2 Observational data and reduction procedures

Our data are derived from 10-inch photographic plates taken with the 48-inch Palomar Schmidt Telescope (Hickson 1977). The emulsions employed were Kodak 127-02 or Kodak 098-04, both used with 2 mm Schott RG-1 glass filter, which corresponds to the red photographic F-band of Oemler (1974). Plates were calibrated using the Palomar spot sensitometer.

Fields containing the clusters originally selected by Hickson (1977), as well as some additional Abell clusters well visible on plates, were scanned in transparency mode using a PDS 1010G micro-densitometer in Rome, producing a digital image for each cluster field having pixel size from 15 to 20 $\mu$m, according to the noise level of the plate and cluster distance. Objects are automatically detected and magnitude values are computed in many circular apertures, producing a magnitude profile from which objects are automatically classified as point-like or diffuse.

Total magnitudes $m_{\rm T}$ are computed from flux integrated in an aperture whose radius is R₁=1.5r₁, where r₁ is the first momentum of the intensity distribution (see Trèvese et al. 1992).

With the above definition, total magnitudes correspond on average to the magnitude $m_{\rm iso}$ computed in a circular aperture determined by the isophote $\mu=24~$mag arcsec^-2, with the advantage that r₁ is less noisy than the corresponding isophotal radius (see Flin et al. 1995).

Magnitude zero points are taken from the literature, as indicated in Sect. 3. For comparison we give also the magnitude computed in a fixed aperture of 5 pixel radius. The corresponding value of the radius in arcsec is given in Table 1 for each cluster.

Figure 1: Abell 147

 

 

Table 1: Cluster data

Abell	N	z	$\alpha(2000)$	$\delta(2000)$	$\phi_{\rm a}(\hbox{$^{\prime\prime}$ })$	n
A147	67	0.0438	01 08.2	+02 09	6.7	155
A260	1	0.0348	01 51.9	+33 09	6.7	85
A272 a		0.0877	01 55.3	+33 56	6.7	58
A278 a		0.0891	01 57.3	+32 13	6.7	176
A1661 b		0.1671	13 01.8	+29 04	5.0	197
A2056 c		0.0804	15 19.2	+28 16	6.7	98
A2073 c		0.124$^{\dag }$	15 25.7	+28 24	6.7	58
A2093 d		0.139$^{\dag }$	15 34.3	+37 02	5.0	48
A2096 d		0.116$^{\dag }$	15 35.4	+37 20	5.0	100
A2124	1	0.0654	15 45.0	+36 03	5.0	99

^aSame calibration as A260.

^bSame calibration as A1700 (Trèvese et al. 1997).

^cSame calibration as A2065 (Trèvese et al. 1997).

^dSame calibration as A2124.

$^{\dag }$ The redshift has been estimated from the z-m₁₀ relation.

The ellipticity and the orientation of each object are computed from the second-order momenta of the intensity distribution.

In each cluster the equatorial coordinates of galaxies for the epoch 2000 have been computed from the rectangular coordinates of scans, using right ascension $\alpha$ and declination $\delta$ of at least 20 galaxies taken from Digitized Sky Survey.

Density maps were constructed using algorithm applying an adaptive filter as described by Pisani (1993) and already involved in plots of maps in the paper Trèvese et al. (1997). Density images were computed using all galaxies brighter than m₃+3. Isodensity maps of each cluster field are shown in Figs. 1 to 10. The accuracy of determined parameters, this is for both photometries and ellipticity of galaxy images, has been discussed in the previous papers as listed in the references.