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2 The sample

At $z \approx $ 0.5, nearly half the Hubble time, the bulk of the morphological classes of galaxies visible today in low density environments are already in place (Griffith et al. 1994), but a significant fraction (40%) of them show peculiarities such as interacting features. Several studies of nearby galaxies (see Schweizer 1992; Reduzzi et al. 1996) conclude that also a large fraction of early-type galaxies shows peculiarities (e.g. fine structures) at the present time. It is useful to remember that while encounters among galaxies in clusters are fast enough to prevent merging (but able to provoke harassment (Moore et al. 1996), interaction in low density environments has a high probability of ending up with a merging event (Barnes 1992). We have selected galaxies showing present/past interaction signatures as typical objects of the low density environment population. Among them, shell galaxies represent a class of objects which show signs of past interaction (minor merging according to Schweizer (1992) or weak-interaction as suggested by Thomson & Wright (1990) and Thomson (1991), and pair members are objects which are still interacting. We describe below the characteristics of the observed samples. Catalogs from which we have selected them give a description of the environment.

The first sample is composed of 21 almost isolated early-type galaxies characterized by the presence of shells, selected from the Malin & Carter (1983) catalogue (Table 1). The second one consists of 30 members of isolated binary systems taken from the Reduzzi & Rampazzo (1995: RR95) catalogue (Table 2) and characterized by various kinds of fine structures. Note that we detect a double nucleus in the galaxy E2400100 (Table 1), unlike other authors that have previously studied this object (Malin & Carter 1983; Carter et al. 1988). In the following we refer to E2400100 as two separate galaxies, indicated as E2400100 A and B.

  
Table 1: Shell galaxies

\begin{tabular}
{crrr@{.}llc}
\hline
&&&\multicolumn{2}{}{}&&\\ Ident. & $\alpha...
 ...us & 0.0093 \\  & $-$21:51:31 & $-$3.5 & 23&4 & I & 10073 \\ \hline\end{tabular}


  
Table 2: Pair galaxies

  
\begin{figure}
\centering

\includegraphics [width=8.5cm]{ds1429f1.eps}\end{figure} Figure 1: Distribution of magnitudes (top panel) and of morphological types (bottom panel) within the two subsamples

Figure 1 shows the distributions of the morphological type and total apparent blue magnitude. A few galaxies in the pair sample are of type later than T=0: their spectra are presented for completeness, both in the present and in the kinematical study (Paper II). Both pairs and shell galaxies are located in low density environments, as mentioned above. In the ESO-LV catalogue (Lauberts & Valentijn 1989), the $N_{\rm tot}$ parameter characterizes the surface density of galaxies within one degree from a specific object. Lauberts & Valentijn (1989) consider that $N_{\rm tot} \geq 9-10$ describes rich environments, e.g. Fornax cluster. With the exclusion of a few objects (namely NGC 1316 in Fornax, NGC 3289, RR 405a and RR 405b) the average $<N_{\rm tot} \gt$ value for the pair members is 2$\pm$1.5, while for shell galaxies is 1.5$\pm$0.9, indicating that they are in very low density environments.

We consider 5 additional galaxies (Table 3), selected from the G93 sample. We use them as spectrophotometric "templates'' to check the transformation of our indices into the Lick-IDS "standard'' system (see Sect. 4).

  
Table 3: "Template galaxies''

\begin{tabular}
{crrc}
\hline
 Ident. & $\alpha$\space (2000) & $B_{\rm T}$\spac...
 ...& 23:55:19 & 12.6 & 0.040 \\  & +05:54:43 & $-$5.0 & 3849 \\ \hline\end{tabular}

We wish to emphasize that the selected sample complements that of G93. The latter author considers "normal'' early-type galaxies, i.e. without fine structures, most of which are in dense environments like the Virgo cluster centre or in groups (Cetus Aries, Libra cloud etc.) (see Tully 1988).


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