2 The sample

The sample analyzed in this work was extracted from the optically selected Virgo Cluster Catalogue (VCC) of Binggeli et al. (1985), which is complete to $B_{\rm T}\leq18$. Galaxies were selected according to the following criteria:

• $B_{\rm T}<18$;
• Hubble type later than S0;
• classified as cluster member by Binggeli et al. (1985, 1993);
• lying at a projected angular distance smaller than 2 degrees from M 87 (cluster-core) or greater than 4 degrees from the position of maximum projected galaxy density given by Sandage et al. (1985) (cluster- periphery), but excluding galaxies within 1.5 degrees of the M 49 sub-cluster;
• to limit the spread of distances within the sample, galaxies in the M and W clouds, and in the Southern Extension ( $\delta(1950) < 5^\circ$) were also excluded.

The sky areas from which galaxies were chosen define two contrasting subsamples to optimise the statistical evaluation of the cluster environment on observed properties (see Fig. 1). The cluster-core subsample is composed of 46 galaxies within the X-ray emitting "atmosphere'' of M 87. The cluster-periphery subsample includes 72 galaxies in the outskirts.

Figure 1: Plot of all VCC galaxies classified as members by Binggeli et al. (1985) taken from Fig. 1 of Sandage et al. (1985). The subsample of galaxies here analyzed (see Sect. 2) are marked with filled symbols of increasing size according to their magnitude. Empty symbols are for Virgo members not included in the ISO sample; circles for late-type galaxies ($\geq$Sa), squares for early types ($\leq$S0a). The 2.0 degree radius circle centred on M 87 contains the cluster-core subsample. The inner boundary of the cluster periphery subsample has a radius of 4 degrees about the position of maximum projected galaxy density. The 1.5 degree radius circle is centered on the position of maximum projected galaxy density of M 49 subcluster.

The resulting sample of 118 galaxies is complete to $B_{\rm T}=18$, and both the cluster-periphery and -core subsamples span the range -21 < M_B< -13. Both subsamples are approximately equally divided between giant spirals on the one hand and dwarf and irregular galaxies on the other. The distribution over Hubble type is summarised in Table 1.

 

 

Table 1: Distribution of sample over Hubble type for the cluster-periphery and cluster-core subsamples.

	S0/a - Sab	Sb - Sc	Scd - Sm	Im	BCD
periphery	9	16	12	20	15
core	16	11	6	10	3
Total	25	27	18	30	18

The parameter of the sample galaxies are given in Table 2, arranged as follows:

• Column 1: VCC denomination (Binggeli et al. 1985).
• Column 2: NGC name.
• Column 3: IC name.
• Column 4: UGC name (Nilson 1973).
• Column 5: CGCG denomination (Zwicky et al. 1961-68)
• Columns 6 and 7: (B2000.0) celestial coordinates, from NED, with a few arcsec accuracy.
• Column 8: morphological type, from the VCC or from Binggeli et al. (1993).
• Column 9: photographic magnitude from the VCC.
• Columns 10 and 11: major (a) and minor (b) optical diameters (arcmin) determined at the surface brightness of $\rm 25th~ mag~arcsec^{-2}$. For galaxies without a $\rm 25th~ mag~arcsec^{-2}$ value in the VCC, the diameter is computed from the "last visible'' isophotal diameter given in the VCC using the relation: Log $~a_{\rm ext}(b_{\rm ext})=0.99 {\rm Log}~a(b)+0.1$.
• Column 12: heliocentric velocity, in km s^-1.
• Column 13: distance, in Mpc. Distances to the various substructures of Virgo are as given in Gavazzi et al. (1999).
• Column 14: cluster membership as defined in Gavazzi et al. (1999).
• Column 15: projected angular separation from the cluster centre (M 87), in degrees.
• Column 16: the model-independent near-IR concentration index parameter C₃₁, from Gavazzi et al. (2000), defined as the ratio between the radii that enclose 75% and 25% of the total light. C₃₁ is a tracer of the light distribution within galaxies: values of C₃₁ < 3 are for pure exponential discs, C₃₁ > 3 for galaxies with bulges.
• Column 17: notes.