2 The sample

The 100 compact groups catalogue by Hickson (1982) has been revised by Hickson et al. (1989) and then by Hickson et al. (1992). By adding a radial velocity criterion Hickson et al. (1992) were able to reject probable non member galaxies. The resulting sample consists of 92 groups each containing three or more "accordant" members, which have radial velocities differing by no more than 1000 km s^-1 from the median velocity of the group. Our sample has been drawn from this latest catalog.

In this paper, the result of the data reduction and calibration of H$_\alpha$ CCD images are presented for 31 Hickson Compact Groups. The remaining 61 HCGs were not in our sample because the adequate H$_\alpha$ interferometric filters were not available during the observations. This is the only criterion used to select the observed groups.

The redshift of observed groups is in the range $0.005 \leq z \leq 0.07$ (Hickson et al. 1992) and their distribution is shown in Fig. 1 (the width of each bin is 0.01).

  

Figure 1: Redshift distribution of the 31 observed groups

  

Table 1: Observed Hickson Compact Groups (Hickson et al. 1992): HCG number, right ascension and declination (1950), number of accordant galaxies, velocity dispersion and mean redshift of group

Table 1 lists the observed HCGs as follows:
Column 1: Name of the groups according to Hickson's catalogue;
Column 2: 1950 right ascension (R.A.) of the centroid of the member galaxies;
Column 3: 1950 declination (Dec.) of the centroid of the member galaxies;
Column 4: Number of accordant members of the group;
Column 5: Velocity dispersion of the group: $\sigma_v$ $({\rm km~s}^{-1})$;
Column 6: Mean Redshift of the group.

The sample is composed of 134 galaxies, 127 out of which have been observed. 52$\%$ of them are Ellipticals and Lenticulars and the remaining 48$\%$ are Spirals and Irregulars. For each observed galaxy we report in Table 2 the heliocentric radial velocity V in units of km s^-1, the total magnitude in the photografic band $B_{\rm T}$, corrected for internal and galactic extinction and the Hubble type, as in Hickson 1993. Galaxies are named with the number of HCG plus letter of galaxy itself.

The distribution of the total $B_{\rm T}$ magnitude of the observed galaxies is shown in Fig. 2 (the width of each bin is 0.5 mag).

  

Figure 2: Distribution of $B_{\rm T}$ magnitude of the 127 observed galaxies

  

Table 2: Principal features of the observed galaxies (Hickson 1993): galaxy name, heliocentric velocity of group, total photographic blue magnitude, morphological type of galaxy