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Astron. Astrophys. 322, 66-72 (1997)

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1. Introduction

The cluster Cl 0500-24 is an optically rich, very compact cluster of galaxies (Giraud 1988) at a medium redshift of [FORMULA] (in Abell et al. (1989) it is listed as supplementary cluster S0506). There are 95-100 galaxies brighter than V = 23 within a radius of [FORMULA] Mpc. The average density, N0.5, defined as the number of bright galaxies, [FORMULA] (where [FORMULA] is the magnitude of the third brightest galaxy) projected within a radius of 0.5 [FORMULA] Mpc is 31-36. This means that Cl 0500-24 appears richer than the well studied giant gravitational arc cluster Abell 370, for which [FORMULA] (see Mellier et al. 1988). Cl 0500-24 contains a large fraction of blue galaxies, and has a quite high line of sight velocity dispersion of [FORMULA] km/s, based on redshifts of 22 cluster galaxies (Giraud 1990). The cluster "apparently has a triple core" (Giraud 1988) so it is to be expected that an X-ray map will identify substructure. Infante et al. (1994) found a bimodal distribution in the galaxy velocities, enhancing this view that the cluster consists of subclumps.

The known redshifts of the galaxies in Cl 0500-24 cover quite a broad range. Infante et al. (1994) showed that the velocity distribution (26 measured galaxies) is bimodal with two peaks at [FORMULA] km/s and [FORMULA] km/s. The velocity difference ([FORMULA] km/s) is much larger than the one-dimensional velocity dispersions of the two sub-clumps ([FORMULA] km/s, [FORMULA] km/s). This double nature of the cluster was supported by Infante et al. (1994)'s group finding algorithms. So it appears that Cl 0500-24 is not one rich cluster, but rather two subclusters along the line of sight.

The most striking feature in Cl 0500-24 is a straight, blue arc with a length of about 14 arcseconds, which is about 22 arcseconds away from the apparent cluster centre. A gravitational lens model for this arc (Wambsganss et al. 1989) finds very naturally a strongly elongated image of a background galaxy at the position of the arc with the correct orientation. This model predicts a mass of about [FORMULA] [FORMULA] for the (projected) inner part of the cluster, i.e. for a circle around the apparent centre C with radius centre-arc (cf. Giraud 1990). Equivalently, a cluster modeled as a singular isothermal sphere with a velocity dispersion of 1200 km/s is necessary to produce an arc with this length at this location. The redshift of the arc was measured to be [FORMULA] (Giraud 1996) confirming the gravitational lens scenario quite satisfactorily. So far no arclets or weak lensing signature have been reported in this cluster.

Here we present X-ray observations of Cl 0500-24 with the ROSAT/HRI (Trümper 1983). We show the morphology as it appears in X-rays, we determine an X-ray profile and the X-ray luminosity, and we estimate the mass of the X-ray emitting gas and the total mass of the cluster. Finally, we discuss our results and compare them with other X-ray/lensing clusters. Throughout this paper we use [FORMULA] km/s/Mpc, [FORMULA] =1 and [FORMULA] =0.5.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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