  We present an analysis of asymmetries in global HI spectra from the extended
GALEX Arecibo SDSS Survey (xGASS), a stellar mass-selected and gas
fraction-limited survey which is representative of the HI properties of
galaxies in the local Universe. We demonstrate that the asymmetry in a HI
spectrum is strongly linked to its signal-to-noise meaning that, contrary to
what was done in previous works, asymmetry distributions for different samples
cannot be compared at face value. We develop a method to account for
noise-induced asymmetry and find that the typical galaxy detected by xGASS
exhibits higher asymmetry than what can be attributed to noise alone, with 37%
of the sample showing asymmetry greater than 10% at an 80% confidence level. We
find that asymmetric galaxies contain, on average, 29% less HI mass compared to
their symmetric counterparts matched in both stellar mass and signal-to-noise.
We also present clear evidence that satellite galaxies, as a population,
exhibit more asymmetric HI spectra than centrals and that group central
galaxies show a slightly higher rate of HI asymmetries compared to isolated
centrals. All these results support a scenario in which environmental
processes, in particular those responsible for gas removal, are the dominant
driver of asymmetry in xGASS.
