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

1. Introduction

The cluster Cl 0500-24 is an optically rich, very compact cluster of galaxies (Giraud 1988) at a medium redshift of (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 Mpc. The average density, N_0.5, defined as the number of bright galaxies, (where is the magnitude of the third brightest galaxy) projected within a radius of 0.5 Mpc is 31-36. This means that Cl 0500-24 appears richer than the well studied giant gravitational arc cluster Abell 370, for which (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 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 km/s and km/s. The velocity difference ( km/s) is much larger than the one-dimensional velocity dispersions of the two sub-clumps ( km/s, 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 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 (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 km/s/Mpc, =1 and =0.5.

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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