We study galaxy pairs (GPs) in the field selected from the 100‐K public release of the Two Degree Field (2dF) galaxy redshift survey. Our analysis provides a well‐defined sample of 1258 GPs, a large data base suitable for statistical studies of galaxy interactions in the local Universe, z≤ 0.1. GPs were selected by radial velocity (ΔV) and projected separation (rp) criteria determined by analysing the star‐formation activity within neighbours. We have excluded pairs in high‐density regions by removing galaxies in groups and clusters. We analyse the star‐formation activity in the pairs as a function of both relative projected distance and relative radial velocity. We found power‐law relations for the mean star‐formation birth parameter and equivalent widths of the galaxies in pairs as a function of rp and ΔV. We find that star formation in GPs is significantly enhanced over that of isolated galaxies with similar redshifts in the field for rp < 25 h−1 kpc and ΔV < 100 km s−1. We detected that, when compared to isolated galaxies of similar luminosity and redshift distribution, the effects of having a companion are more significant on the star‐formation activity of bright galaxies in pairs, unless the pairs are formed by similar luminosity galaxies. In this case, the star formation is enhanced in both components. The ratio between the fractions of star‐forming galaxies in pairs and in isolation is a useful tool to unveil the effects of having a close companion. We found that about 50 per cent of GPs do not show signs of important star‐formation activity (independently of their luminosities), supporting the hypothesis that the internal properties of the galaxies play a crucial role in the triggering of star formation by interactions.

We analyse star formation rates derived from photometric and spectroscopic data of galaxies in pairs in different environments using the 2dF Galaxy Redshift Survey (2dFGRS) and the Sloan Digital Sky Survey (SDSS). The two samples comprise several thousand pairs, suitable to explore into detail the dependence of star formation activity in pairs on orbital parameters and global environment. We use the projected galaxy density derived from the fifth nearest neighbour of each galaxy, with convenient luminosity thresholds to characterise environment in both surveys in a consistent way. Star formation activity is derived through the $\eta$ parameter in 2dFGRS and through the star formation rate normalised to the total mass in stars, $SFR/M^*$, given by Brinchmann et al. (2004) in the second data release SDSS-DR2. For both galaxy pair catalogs, the star formation birth rate parameter is a strong function of the global environment and orbital parameters. Our analysis on SDSS pairs confirms previous results found with the 2dFGRS where suitable thresholds for the star formation activity induced by interactions are estimated at a projected distance $r_{\rm p} = 100 \kpc$ and a relative velocity $\Delta V = 350$ km $s^{-1}$. We observe that galaxy interactions are more effective at triggering important star formation activity in low and moderate density environments with respect to the control sample of galaxies without a close companion. Although close pairs have a larger fraction of actively star-forming galaxies, they also exhibit a greater fraction of red galaxies with respect to those systems without a close companion, an effect that may indicate that dust stirred up during encounters could be affecting colours and, partially, obscuring tidally-induced star formation.Comment: accepted MNRA

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 −1 kpc show significant SF activity in comparison with other group member galaxies. Similar results were found for galaxy pairs in the field (r p < 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.