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 9,825
Smithsonian Hectospec Lensing Survey (SHELS) galaxies with R<20.3. To examine
the evidence for tidal triggering, we identify a volume-limited sample of major
(|\Delta M_R|<1.75, corresponding to mass ratio >1/5) pair galaxies with $M_R <
-20.8 in the redshift range z=0.08-0.31. The size and completeness of the
spectroscopic survey allows us to focus on regions of low local density. The
spectrophotometric calibration enables the use of the 4000 Ang break (D_n4000),
the H\alpha specific star formation rate (SSFR_{H\alpha}), and population
models to characterize the galaxies. We show that D_n4000 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 D_n4000 demonstrates the expected anti-correlation
between pair-wise projected separation and a set of star formation indicators
explored in previous studies. We measure the frequency of triggered star
formation by comparing the SSFR_{H\alpha} in the volume-limited sample in
regions of low local density: 32 +/-7% of the major pair galaxies have
SSFR_{H\alpha} 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.
