Galaxy pairs in the 2dF survey -I. Effects of interactions on star formation in the field

Abstract

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 (r p) 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 r p 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.