Isolating triggered star formation

Galaxy pairs provide a potentially powerful means of studying triggered star formation from galaxy interactions. We use a large cosmological N-body simulation coupled with a well-tested semianalytic substructure model to demonstrate that the majority of galaxies in close pairs reside within cluster-or group-sized halos and therefore represent a biased population, poorly suited for direct comparison to field'' galaxies. Thus, the frequent observation that some types of galaxies in pairs have redder colors than field galaxies is primarily a selection effect. We use our simulations to devise a means to select galaxy pairs that are isolated in their dark matter halos with respect to other massive subhalos (N = 2 halos) and to select a control sample of isolated galaxies (N 1 halos) for comparison. We then apply these selection criteria to a volume-limited subset of the 2dF Galaxy Redshift Survey with M-B,M-j <= - 19 and obtain the first clean measurement of the typical fraction of galaxies affected by triggered star formation and the average elevation in the star formation rate. We find that 24% (30.5%) of these L* and sub-L* galaxies in isolated 50 (30) h(-1) kpc pairs exhibit star formation that is boosted by a factor of greater than or similar to 5 above their average past value, while only 10% of isolated galaxies in the control sample show this level of enhancement. Because only 12% (16%) of model galaxies in these pairs have had recent close passes, the data are broadly consistent with a scenario in which most or all close passes of isolated pairs result in triggered star formation. The isolation criteria we develop provide a means to constrain star formation and feedback prescriptions in hydrodynamic simulations and a very general method of understanding the importance of triggered star formation in a cosmological context.