2 The project

2.1 The sample

The sample has been selected from the catalogue given in De Grandi et al. (1999) by choosing only clusters at declination $<0^\circ$, with X-ray fluxes measured in the 0.5 - 2.0 keV energy band larger than 10^-11 erg cm^-2 s^-1, and with extended X-ray emission (i.e., sources with probability to be point-like, as computed by De Grandi et al. 1997, smaller than 1%). The resulting sample of 20 clusters is reported in Table 1. Columns list the main X-ray and optical properties for each source as follows: Column (1) -- Cluster name. Column (2) -- X-ray position: J2000.0 right ascension (hh mm ss.s). Column (3) -- X-ray position: J2000.0 declination (dd mm ss.s). Column (4) -- Cluster red-shift. Column (5) -- Unabsorbed X-ray flux computed in the 0.5-2.0 keV band in units of 10^-11 erg cm^-2 s^-1. Column (6) -- Bautz-Morgan type. Column (7) -- Optical richness. Column (8) -- Status of observations (Obs. = observed).

   

Table 1: The sample. Data relative to X-ray flux and red-shift are from De Grandi et al. (1999). Data relative to optical richness and morphology are from Abell et al. (1989). Data labelled with ^* are our estimate

Name	$\alpha_{2000}$	$\delta_{2000}$	Redshift	Flux	B.M.	Rich.	Status
A0085	00h41m50.11s	-09d18m17.5s	0.05560	4.092+0.256-0.244	I	2	Obs.
A0119	00h56m11.69s	-01d14m52.5s	0.04420	2.406+0.854-0.708	II-III	2	Obs.
A0133	01h02m42.21s	-21d52m43.5s	0.05660	1.578+0.181-0.166	I*	2	Obs.
A3158	03h42m53.06s	-53d37m43.0s	0.05910	2.250+0.206-0.184	I-II	2	-
A3186	03h52m25.09s	-74d01m02.5s	0.12700	1.041+0.177-0.157	I-II	1	Obs.
EXO0422-086	04h25m51.02s	-08d33m38.5s	0.03971	1.870+0.188-0.179	I*	-	Obs.
A3266	04h30m58.82s	-61d27m52.5s	0.05890	3.001+0.869-0.717	I	2	Obs.
A0496	04h33m37.07s	-13d15m20.0s	0.03284	4.652+0.423-0.389	I	1	Obs.
A3376	06h01m37.77s	-40d00m31.0s	0.04550	1.504+0.519-0.429	I	1	-
A3391	06h26m20.10s	-53d41m44.5s	0.05310	1.313+0.183-0.152	I	0	-
A3395	06h27m38.83s	-54d26m38.5s	0.04980	1.122+0.289-0.209	I	3	Obs.
A3667	20h12m35.08s	-56d50m30.5s	0.05560	3.289+0.927-0.668	II	2	Obs.
A3695	20h34m46.86s	-35d49m07.5s	0.08930	1.247+0.315-0.243	I	2	Obs.
A3822	21h54m10.21s	-57d52m05.5s	0.07590	1.099+0.202-0.170	II-III	2	Obs.
A3827	22h01m58.85s	-59d57m37.0s	0.09840	1.517+0.173-0.160	I	2	-
A2420	22h10m20.09s	-12d10m49.0s	0.08380	1.172+0.240-0.199	I	2	Obs.
A3921	22h50m03.61s	-64d26m30.0s	0.09360	1.201+0.259-0.226	II	2	Obs.
RX-J2344.2-422	23h44m15.98s	-04d22m24.5s	0.07860	1.214+0.302-0.232	I*	-	-
A4038	23h47m41.78s	-28d08m26.5s	0.02920	2.751+0.232-0.221	III	2	Obs.
A4059	23h57m00.02s	-34d45m24.5s	0.04600	1.974+0.189-0.178	I	1	Obs.

Figure 1: The efficiency of the g, r, i filters as function of the wavelength. In comparison, a typical early type galaxy spectrum is superimposed at two different red-shifts: z=0 and z=0.12, the extremes of the catalogue redshift range

Figure 2: The observed field of the cluster Abell 496 with the cD galaxy in the NE corner of the image. The mosaic is composed of 4 adjoining, and slightly overlapping fields: their identification number (Table 2) increases moving from NE (field 0) to SO corner (field 3). The angular distance between NE and SO corners is 30 arcmin

2.2 Imaging

CCD observations of the sample clusters were carried out since December 1994 at La Silla with the 1.5 m Danish Telescope equipped with DFOSC camera. For each cluster we observed a mosaic composed of 3 or 4 slightly overlapping fields (Fig. 2 shows the mosaic of Abell 496). In each mosaic the centre of the first field corresponds to the centre of the X-ray isophotes (see the Fig. 1 in Molinari et al. 1998). The other fields are centred along a radial direction. For each mosaic, the typical total observed area is 250 arcmin²with a typical maximum angular distance of 30 arcmin (equal to a linear distance of 2.5 Mpc at z=0.05). For each cluster the observation of the most external field is used mainly to evaluate the background. Each field is observed with the g, r, i filters of the Gunn photometric system (Thuan & Gunn 1976; Wade et al. 1979). The spectral response is illustrated in Fig. 1 along with the observed spectrum of an elliptical galaxy. Observations of each field consist of 3600 s exposure as a result of the integration of 4 $\times$ 900 s different exposures. Up to date, we have collected photometric observations of 15 out of 20 clusters of the sample (Table 1). Spectroscopic observations are also being planned and will likely start shortly before completion of the photometric sample. This paper will deal, in particular, with the data analysis carried out for the cluster Abell 496. However it reflects the method we will also use for the other clusters.