# Atomic and molecular gas in IllustrisTNG galaxies at low redshift

## TL;DR

This paper compares the atomic and molecular hydrogen content of IllustrisTNG galaxies at low redshift with observations, finding good agreement in some metrics but also notable discrepancies, and demonstrates the improvements over previous simulations.

## Contribution

It introduces a post-processing framework for estimating HI and H2 in large cosmological simulations and compares these to observational data at low redshift.

## Key findings

- Good agreement in gas fractions and HI mass-size relation.
- H2 correlates with star formation rate with a consistent depletion time.
- Overestimation of total neutral gas abundance at z=0 by about a factor of two.

## Abstract

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.