Enhanced atomic gas fractions in recently merged galaxies: quenching is not a result of post-merger gas exhaustion

We present a detailed assessment of the global atomic hydrogen gas fraction (f gas = logM H I /M ) in a sample of post-merger galaxies identified in the Sloan Digital Sky Survey (SDSS). Archival H I measurements of 47 targets are combined with new Arecibo observations of a further 51 galaxies. The stellar mass range of the post-merger sample, our observing strategy, detection thresholds and data analysis procedures replicate those of the extended GALEX Arecibo SDSS Survey (xGASS) which can therefore be used as a control sample. Our principal results are (1) the post-merger sample shows a 50 per cent higher HI detection fraction compared with xGASS; (2) accounting for non-detections, the median atomic gas fraction of the post-merger sample is larger than the control sample by 0.3-0.6 dex; and (3) the median atomic gas fraction enhancement ( f gas ), computed on a galaxy-by-galaxy basis at fixed stellar mass, is 0.51 dex. Our results demonstrate that recently merged galaxies are typically a factor of 3 more H I rich than control galaxies of the same M . If the control sample is additionally matched in star formation rate, the median H I excess is reduced to f gas = 0.2 dex, showing that the enhanced atomic gas fractions in post-mergers are not purely a reflection of changes in star formation activity. We conclude that merger-induced starbursts and outflows do not lead to prompt quenching via exhaustion/expulsion of the galactic gas reservoirs. Instead, we propose that if star formation ceases after a merger, it is more likely due to an enhanced turbulence which renders the galaxy unable to effectively form new stars.