TRIGGERED STAR FORMATION IN GALAXY PAIRS AT z = 0.08-0.38

We measure the strength, frequency, and timescale of tidally triggered star formation at redshift z = 0.08-0.38 in a spectroscopically complete sample of galaxy pairs drawn from the magnitude-limited redshift survey of 9825 Smithsonian Hectospec Lensing Survey galaxies with R < 20.3. To examine the evidence for tidal triggering, we identify a volume-limited sample of major (|ΔMR | < 1.75, corresponding to mass ratio >1/5) pair galaxies with MR < -20.8 in the redshift range z = 0.08-0.31. The size and completeness of the spectroscopic survey allow us to focus on regions of low local density. The spectrophotometric calibration enables the use of the 4000 A break (Dn 4000), the Hα specific star formation rate (SSFRHα), and population models to characterize the galaxies. We show that Dn 4000 is a useful population classification tool; it closely tracks the identification of emission line galaxies. The sample of major pair galaxies in regions of low local density with low Dn 4000 demonstrates the expected anti-correlation between pairwise projected separation and a set of star formation indicators explored in previous studies. We measure the frequency of triggered star formation by comparing the SSFRHα in the volume-limited sample in regions of low local density: 32% ± 7% of the major pair galaxies have SSFRHα at least double the median rate of the unpaired field galaxies. Comparison of stellar population models for pair and for unpaired field galaxies implies a timescale for triggered star formation of ∼300-400 Myr.