Galaxy pairs in the Sloan Digital Sky Survey - III. Evidence of induced star formation from optical colours

We have assembled a large, high-quality catalogue of galaxy colours from the Sloan Digital Sky Survey Data Release 7 and have identified 21 347 galaxies in pairs spanning a range of projected separations (r(p) < 80 h-1(70) kpc), relative velocities (delta v < 10 000 km s-1, which includes projected pairs that are essential for quality control) and stellar mass ratios (from 1:10 to 10:1). We find that the red fraction of galaxies in pairs is higher than that of a control sample matched in stellar mass and redshift, and demonstrate that this difference is likely due to the fact that galaxy pairs reside in higher density environments than non-paired galaxies. We detect clear signs of interaction-induced star formation within the blue galaxies in pairs, as evidenced by a higher fraction of extremely blue galaxies, along with blueward offsets between the colours of paired versus control galaxies. These signs are strongest in close pairs (r(p) < 30 h-1(70) kpc and delta v < 200 km s-1), diminish for more widely separated pairs (r(p) > 60 h-1(70) kpc and delta v < 200 km s-1) and disappear for close projected pairs (r(p) < 30 h-1(70) kpc and delta v > 3000 km s-1). These effects are also stronger in central (fibre) colours than in global colours and are found primarily in low- to medium-density environments. Conversely, no such trends are seen in red galaxies, apart from a small reddening at small separations, which may result from residual errors with photometry in crowded fields. When interpreted in conjunction with a simple model of induced starbursts, these results are consistent with a scenario in which close pericentre passages trigger induced star formation in the centres of galaxies which are sufficiently gas rich, after which time the galaxies gradually redden as they separate and their starbursts age.