Galaxy pairs in the Sloan Digital Sky Survey – X. Does gas content alter star formation rate enhancement in galaxy interactions?

Abstract

New spectral line observations, obtained with the Jansky Very Large Array\n(VLA), of a sample of 34 galaxies in 17 close pairs are presented in this\npaper. The sample of galaxy pairs is selected to contain galaxies in close,\nmajor interactions (i.e., projected separations $<$30 kpc/h, and mass ratios\nless extreme than 4:1), while still having a sufficiently large angular\nseparation that the VLA can spatially resolve both galaxies in the pair. Of the\n34 galaxies, 17 are detected at $> 3\\sigma$. We compare the HI gas fraction of\nthe galaxies with the triggered star formation present in that galaxy. When\ncompared to the star formation rates (SFRs) of non-pair galaxies matched in\nmass, redshift, and local environment, we find that the star formation\nenhancement is weakly positively correlated ($\\sim 2.5\\sigma$) with HI gas\nfraction. In order to help understand the physical mechanisms driving this weak\ncorrelation, we also present results from a small suite of binary galaxy merger\nsimulations with varying gas fractions. The simulated galaxies indicate that\nlarger initial gas fractions are associated with lower levels of\ninteraction-triggered star formation (relative to an identical galaxy in\nisolation), but also show that high gas fraction galaxies have higher absolute\nSFRs prior to an interaction. We show that when interaction-driven SFR\nenhancements are calculated relative to a galaxy with an average gas fraction\nfor its stellar mass, the relationship between SFR and initial gas fraction\ndominates over the SFR enhancements driven by the interaction. Simulated galaxy\ninteractions that are matched in stellar mass but not in gas fraction, like our\nVLA sample, yield the same general positive correlation between SFR enhancement\nand gas fraction that we observe.\n