The neutral gas content of post-merger galaxies

Abstract

Measurements of the neutral hydrogen gas content of a sample of 93\npost-merger galaxies are presented, from a combination of matches to the\nALFALFA.40 data release and new Arecibo observations. By imposing completeness\nthresholds identical to that of the ALFALFA survey, and by compiling a mass-,\nredshift- and environment-matched control sample from the public ALFALFA.40\ndata release, we calculate gas fraction offsets (Delta f_gas) for the\npost-mergers, relative to the control sample. We find that the post-mergers\nhave HI gas fractions that are consistent with undisturbed galaxies. However,\ndue to the relative gas richness of the ALFALFA.40 sample, from which we draw\nour control sample, our measurements of gas fraction enhancements are likely to\nbe conservative lower limits. Combined with comparable gas fraction\nmeasurements by Fertig et al. in a sample of galaxy pairs, who also determine\ngas fraction offsets consistent with zero, we conclude that there is no\nevidence for significant neutral gas consumption throughout the merger\nsequence. From a suite of 75 binary merger simulations we confirm that star\nformation is expected to decrease the post-merger gas fraction by only 0.06\ndex, even several Gyr after the merger. Moreover, in addition to the lack of\nevidence for gas consumption from gas fraction offsets, the observed HI\ndetection fraction in the complete sample of post-mergers is twice as high as\nthe controls, which suggests that the post-merger gas fractions may actually be\nenhanced. We demonstrate that a gas fraction enhancement in post-mergers,\nrelative to a stellar mass-matched control sample, would indeed be the natural\nresult of merging randomly drawn pairs from a parent population which exhibits\na declining gas fraction with increasing stellar mass.\n