Astron. Astrophys. 363, 440-450 (2000)

1. Introduction

Clusters of galaxies are the largest bound systems in the Universe, and as such they are the largest objects where detailed studies of their gravitational potential are possible. Given their large sizes, 3 to 6 Mpc in extent, they are also thought to be representative of the Universe in terms of the baryonic fraction which is directly related to the density of the universe and the predictions of the Big Bang nucleosynthesis theory. Studies so far have found that the baryonic fractions in clusters favour a low matter density universe given the predictions of baryon densities given by the nucleosynthesis theory (e.g. White et al. 1993). Recently, detailed and independent estimates of cluster total mass distributions have become available; the mass-tracers used and the observational techniques employed can be summarised as follows:

• Cluster Galaxies : these have a long tradition of providing mass estimates via application of the Virial Theorem to the observed dispersion in their radial velocities. The method rests upon the assumption that the galaxies are in dynamical equilibrium.

• Hot Intracluster Gas : as well as being an important mass component of clusters, its X-ray emission provides an ideal tracer - through the hydrostatic equation - of the total underlying mass. The assumption that the gas is in hydrostatic equilibrium with the cluster's gravitational potential is thought to be reasonably secure for the central few Mpc (Evrard et al. 1996; Schindler 1996) and the gas density and temperature profiles required to solve the hydrostatic equation are readily available from the X-ray data.

• Gravitational Lensing : here the lensing action of the cluster on background sources, as revealed in deep high resolution imagery (Tyson et al. 1990; Fort & Mellier 1994 and references there in), is used to provide a direct measure of the shape and depth of the cluster potential and hence the projected mass distribution (Kaiser & Squires 1993; Broadhurst et al. 1995 etc.). Unlike the first 2 methods, this approach is not reliant upon assumptions of hydrostatic or dynamical equilibrium.

For detailed studies in the X-ray and optical, we need a nearby cluster, though gravitational lensing effects are diminished for low redshift clusters. An ideal cluster for this kind of detailed and independent estimates of mass distributions, would be one of the lowest redshift clusters with obvious lensing effects such as a giant arc. In this paper, we will analyse the X-ray and optical data for one of the nearby lensing clusters.

Abell 2104 is a rich cluster (richness class 2) at a redshift of 0.155 (Allen et al. 1992). It was found to have a high X-ray luminosity from the ROSAT all-sky survey data (Pierre et al. 1994). Subsequent optical followup observations with the CFHT revealed an arc embedded in the halo of the central cD galaxy away from the centre (Pierre et al. 1994). The arc spans in length and it is amongst the reddest known arcs. Fig. 2 shows a close up picture of the arc. Given the small arc radius, it is important to have a high resolution X-ray observation with an instrument such as the ROSAT /HRI to probe the gravitational potential within the arc radius.

Fig. 1. Optical field of Abell 2104, observed at the CFHT in R band. Overlaid are the ROSAT HRI contours with levels  counts s- 1 arcsec-2. The X-ray image was rebinned into pixels and smoothed with a Gaussian.

Fig. 2. A close up image of the central regions of Abell 2104 showing the giant arc.

The optical data including photometry and spectroscopy will be analysed in Sect. 2. The spatial and spectroscopic analysis of the X-ray data from ROSAT and ASCA will be given in Sect. 3. The independent mass estimates using different methods as well as a comparisons will be given Sect. 4.

Throughout the paper we adopt a cosmological model with km s^-1Mpc^-1, and . Celestial coordinates are in J2000.

© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000
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