Accepted for publication in A&

The Fundamental Plane of Galaxy Clusters

Velocity dispersion $\sigma$, radius $R$ and luminosity $L$ of elliptical galaxies are known to be related, leaving only two degrees of freedom and defining the so-called ``fundamental plane". In this {\em Letter} we present observational evidence that rich galaxy clusters exhibit a similar behaviour. Assuming a relation $L \propto R^{\alpha}\sigma^{2 \beta}$, the best-fit values of $\alpha$ and $\beta$ are very close to those defined by galaxies. The dispersion of this relation is lower than 10 percent, i.e. significantly smaller than the dispersion observed in the $L-\sigma$ and $L-R$ relations. We briefly suggest some possible implications on the spread of formation times of objects and on peculiar velocities of galaxy clusters.Comment: 11pp., 4 figures (available on request), LaTeX, BAP-04-1993-015-OA

ATCA observations of the galaxy cluster Abell 3921 - I. Radio emission from the central merging sub-clusters

We present the analysis of our 13 and 22 cm ATCA observations of the central region of the merging galaxy cluster A3921 (z=0.094). We investigated the effects of the major merger between two sub-clusters on the star formation (SF) and radio emission properties of the confirmed cluster members. The origin of SF and the nature of radio emission in cluster galaxies was investigated by comparing their radio, optical and X-ray properties. We also compared the radio source counts and the percentage of detected radio galaxies with literature data. We detected 17 radio sources above the flux density limit of 0.25 mJy/beam in the central field of A3921, among which 7 are cluster members. 9 galaxies with star-forming optical spectra were observed in the collision region of the merging sub-clusters. They were not detected at radio wavelengths, giving upper limits for their star formation rate significantly lower than those typically found in late-type, field galaxies. Most of these star-forming objects are therefore really located in the high density part of the cluster, and they are not infalling field objects seen in projection at the cluster centre. Their SF episode is probably related to the cluster collision that we observe in its very central phase. None of the galaxies with post-starburst optical spectra was detected down our 2$\sigma$ flux density limit, confirming that they are post-starburst and not dusty star-forming objects. We finally detected a narrow-angle tail (NAT) source associated with the second brightest cluster galaxy (BG2), whose diffuse component is a partly detached pair of tails from an earlier period of activity of the BG2 galaxy.Comment: 17 pages, 9 figures, accepted for publication in A&A, date of acceptance 29/06/2006. A version of the paper with higher resolution images can be downloaded at: http://astro.uibk.ac.at/~c.ferrari/ATCA_Paper/A3921_ATCA.pd

Properties of Very Luminous Galaxies

Recent analysis of the SSRS2 data based on cell-counts and two-point correlation function has shown that very luminous galaxies are much more strongly clustered than fainter galaxies. In fact, the amplitude of the correlation function of very luminous galaxies ($L > L^*$) asymptotically approaches that of $R \ge 0$ clusters. In this paper we investigate the properties of the most luminous galaxies, with blue absolute magnitude $M_B \le -21$. We find that: 1) the population mix is comparable to that in other ranges of absolute magnitudes; 2) only a small fraction are located in bona fide clusters; 3) the bright galaxy-cluster cross-correlation function is significantly higher on large scales than that measured for fainter galaxies; 4) the correlation length of galaxies brighter than \MB $\sim -20.0$, expressed as a function of the mean interparticle distance, appears to follow the universal dimensionless correlation function found for clusters and radio galaxies; 5) a large fraction of the bright galaxies are in interacting pairs, others show evidence for tidal distortions, while some appear to be surrounded by faint satellite galaxies. We conclude that very luminous optical galaxies differ from the normal population of galaxies both in the clustering and other respects. We speculate that this population is highly biased tracers of mass, being associated to dark halos with masses more comparable to clusters than typical loose groups.Comment: 29 pages (6 figures) + 2 tables; paper with all figures and images available at http://boas5.bo.astro.it/~cappi/papers.html; The Astronomical Journal, in pres

High-Order Correlations of Rich Galaxy Clusters

We analyse the two--dimensional all--sky distribution of rich Abell and ACO galaxy clusters by using counts in cells and measuring the high--order area--averaged angular correlation functions. Confirming previous results, we find a well defined hierarchical relation between the two and three--point correlation functions, remarkably constant with scale. In the angular range $2^\circ \le \theta \le 4^\circ$, the southern sample, limited at $b_{II} \le -40^\circ$ and including both Abell and ACO clusters, shows a remarkable hierarchical behavior up to the 6th order, while northern Abell clusters give positive correlations in the same range only up to the 4th order. The inferred deprojected values of the 3--D coefficients $S_J$, where $S_J = \bar{\xi}_J / {\bar{\xi}_2}^{J-1}$, are similar to those measured for the galaxy distribution, and consistent with theoretical predictions. These results are confirmed to the 4th order by our analysis of a 3--D sample of Abell and ACO clusters. Assuming that selection effects and / or the absence of a cluster fair sample are the reason of the difference between the two galactic hemispheres, and between Abell and ACO clusters, our results indicate that the statistical properties of the cluster distribution originate from the underlying galaxy distribution and show that the biasing between clusters and galaxies is non--linear.Comment: 29 pp., (ApJ, accepted for publication). This is a uuencoded compressed postscript file including figure

Abell 2384: the galaxy population of a cluster post-merger

We combine multi-object spectroscopy from the 2dF and EFOSC2 spectrographs with optical imaging of the inner 30'x30' of A2384 taken with the ESO Wide Field Imager. We carry out a kinematical analysis using the EMMIX algorithm and biweight statistics. We address the possible presence of cluster substructures with the Dressler-Shectman test. Cluster galaxies are investigated with respect to [OII] and H{\alpha} equivalent width. Galaxies covered by our optical imaging observations are additionally analysed in terms of colour, star formation rate and morphological descriptors such as Gini coefficient and M20 index. We study cluster galaxy properties as a function of clustercentric distance and investigate the distribution of various galaxy types in colour-magnitude and physical space. The Dressler-Shectman test reveals a substructure in the east of the 2dF field-of-view. We determine the mass ratio between the northern and southern subcluster to be 1.6:1. In accordance with other cluster studies, we find that a large fraction of the disk galaxies close to the cluster core show no detectable star formation. Probably these are systems which are quenched due to ram-pressure stripping. The sample of quenched disks populates the transition area between the blue cloud and the red sequence in colour-magnitude space. We also find a population of morphologically distorted galaxies in the central cluster region. The substructure in the east of A2384 might be a group of galaxies falling onto the main cluster. We speculate that our sample of quenched spirals represents an intermediate phase in the ram-pressure driven transformation of infalling field spirals into cluster S0s. This is motivated by their position in colour-magnitude space. The occurrence of morphologically distorted galaxies in the cluster core complies with the hypothesis of A2384 representing a post merger system.Comment: 14 pages, 18 figures, A&A accepte