Astron. Astrophys. 355, 848-862 (2000)

1. Introduction

Clusters of galaxies are complex, evolving systems which present numerous observational and theoretical challenges, yet the scientific payoff of a detailed understanding of these structures is also great. The determination of the total gravitationally-bound mass, the relative distribution of visible (galaxies and hot gas) and dark matter, and the dynamics of galaxies in clusters all provide essential information for testing models of galaxy formation and evolution.

There exist numerous uncertainties in deriving estimates of the mass distribution within clusters from optical observations alone (Merritt & Gebhardt 1995). X-ray observations of the hot gas in clusters permit significant progress in our understanding of cluster dark matter distributions. The two approaches are complementary, but for both methods strong assumptions are required for recovering the mass distribution. The optical approach (through the virial analysis) requires one to adopt hypotheses concerning the orbital distribution of member galaxies, while the X-ray approach assumes that the hot gas is in hydrostatic equilibrium within (presumably) a single cluster gravitational potential. A comparison of both methods, when possible, provides the opportunity to test the underlying hypotheses and to better constrain the model parameters (Henry et al. 1993). With this aim in mind, we have initiated a combined X-ray and optical observational program, including both imaging and multi-object spectroscopy at ESO and at the CFHT, for a selected sample of clusters at intermediate redshifts. Our sample was defined in order to cover a variety of clusters, which could be taken as representative of a range of dynamical states.

Abell 521 is a distant (Abell distance class 6), relatively rich (Abell richness class 1), southern cluster, morphologically classified as Bautz-Morgan Type III (Abell 1958; Abell et al. 1989). This cluster was shown by the HEAO-1 survey to be a bright X-ray source (Kowalski et al. 1984). In images taken to select spectroscopic targets in the field of this cluster, we detected several arc candidates (Maurogordato et al. 1996). This was not surprising, as it is suggested by theoretical studies of the gravitational lensing phenomenon that luminous arcs should be frequently found in the centers of distant X-ray luminous clusters (Le Fèvre et al. 1995). The detection of luminous arcs in Abell 521, if real , is important because they provide an independent means to estimate the total mass of the cluster, without requiring explicit models of the mass distribution (Fort & Mellier 1994). We were thus motivated to focus on this cluster for further analysis, mainly through multi-object spectroscopy, to probe its dynamical state, with long-slit spectroscopy, to test the reality of the arc and arclet candidates, and with multi-band photometry of the central region, with X-Ray imaging and spectroscopy being conducted in parallel. The combined X-ray and optical analysis of the cluster will be presented in Arnaud et al. (2000); a photometric redshift analysis of the arc candidates from multicolor photometry will appear in Pelló et al. (2000).

Sect. 2 of this paper presents the spectroscopic data we have obtained to date in our observational campaign. Sect. 3 is an analysis of the velocity distribution. In Sect. 4 we discuss the nature of the brightest cluster galaxy, and the reality of the gravitational arc candidates which have been previously suggested. In the following, unless explicitly specified, we have used , and .

© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000
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