The neutral gas content of post-merger galaxies
  Measurements of the neutral hydrogen gas content of a sample of 93
post-merger galaxies are presented, from a combination of matches to the
ALFALFA.40 data release and new Arecibo observations. By imposing completeness
thresholds identical to that of the ALFALFA survey, and by compiling a mass-,
redshift- and environment-matched control sample from the public ALFALFA.40
data release, we calculate gas fraction offsets (Delta f_gas) for the
post-mergers, relative to the control sample. We find that the post-mergers
have HI gas fractions that are consistent with undisturbed galaxies. However,
due to the relative gas richness of the ALFALFA.40 sample, from which we draw
our control sample, our measurements of gas fraction enhancements are likely to
be conservative lower limits. Combined with comparable gas fraction
measurements by Fertig et al. in a sample of galaxy pairs, who also determine
gas fraction offsets consistent with zero, we conclude that there is no
evidence for significant neutral gas consumption throughout the merger
sequence. From a suite of 75 binary merger simulations we confirm that star
formation is expected to decrease the post-merger gas fraction by only 0.06
dex, even several Gyr after the merger. Moreover, in addition to the lack of
evidence for gas consumption from gas fraction offsets, the observed HI
detection fraction in the complete sample of post-mergers is twice as high as
the controls, which suggests that the post-merger gas fractions may actually be
enhanced. We demonstrate that a gas fraction enhancement in post-mergers,
relative to a stellar mass-matched control sample, would indeed be the natural
result of merging randomly drawn pairs from a parent population which exhibits
a declining gas fraction with increasing stellar mass.
