Aims. We present the results from a comprehensive spectroscopic survey of the WINGS (WIde-field Nearby Galaxy-cluster Survey) clusters, a program called WINGS-SPE. The WINGS-SPE sample consists of 48 clusters, 22 of which are in the southern sky and 26 in the north. The main goals of this spectroscopic survey are: (1) to study the dynamics and kinematics of the WINGS clusters and their constituent galaxies, (2) to explore the link between the spectral properties and the morphological evolution in different density environments and across a wide range of cluster X-ray luminosities and optical properties. Methods. Using multi-object fiber-fed spectrographs, we observed our sample of WINGS cluster galaxies at an intermediate resolution of 6-9 Å and, using a cross-correlation technique, we measured redshifts with a mean accuracy of ∼45 km s −1 . Results. We present redshift measurements for 6137 galaxies and their first analyses. Details of the spectroscopic observations are reported. The WINGS-SPE has ∼30% overlap with previously published data sets, allowing us both to perform a complete comparison with the literature and to extend the catalogs. Conclusions. Using our redshifts, we calculate the velocity dispersion for all the clusters in the WINGS-SPE sample. We almost triple the number of member galaxies known in each cluster with respect to previous works. We also investigate the X-ray luminosity vs. velocity dispersion relation for our WINGS-SPE clusters, and find it to be consistent with the form L x ∝ σ 4 v .

We apply a friends-of-friends algorithm to the combined Updated Zwicky Catalog and Southern Sky Redshift Survey to construct a catalog of 1168 groups of galaxies; 411 of these groups have five or more members within the redshift survey. The group catalog covers 4.69 sr, and all groups exceed the number density contrast threshold, / = 80. We demonstrate that the groups catalog is homogeneous across the two underlying redshift surveys; the catalog of groups and their members thus provides a basis for other statistical studies of the large-scale distribution of groups and their physical properties. The median physical properties of the groups are similar to those for groups derived from independent surveys, including the ESO Key Programme and the Las Campanas Redshift Survey. We include tables of groups and their members.

For the first time, we construct a catalog of compact groups selected from a complete, magnitude-limited redshift survey. We select groups with N ≥ 3 members based on projected separation and association in redshift space alone. We evaluate the characteristics of the Redshift Survey Compact Groups (RSCG's). Their physical properties (membership frequency, velocity dispersion, density) are similar to those of the Hickson [ApJ, 255, 382 (1982)] Compact Groups. Hickson's isolation criterion is a strong function of the physical and angular group radii and is a poor predictor of the group environment. In fact, most RSCG's are embedded in dense environments. The luminosity function for RSCG's is mildly inconsistent with the survey luminosity function -the characteristic luminosity is brighter and the faint end shallower for the RSCG galaxies. We construct a model of the selection function of compact groups. Using this selection function, we estimate the abundance of RSCG's; for groups with N ≥ 4 members the abundance is 3.8 × 10 −5 h 3 Mpc −3 . For all RSCG's (N ≥ 3) the abundance is 1.4 × 10 −4 h 3 Mpc −3 .

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Abstract. We present an objective and automated procedure for detecting clusters of galaxies in imaging galaxy surveys . Our Voronoi Galaxy Cluster Finder (VGCF) uses galaxy positions and magnitudes to find clusters and determine their main features: size, richness and contrast above the background. The VGCF uses the Voronoi tessellation to evaluate the local density and to identify clusters as significative density fluctuations above the background. The significance threshold needs to be set by the user, but experimenting with different choices is very easy since it does not require a whole new run of the algorithm. The VGCF is non-parametric and does not smooth the data. As a consequence, clusters are identified irrespective of their shape and their identification is only slightly affected by border effects and by holes in the galaxy distribution on the sky. The algorithm is fast, and automatically assigns members to structures. A test run of the VGCF on the PDCS field centered at α = 13 h 26 m and δ = +29• 52 (J2000) produces 37 clusters. Of these clusters, 12 are VGCF counterparts of the 13 PDCS clusters detected at the 3σ level and with estimated redshifts from z = 0.2 to z = 0.6. Of the remaining 25 systems, 2 are PDCS clusters with confidence level < 3σ and redshift z ≤ 0.6. Inspection of the 23 new VGCF clusters indicates that several of these clusters may have been missed by the matched filter algorithm for one or more of the following reasons: a) they are very poor, b) they are extremely elongated, c) they lie too close to a rich and/or low redshift cluster.

