On the relationship between gas content, star formation, and global H i asymmetry of galaxies on the star-forming main-sequence

Observations have revealed that disturbances in the cold neutral atomic hydrogen (H I) in galaxies are ubiquitous, but the reasons for these disturbances remain unclear. While some studies suggest that asymmetries in integrated H I spectra (global H I asymmetry) are higher in H I-rich systems, others claim that they are preferentially found in H I-poor galaxies. In this work, we utilize the Arecibo Legacy Fast ALFA (ALFALFA) and extended GALEX Arecibo SDSS Survey (xGASS) surveys, plus a sample of post-merger galaxies, to clarify the link between global H I asymmetry and the gas properties of galaxies. Focusing on star-forming galaxies in ALFALFA, we find that elevated global H I asymmetry is not associated with a change in the H I content of a galaxy, and that only the galaxies with the highest global H I asymmetry show a small increase in specific star formation rate (sSFR). However, we show that the lack of a trend with H I content is because ALFALFA misses the ‘gas-poor’ tail of the star-forming main-sequence. Using xGASS to obtain a sample of star-forming galaxies that is representative in both sSFR and H I content, we find that global H I asymmetric galaxies are typically more gas-poor than symmetric ones at fixed stellar mass, with no change in sSFR. Our results highlight the complexity of the connection between galaxy properties and global H I asymmetry. This is further confirmed by the fact that even post-merger galaxies show both symmetric and asymmetric H I spectra, demonstrating that merger activity does not always lead to an asymmetric global H I spectrum.