2 Sample properties

Our sample consists of 148 early-type galaxies which were observed spectroscopically. The selection of the sample galaxies is intended to complete a larger magnitude-limited sample of 530 galaxies with $B_{T} \le 13.5$ mag. The southern part of the sample observed here was compiled from a variety of catalogues including the list of Faber et al. (1989), the 3rd Reference Catalogue of Bright Galaxies (de Vaucouleurs et al. 1991, RC3) and the ESO Lauberts-Valentijn Catalogue (Lauberts & Valentijn 1989, ESO-LV). The apparent blue magnitudes B_T were taken from the RC3 and, when not available therein, from the ESO-LV. The absolute magnitudes M_B were computed from the corrected B_T⁰ magnitudes (see Table 5) taken first from the list of Faber et al. (1989) and subsequently from the RC3 and the ESO-LV using the heliocentric velocities derived here (see Table 5). A value of H₀=50 km s^-1 Mpc^-1 was used throughout the paper. The values of Faber et al. (1989) include additionally the K-correction following Pence (1976). The RC3 values are K-corrected following de Vaucouleurs et al. (1976, RC2). These latter corrections are usually small due to the low redshifts of the sample galaxies.

No detailed spectroscopy was previously available in the literature for most of the 148 galaxies studied here. For 85% of the sample the Lick/IDS indices measured here are first-time measurements and for 30% of the galaxies no velocity dispersion values were previously known. In particular, concomitant information on both the velocity dispersion and the line-strength indices was available for very few galaxies, specially for indices other than Mg₂.

The histograms of B_T and M_B are shown in Fig. 1. The distribution of M_B peaks at $M_{B} \approx -21.5$ mag with only little contribution from lower luminosity ellipticals with M_B > -20 and bright galaxies with M_B < -23 mag.

The observed sample comprises about 50 per cent elliptical and cD-type galaxies and 50 per cent E/S0 and S0 galaxies. A more detailed subdivision into morphological types can be found in Table 1.

 

 

Table 1: Galaxy types. T: numerical type according to RC3 or ESO-LV, T=-5stands for the interval $-5.5 < T \le -4.5$, for example. The fraction is given in per cent.

Type	T	number	fraction
E	-5	47	31
cD	-4	30	20
E/S0	-3	58	38
S0	-2	15	10
later	$\ge$-1	2	1

The galaxies inhabit a variety of types of environments, from low- to high-density neighbourhoods. To describe the environmental density of a galaxy in our sample, we use the local surface density of galaxies (NG_T), as given by the ESO-LV catalogue. It is derived as the number of the galaxies with an angular diameter larger than 1 arcmin on the ESO Quick Blue survey plates per square degree inside a radius of one degree around the considered galaxy. The considered galaxy itself is not included. Although being a projected surface density, it is a good indicator of the true volume density as discussed in ESO-LV. The density indicator NG_T is available for 92 per cent of our sample. Another possible density indicator would be that supplied by the Nearby Galaxies Catalogue (Tully 1988) ( $\rho_{{\rm Tully}}$). In this case the density is calculated using all companions brighter than -16 mag in the vicinity of the considered galaxy using redshift information. It reflects a volume density given in galaxies Mpc^-3. Unfortunately $\rho_{{\rm Tully}}$is available only for a small fraction of our sample galaxies ($\approx$25 per cent), therefore we decided to use NG_T.

Figure 1: Histograms of BT (shaded area) and MB (unshaded area) for the sample of 148 galaxies observed in this study. H0= 50 km s-1 Mpc-1 was used to calculate MB.

Figure 2 shows the distribution of the environmental densities, as given by the NG_T parameter, for our sample galaxies. As a reference, approximate values of the environmental densities for Local Group (LG), Fornax and Virgo galaxies are indicated. Note that the quoted densities refer to the central regions of the respective environments. The transition from field/small group to cluster seems to be at $NG_{T}\simeq 9$ (ESO-LV). Taking this value as dividing line, 25 per cent of our sample galaxies are located in clusters and 75 per cent reside in environments with lower densities.