On the relationship between gas content, star-formation, and global HI
  asymmetry of galaxies on the star-forming main-sequence
  Observations have revealed that disturbances in the cold neutral atomic
hydrogen (HI) in galaxies are ubiquitous, but the reasons for these
disturbances remain unclear. While some studies suggest that asymmetries in
integrated HI spectra (global HI asymmetry) are higher in HI-rich systems,
others claim that they are preferentially found in HI-poor galaxies. In this
work, we utilise the ALFALFA and xGASS surveys, plus a sample of post-merger
galaxies, to clarify the link between global HI asymmetry and the gas
properties of galaxies. Focusing on star-forming galaxies in ALFALFA, we find
that elevated global HI asymmetry is not associated with a change in the HI
content of a galaxy, and that only the galaxies with the highest global HI
asymmetry show a small increase in specific star-formation rate (sSFR).
However, we show that the lack of a trend with HI 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 HI content, we find that global HI 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 HI asymmetry. This is further confirmed by the fact that even
post-merger galaxies show both symmetric and asymmetric HI spectra,
demonstrating that merger activity does not always lead to an asymmetric global
HI spectrum.
