Abstract
We have recently developed a 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 resulting HI and H2 content of
IllustrisTNG galaxies to observations. We mostly restrict this comparison to $z
0$ 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.
Description
Atomic and molecular gas in IllustrisTNG galaxies at low redshift
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