  Understanding what shapes the cold gas component of galaxies, which both
provides the fuel for star formation and is strongly affected by the subsequent
stellar feedback, is a crucial step towards a better understanding of galaxy
evolution. Here, we analyse the HI properties of a sample of 46 Milky Way
halo-mass galaxies, drawn from cosmological simulations (EMP-Pathfinder and
FIREbox). This set of simulations comprises galaxies evolved self-consistently
across cosmic time with different baryonic sub-grid physics: three different
star formation models [constant star formation efficiency (SFE) with different
star formation eligibility criteria, and an environmentally-dependent,
turbulence-based SFE] and two different feedback prescriptions, where only one
sub-sample includes early stellar feedback. We use these simulations to assess
the impact of different baryonic physics on the HI content of galaxies. We find
that the galaxy-wide HI properties agree with each other and with observations.
However, differences appear for small-scale properties. The thin HI discs
observed in the local Universe are only reproduced with a turbulence-dependent
SFE and/or early stellar feedback. Furthermore, we find that the morphology of
HI discs is particularly sensitive to the different physics models: galaxies
simulated with a turbulence-based SFE have discs that are smoother and more
rotationally symmetric, compared to those simulated with a constant SFE;
galaxies simulated with early stellar feedback have more regular discs than
supernova-feedback-only galaxies. We find that the rotational asymmetry of the
HI discs depends most strongly on the underlying physics model, making this a
promising observable for understanding the physics responsible for shaping the
interstellar medium of galaxies.
