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Astron. Astrophys. 353, 124-128 (2000)

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1. Introduction

Cl0024+17 is one of the most interesting distant ([FORMULA], Gunn & Oke 1975) galaxy clusters featuring gravitational lensing effects. In this cluster eight, partly very detailed images of a single background galaxy have been identified (Colley et al. 1996, Tyson et al. 1998) which allows very detailed modeling of the underlaying mass distribution of the cluster center. It is also the cluster in which the first very large scale gravitational shear field has been detected and characterized (Bonnet et al. 1994) with a significant shear signal out to a radius of almost 3 [FORMULA] Mpc (the typical virial radius of very rich clusters).

The cluster has been discovered by Humason & Sandage (1957) and was one of the first targets to display the so-called Butcher-Oemler effect (Butcher & Oemler 1978). Dressler & Gunn (1982), Dressler et al. (1985) and Schneider et al. (1986) have found that the cluster is very rich, but has a large number of blue galaxies, confirming now with a redshift survey the earlier found Butcher-Oemler effect.

Gravitational arcs were discovered in the cluster by Koo (1988) and subsequently studied by a number of authors (Mellier et al. 1991, Kassiola et al. 1992, Kassiola et al. 1994, Smail et al. 1997, Wallington et al. 1995, Colley et al. 1996, Tyson et al. 1998). The most impressive arc system is located at a radius of about 35 arcsec ([FORMULA] kpc) presumably very close to the critical radius of the cluster lens. The redshift of the lensed background galaxy, which is very difficult to determine due to the lack of covenient emission lines for the redshift of the source (e.g. Mellier et al. 1991), has recently been measured by Broadhurst et al. (1999) to be [FORMULA].

Various mass estimates have been conducted for the cluster. From the measured line-of-sight velocity dispersion of [FORMULA] km s-1 and an optical core radius of [FORMULA] kpc, Schneider et al. (1986) calculate a gravitational mass of [FORMULA] [FORMULA] within a radius of [FORMULA] Mpc. On a much larger scale Bonnet et al. (1994) find a lensing mass of about [FORMULA][FORMULA] within a radius of [FORMULA] Mpc. A recent analysis of the shear field of Cl0024+17 is also included in the work by van Waerbeke et al. (1997). Mass estimates for the central region of the cluster are discussed in Sect. 4.

In the following we will be using a Hubble constant of [FORMULA] km s-1 Mpc- 1 and [FORMULA] and indicate the scaling of important parameters with [FORMULA] km s- 1 Mpc-1. For this cosmology 1 arcmin at the cluster redshift corresponds to a comoving scale of [FORMULA] kpc. In Sect. 2 we describe the observations and the morphological analysis. Sect. 3 provides mass estimates and in Sect. 4 we discuss these results in comparison with the lensing properties of the cluster.

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© European Southern Observatory (ESO) 2000

Online publication: December 8, 1999