1 Introduction

Clusters of galaxies are excellent probes for cosmological theories. Different cosmological models predict vastly different properties for the clusters as a function of redshift. Therefore relations of cluster properties, their distributions and evolution can be used to constrain cosmological parameters. Selection according to X-ray luminosity is a good way to find the highest mass concentrations because of a relatively well defined correlation between the X-ray luminosity and the total cluster mass (Reiprich & Böhringer 1999; Schindler 1999). A second advantage of high luminosity clusters is, of course, that they require shorter exposure times than their fainter counterparts to get the same numbers of photons. Therefore many X-ray studies have concentrated so far on luminous clusters.

But also low-luminosity clusters are interesting for different reasons. For example, low-luminosity clusters are the links between massive clusters and groups of galaxies. Groups differ in various properties from clusters. Their $L_{\rm X}-T$ relation is steeper than the one for clusters (compare Ponman et al. 1996 with Arnaud & Evrard 1999), their gas mass fraction is smaller (compare Pildis et al. 1995 and Ettori & Fabian 1999 or Mohr et al. 1999) and their silicon abundance is lower (Fukazawa et al. 1998). To investigate the physics causing these differences it is important to understand how these properties change with mass or X-ray luminosity.

Low-luminosity clusters are also needed to determine the slope of various relations well. With a long leverage the slope of a relation can be determined more accurately, therefore the luminosities should preferably span several orders of magnitude. For other studies, e.g. the determination of the correlation function or distribution of properties, large samples of clusters are desirable to get the most accurate results. Therefore, when large numbers are required also faint clusters must be taken into account.

 

 

Table 1: X-ray and optical observations of the three clusters

cluster	ROSAT/HRI		ESO 3.6 m/MEFOS
	exposure time	date	exposure time	date
A901	12680 s	15/05/97 - 01/06/97	9000 s	15/05/96
A1437	16210 s	30/06/97 - 02/07/97	-	-
A3570	18860 s	04/02/98 - 05/02/98	5400 s	15/05/96

Nearby low-luminosity clusters sometimes provide problems. Not only is the countrate determination more sensitive to the background subtraction, but also contamination by other fore- and background sources can become critical, in particular when the cluster shows asymmetries and substructure. The degree of source confusion affecting morphological studies and countrate determinations depends on the sensitivity and the resolution of the detector. Already the factor of 5 between the point spread function (PSF) of the ROSAT/PSPC and the ROSAT/HRI can make a big difference, in particular for low-flux clusters (compare the ROSAT/PSPC observation of Cl0939+4713 (Schindler & Wambsganss 1996) and the corresponding ROSAT/HRI observation (Schindler et al. 1998). In the RASS (Voges et al. 1996) the spatial resolution is even more critical because the source is not always in the centre of the field-of-view, where the detector has the best spatial resolution, like in most pointed observations. But the source is observed at different off-axis angles as the detector scans the sky. The radius including 50% of the photons increases by about a factor of 13 when moving from the centre to the border of the ROSAT/PSPC. As the final image of a source is composed of many PSFs the final 50% radius in the RASS is about 80 $\hbox{$^{\prime\prime}$ }$, i.e. about six times larger than the on-axis PSPC resolution (depending also on the energy).

Clusters which have low fluxes because they are far away, are also very interesting, in particular for cosmological applications. But their extent in the sky is much smaller than for nearby clusters and thus they are less affected by source confusion. Therefore, we concentrate this investigation on nearby clusters with low to intermediate luminosity.

Three clusters - A901, A1437 and A3570 - were selected for a detailed look with the high resolution of the ROSAT/HRI and for optical observations. Each of the clusters shows irregular structure in the RASS. This investigation does of course not show representative RASS clusters, but it is a "worst-case'' study. For RASS clusters with high flux and/or regular shape countrate determination and morphological analyses are straightforward and reliable.

The paper is organised as follows. After a description of the observations (Sect. 2) we present the analysis of the optical data in Sect. 3 and the analysis of the X-ray data in Sect. 4. Finally, Sect. 5 gives our summary and conclusions. Throughout this paper we use H₀ = 50 km s^-1/Mpc and q₀=0.5.