Galaxy pairs in the 2dF survey ¿ II. Effects of interactions on star formation in groups and clusters

We assess the effects of galaxy-galaxy interactions on star formation (SF) by analysing pairs in groups and clusters of galaxies with virial masses in the range 10 13 -10 15 M ○. extracted from the 2dF Galaxy Redshift Survey (2dFGRS). We find that galaxy pairs with relative separation r p < 15 h - kpc show significant SF activity in comparison with other group member galaxies. Similar results were found for galaxy pairs in the field (rp < 25 h -1 kpc) so that the nature of SF driven by galaxy interactions is nearly independent of environment, in spite of the general lower level of SF activity in systems of galaxies. The above results reflect, on the one hand, the local nature of SF induced by tidal interactions and, on the other, the role played by the internal properties of galaxies. By using a 2dFGRS mock catalogue we estimate the contamination by spurious pairs, finding that our statistics are dominated by real pairs, in particular for close relative separations, for which our study indicates significant SF activity. We obtain a similar radial and relative velocity distribution of the pairs with respect to the group centre compared with those of other typical group members, so that galaxy pairs have no particular location and dynamics within groups. Except for pairs with small relative separations, which show significant SF activity, we find that pairs in groups are systematically redder and have a lower present-day SF activity than other group members.