Atomic and molecular gas in IllustrisTNG galaxies at low redshift
  We have recently developed a post-processing framework to estimate the
abundance of atomic and molecular hydrogen (HI and H2, respectively) in
galaxies in large-volume cosmological simulations. Here we compare the HI and
H2 content of IllustrisTNG galaxies to observations. We mostly restrict this
comparison to $z \approx 0$ and consider six observational metrics: the overall
abundance of HI and H2, their mass functions, gas fractions as a function of
stellar mass, the correlation between H2 and star formation rate, the spatial
distribution of gas, and the correlation between gas content and morphology. We
find generally good agreement between simulations and observations,
particularly for the gas fractions and the HI mass-size relation. The H2 mass
correlates with star formation rate as expected, revealing an almost constant
depletion time that evolves up to z = 2 as observed. However, we also discover
a number of tensions with varying degrees of significance, including an
overestimate of the total neutral gas abundance at z = 0 by about a factor of
two and a possible excess of satellites with no or very little neutral gas.
These conclusions are robust to the modelling of the HI/H2 transition. In terms
of their neutral gas properties, the IllustrisTNG simulations represent an
enormous improvement over the original Illustris run. All data used in this
paper are publicly available as part of the IllustrisTNG data release.
