PRIMUS: Enhanced Specific Star Formation Rates In Close Galaxy Pairs

Abstract: Tidal interactions between galaxies can trigger star formation, which contributes to the global star formation rate density of the universe and could be a factor in the transformation of blue, star-forming galaxies to red, quiescent galaxies over cosmic time. We investigate tidally-triggered star formation in isolated close galaxy pairs drawn from the Prism Multi-Object Survey (PRIMUS), a low-dispersion prism redshift survey that has measured ~120,000 robust galaxy redshifts over 9.1 deg^2 out to z ~ 1. We select a sample of galaxies in isolated galaxy pairs at redshifts 0.25 < z < 0.75, with no other objects within a projected separation of 300 h^-1 kpc and dz/(1+z) = 0.01, and compare them to a control sample of isolated galaxies to test for systematic differences in their rest-frame FUV-r and NUV-r colors as a proxy for relative specific SFR. We find that galaxies in r_p < 50 h^-1 kpc pairs have bluer dust-corrected UV-r colors on average than the control galaxies by -0.134 +/- 0.045 magnitudes in FUV-r and -0.075 +/- 0.038 magnitudes in NUV-r, corresponding to a ~15-20% increase in SSFR. This indicates an enhancement in SSFR due to tidal interactions. We also find that this relative enhancement is greater for a subset of r_p < 30 h^-1 kpc pair galaxies, for which the average colors offsets are -0.193 +/- 0.065 magnitudes in FUV-r and -0.159 +/- 0.048 magnitudes in NUV-r, corresponding to a ~25-30% increase in SSFR. We test for evolution in the enhancement of tidally-triggered star formation with redshift across our sample redshift range and find marginal evidence for a decrease in SSFR enhancement from 0.25 < z < 0.5 to 0.5 < z < 0.75. This indicates that a change in enhanced star formation triggered by tidal interactions in low density environments is not a contributor to the decline in the global star formation rate density across this redshift range.