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Astron. Astrophys. 343, 760-774 (1999)

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1. Introduction

As the largest gravitationally bound systems in the Universe, clusters of galaxies have attracted much interest since the pioneering works of Zwicky, who evidenced the existence of dark matter in these objects, and later of Abell (1958), who achieved the first large catalogue of clusters. Clusters of galaxies are now studied through various complementary approaches, e.g. optical imaging and spectroscopy, which allow in particular to derive the distribution and kinematical properties of the cluster galaxies, and to estimate the luminosity function, and X-ray spectral imaging, which gives informations on the physical properties of the X-ray gas embedded in the cluster, and with some hypotheses can lead to estimate the total cluster binding mass.

As a complementary approach to large cluster surveys at small redshifts such as the ESO Nearby Abell Cluster Survey (ENACS, Katgert et al. 1996), we have chosen to analyze in detail a few low-z clusters of galaxies, by combining optical data (imaging and spectroscopy of a large number of galaxies) and X-ray data from the ROSAT archive. We present here complementary results on ABCG 85, which our group has already analyzed under various aspects (see references below).

ABCG 85 has a redshift of z[FORMULA]0.0555, corresponding to a spatial scale of 97.0 kpc/arcmin (for [FORMULA] km s-1 Mpc-1, value that will be used hereafter, together with q0=0). Its center is defined hereafter as the center of the diffuse X-ray component: [FORMULA], [FORMULA] (Pislar et al. 1997). A wealth of data is now available for this cluster: a photometric catalogue of 4232 galaxies obtained by scanning a [FORMULA] band photographic plate in a square region [FORMULA] (5.83 Mpc at the cluster redshift) from the cluster center, calibrated with V and R band CCD imaging taken in the very center (Slezak et al. 1998) and a spectroscopic catalogue of 551 galaxies in a roughly circular region of [FORMULA] radius in the direction of ABCG 85, among which 305 belong to the cluster (Durret et al. 1998a). As discussed in our previous papers (Pislar et al. 1997, Lima-Neto et al. 1997, Durret et al. 1998b), there exists in fact a complex of clusters ABCG 85/87/89 in this direction. In X-rays, ABCG 85 shows a homogeneous body, onto which are superimposed various structures: an excess towards the north-west and south-west, a south region superimposed on it, and several blobs forming a long filament towards the south-east; the velocity data confirm the existence of groups and clusters superimposed along the line of sight (see a complete description in Durret et al. 1998b) and show that this X-ray filament seems to be made of blobs falling onto the main cluster.

We present here results on ABCG 85 which may have cosmological implications on the formation of galaxies, clusters and large scale structures. The properties of emission versus non-emission line galaxies will be discussed in Sect. 2 and compared to recent results on emission line galaxies in clusters by Mohr et al. (1996) for ABCG 576 (redshift z=0.038), Biviano et al. (1997) for the large ENACS sample at low redshift (0.035[FORMULA]z[FORMULA]0.121), and Carlberg et al. (1996) for the CNOC sample at higher redshift (0.1709[FORMULA]z[FORMULA]0.5466). The cluster luminosity function in the R band will be derived in Sect. 3 and its shape will be compared to that found in other clusters. We will present in Sect. 4 the dynamical properties of the cluster, by estimating the dynamical mass from optical data with various methods and comparing these results with the dynamical mass derived from X-ray data. Finally, conclusions will be drawn in Sect. 5.

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© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999