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Astron. Astrophys. 356, 815-826 (2000)

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1. Introduction

In the framework of hierarchical clustering, the Universe is believed to be made of galaxies distributed in sheets encircling voids or filaments, at the intersection of which clusters of galaxies are located. Such models can be tested through the analysis of clusters, which are likely to keep a "memory" of their formation. This is suggested for example by the alignment effects observed in some clusters, such as for example Abell 3558 (Dantas et al. 1997) or Abell 85 (Durret et al. 1998), where the cD, the brightest galaxies, the X-ray emitting gas and possibly even larger scale structures (in the case of Abell 85) all appear aligned along the same direction. Multi-wavelength studies of clusters of galaxies also allow us to draw a global and coherent portrait of these objects, which we can then use to address other questions of interest, such as the influence of mergers and environmental effects at various scales on the properties of both galaxies and X-ray gas. Large scale (i.e. cluster size) mergers are quite often observed from substructures detected in the X-ray gas; smaller scale mergers (i.e. group size) such as the infall of dwarf galaxies onto groups surrounding bright galaxies can be derived from various methods such as those of Serna & Gerbal (1996) or Gurzadyan & Mazure (1998), which require optical velocity and magnitude catalogues; the existence of subclustering also has an influence on the shape of the galaxy luminosity function, which in some cases appears to show a deficit of faint galaxies often interpreted as due to accretion of dwarf galaxies onto larger galaxies or groups (e.g. in Coma, Lobo et al. 1997, Adami et al. 2000). It therefore appears important to analyze cluster properties in detail before using them in other studies. Note in particular that the existence of substructures can lead to overestimate cluster velocity dispersions, and hence M/L ratios and the value of [FORMULA] in clusters.

With the improvement of both observational means (better X-ray detectors, optical multi-object spectroscopy) and modern methods of analysis (some of which are described below), an ever increasing number of clusters showing evidence for merging and environmental effects has been found. A rather general picture has therefore emerged for clusters, with a main relaxed body on to which groups of various sizes can be falling.

Our approach these last years has been to study a small sample of nearby clusters in detail. These have the advantage of being bright, and can therefore be observed in detail within a reasonable amount of telescope time. Besides, they are free of evolution effects. We present here a detailed multi-wavelength study of Abell 496, based on optical (extensive redshift and photometric catalogues) and X-ray (ROSAT PSPC) data.

Abell 496 is a richness class 1 (Abell 1958) cD type (Struble & Rood 1987) cluster at a redshift of 0.0331. For a Hubble constant H0=50 km s-1 Mpc-1, the corresponding scale is 55.0 kpc/arcmin and the distance modulus is 36.52. At optical wavelengths, an adaptive kernel map of the central region (in a 60[FORMULA]60 arcmin2 square) has revealed a somewhat complicated structure, with a strong concentration of galaxies in the north-south direction (Kriessler & Beers 1997). Note however that this map does not include redshift information. Thorough investigations of the X-ray properties of Abell 496 can be found in Mohr et al. (1999) and Markevitch et al. (1999); their results will be compared to ours in Sect. 4.5.

The paper is organized as follows: we present the data in Sect. 2; the structures along the line of sight derived from the velocity catalogue are described in Sect. 3; the optical properties of the Abell 496 cluster itself are described in Sect. 4; the X-ray cluster properties are described in Sect. 5; a summary and conclusions are given in Sect. 6.

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© European Southern Observatory (ESO) 2000

Online publication: April 17, 2000