The colors of pairs members follow the so-called Holmberg effect which is a correlation between the colors of the pairs members (Holmberg 1954; Demin et al. 1984). In Fig. 7 (click here) we show the (B-V) plot for each component of the pairs. Late-type pairs are marked as filled circles, early-type pairs as open circles and mixed-pairs as stars. Reduzzi & Rampazzo (1995) using a more complete sample found that while the colors of the members correlate significantly for EE and SS pairs, the correlation for ES is less significant. It suggests that pairs of different morphologies should be analyzed differently. In pairs with two gas-rich components (SS) tidal forces perturb the gas and the stars over the entire disk causing severe damage to both galaxies. SS pairs such as CPG551 and CPG587 have strong tails probably caused by the slow encounter between the galaxies. Other SS pairs have barred galaxies probably also created by tidal effects. However, our sample is too small to allow any statistical analysis.
Figure 7: B-V plot for each component . Filled circles are late-type pairs, open circles are early-type pairs and stars are mixed-pairs
Four of our pairs are now confirmed to have a lenticular galaxy as a component. Rampazzo et al. (1992 and 1995) had also found an excess of lenticulars in their sample. In fact, they had estimated that of all pairs classified as mixed pairs would in fact be disky pairs, i.e. formed by a spiral galaxy and a lenticular galaxy. This result suggests that S0's could be the result of interaction. Charlton et al. (1992) have proposed a toy-model for evolution of morphological types in a merger scenario which also agrees with our results. They have considered that mergers between unequal mass galaxies make S0's and mergers between equal mass galaxies make ellipticals. In their model S-S0 pairs are formed from an initial group of 3-5 galaxies.
The distortion on the isophotal shapes of elliptical galaxies are frequently used in studies of environmental effects as evidences of tidal effects. For instance, Bender (1988) has suggested that elliptical galaxies can be classified according to their flatness and Bender et al. (1989) suggested that boxy isophotes are signs of past merger (Bender et al. 1989). Caon & Einasto (1995) have suggested that this property is more sensitive to the environment rather than the bulge/disk ratio. They found that disky elliptical galaxies are located in lower density environments and boxy ellipticals in higher densities. However, more recently, Andreon (1996) using a more complete sample of galaxies in the Coma cluster has suggested that diskiness and boxiness has no environmental origin. However, because our sample of galaxies is made of isolated pairs and that galaxies in clusters are more influenced by the cluster as a whole than by neighboring galaxies (Makino & Hut 1997) it is possible that tidal effects on the internal properties of galaxies be more effective in pairs than in clusters. In this study we verified that 6 galaxies show deviations from their elliptical isophotes or inner substructures; the most interesting results were obtained for 2 S0 galaxies. One of them, CPG577b, shows inner shells and the other one, CPG583a, exhibits X-structures. However, because of the small size of our sample we are unable to draw any conclusion on the environmental origin of these features. However, our results agree with previous ones and show that X-structures is one of the less frequently observed forms of fine structure (less than 10% of our sample show this structure).
It is well known that interaction can trigger both
thermal and non-thermal activity, i.e. starburst and
AGN (Dahari 1985;
Gallimore & Keel 1993;
Keel & van
Liu & Kennicutt 1995). However, while many
active galaxies show signs of interactions, not all
interacting galaxies show signs of activity. An intuitive
solution for this dichotomy is the fact that tidal damage
is long-lived (few Gyr) while any form of activity is
(108 yr). Keel et al. (1985) showed that Seyfert activity is over-represented in early stage interactions. Liu & Kennicutt (1995) work on merging systems found a very large range of spectral properties, ranging from completely evolved stellar populations to starburst and post-starburst systems. However, samples of galaxies such as the one selected by Bergvall & Johansson (1995) lack of such effects (N. Bergvall private communication). This is may be due to the fact that Bergvall & Johansson's sample includes weak cases of interaction and pairs with large separations, which is particularly different from the sample that we are analyzing. Some pairs of galaxies of our sample are in stages of interaction which present tidal damage and also activity. There is strong tidal damage on late-type galaxies such as CPG083, CPG551, CPG580 and even on early-type galaxies such as CPG099. There is Seyfert activity in CPG083 and CPG575 (Hippelein 1989 and Marquez & Moles 1994) and several star formation regions in CPG551a and CPG580b. However, one has to take into account that pairs of different morphologies may have passed to different evolution processes and therefore show different effects. Moreover, activities in disturbed disks have a longer lifetime due to a larger available gas supply in a disturbed disk (Keel 1996).