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We describe the ROSAT All-Sky Survey-Center for Astrophysics Loose Systems (RASSCALS), the largest X-ray and optical survey of low mass galaxy groups to date. We draw 260 groups from the combined Center for Astrophysics and Southern Sky Redshift Surveys, covering one quarter of the sky to a limiting Zwicky magnitude of m z = 15.5. We detect 61 groups (23%) as extended X-ray sources.The statistical completeness of the sample allows us to make the first measurement of the X-ray selection function of groups, along with a clean determination of their fundamental scaling laws. We find robust evidence of similarity breaking in the relationship between the X-ray luminosity and velocity dispersion. Groups with σ p < 340 km s −1 are overluminous by several orders of magnitude compared to the familiar L X ∝ σ 4 law for higher velocity dispersion systems. An understanding of this break depends on the detailed structure of groups with small velocity dispersions σ p < 150 km s −1 .After accounting for selection effects, we conclude that only 40% of the optical groups are extended X-ray sources. The remaining 60% are either accidental superpositions, or systems devoid of X-ray emitting gas. Combining our results with group statistics from N-body simulations, we find that the fraction of real, bound systems in our objectively selected optical catalog is between 40%-80%.The X-ray detections have a median membership of 9 galaxies, a median recession velocity cz = 7250 km s −1 , a median projected velocity dispersion σ p = 400 km s −1 , and a median X-ray luminosity L X = 3 × 10 42 h −2 100 erg s −1 , where the Hubble constant is H 0 = 100h 100 km s −1 Mpc −1 . We include a catalog of these properties, or the appropriate upper limits, for all 260 groups.

Abstract. We present the results of a new spectroscopic and photometric survey of the hot X-ray cluster 1E0657-56, at z = 0.296. We determine the presence of a low velocity dispersion subcluster, which is offset from the main cluster position by 0.7 Mpc and 600 km s −1 . We determine the virial masses and total luminosities of the cluster and its subcluster, and solve for the two-body dynamical model. With additional constraints from the results of the analysis of the cluster X-ray emission by Markevitch et al. (2001), we find that the subcluster passed through the cluster centre 0.15 Gyr ago. Taken at face value the mass of the subcluster is typical of a loose group. It is however difficult to establish the pre-merger mass of the colliding system. We provide tentative evidence that the subcluster is in fact the remnant core of a moderately massive cluster, stripped by the collision with 1E0657-56. The main cluster dynamics does not seem to have suffered from this collision. On the contrary, the cluster X-ray properties seem to have been significantly affected. We also discuss the effect of the subcluster collision in relation to starburst events and the cluster radio halo.Key words. galaxies: clusters: individual: 1E0657-56 -galaxies: clusters: general -galaxies: distances and redshifts IntroductionThe evolution of clusters of galaxies as seen in numerical simulations is characterized by the asymmetric accretion of mass clumps from surrounding filaments (e.g. Diaferio et al. 2001). Nearby clusters are characterized by a variety of morphologies, indicative of different dynamical properties. Distant clusters, at redshifts z > 0.8, are often characterized by an elongated distribution, traced by several, apparently distinct, galaxy clumps. Such is the case of Cl 0023+0423 at z = 0.84 (Lubin et al. 1998), RX J1716.6+6708 at z = 0.81 (Gioia et al. 1999), MS 1054-03 at z = 0.84 (van Dokkum et al. 2000). Other distant clusters are found to have nearby companions, possibly in a pre-merger phase (Lubin et al. 2000;Rosati et al. 1999;Haines et al. 2001;Pentericci et al. 2000). Most of these high-z clusters are X-ray selected, and are therefore expected to be very massive.1E0657-558 is in many respects a low-redshift (z = 0.296) analogue of these high-z clusters. It is X-ray selected, it has an elongated morphology, and there is evidence for an additional subclump located to the West Send offprint requests to: A. Biviano, e-mail: biviano@ts.astro.it Based on observations collected at the European Southern Observatory (La Silla, Chile), Proposal ID: 64.0-0664.with respect to the main cluster region (see Sect. 3.1). Its high X-ray luminosity and temperature (Tucker et al. 1998, hereafter T98; Liang et al. 2000a;Markevitch et al. 2001, hereafter M01) as well as its high velocity dispersion (T98) strongly suggest it to be a very massive cluster. A detailed dynamical study of 1E0657-558 could therefore help us understand the dynamics of its more distant analogues.1E0657-558 is also very interesting per se. After detection in X...