Tidally Triggered Star Formation in Close Pairs of Galaxies: Major and Minor Interactions

Abstract

We study star formation in a sample of 345 galaxies in 167 pairs and compact groups drawn from the original CfA2 Redshift Survey and from a follow-up search for companions. We construct our sample with attention to including pairs with luminosity contrast |Δm_R| >= 2. These 57 galaxies with |Δm_R| >= 2 provide a set of nearby representative cases of minor interactions, a central feature of the hierarchical galaxy formation model. Here we report the redshifts and positions of the 345 galaxies in our sample, and of 136 galaxies in apparent pairs that are superpositions. In the pairs sample as a whole, there are strong correlations between the equivalent width of the Hαemission line and the projected spatial and the line-of-sight velocity separation of the pair. For pairs of small luminosity contrast, |Δm_R| < 2, the member galaxies show a correlation between the equivalent width of Hαand the projected spatial separation of the pair. However, for pairs with large luminosity contrast, |Δm_R| >= 2, we detect no correlation between the equivalent width of Hαand the projected spatial separation. The relative luminosity of the companion galaxy is more important in a gravitational tidal interaction than the intrinsic luminosity of the galaxy. Central star formation across the entire pairs sample depends strongly on the luminosity ratio, |Δm_R|, a reasonable proxy for the mass ratio of the pair; pairs composed of similarly luminous galaxies produce the strongest bursts of star formation. Pairs with |Δm_R| >= 2 rarely have EW(Hα) >~ 70 Ang.