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

Abstract

We present a detailed assessment of the global atomic hydrogen gas fraction ( f gas =log[M HI /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 H i 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; 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.

Prevalence of neutral gas in centres of merging galaxies−II: nuclear H i and multiwavelength properties

“…Hence, the dependence of N (H i) on M * could be due to higher concentration of H i gas in centres of wet or gas-rich mergers. However, this may also reflect the general trend of increasing detection rate of H i emission and atomic gas fraction with decreasing stellar mass observed in both isolated and post-merger galaxies Ellison et al 2018).…”

“…Hibbard & van Gorkom 1996;Tacconi et al 1999), statistical studies of the cold gas in galaxy pairs and mergers have typically focused on the average gas properties (like gas mass and fraction) derived using single dish radio observations (e.g. Ellison et al 2015Ellison et al , 2018. Simulations of mergers, however, predict concentration of stars and gas in centres of galaxies resulting from tidal torques (e.g.…”

xGASS: Robust quantification of asymmetries in global H i spectra and their relationship to environmental processes

“…We compute the gas fraction offset (∆ f gas , Ellison et al 2018) as the difference between the logarithmic gas fraction of an asymmetric galaxy and the median logarithmic gas Figure 8. The S/N − A fr parameter space for xGASS.…”

Interacting galaxies in the IllustrisTNG simulations – II: star formation in the post-merger stage

“…Therefore, f gas can be thought of as the total gas fraction available to form stars during the merger. Note that f gas accounts for all gas phases and therefore should not be compared to the molecular and cold gas fractions reported by observational studies (e.g., Combes et al 2013;Ellison et al 2018;Tacconi et al 2018). Figure 10 shows the distribution of f gas for the postmergers in our sample.…”