Global HI asymmetries in IllustrisTNG: a diversity of physical processes
  disturb the cold gas in galaxies
  Observations of the cold neutral atomic hydrogen (HI) in and around disc
galaxies have revealed that spatial and kinematic asymmetries are commonplace,
and are reflected in the global HI spectra. We use the TNG100 box from the
IllustrisTNG suite of cosmological simulations to study the conditions under
which these asymmetries may arise in current theoretical galaxy formation
models. We find that more than 50% of the sample has at least a 10% difference
in integrated flux between the high- and low-velocity half of the spectrum,
thus the typical TNG100 galaxy has an HI profile that is not fully symmetric.
We find that satellite galaxies are a more asymmetric population than centrals,
consistent with observational results. Using halo mass as a proxy for
environment, this trend appears to be driven by the satellite population within
the virial radius of haloes more massive than $10^{13} M_{\odot}$, typical of
medium/large groups. We show that, while the excess of HI asymmetry in group
satellites is likely driven by ram pressure, the bulk of the asymmetric HI
profiles observed in TNG100 are driven by physical processes able to affect
both the central and satellite populations. Our results highlight how
asymmetries are not driven solely by environment, and multiple physical
processes can produce the same asymmetric shape in global HI spectra.
