Kavli Affiliate: Nickolay Y. Gnedin
| First 5 Authors: Ava Polzin, Andrey V. Kravtsov, Vadim A. Semenov, Nickolay Y. Gnedin,
| Summary:
We use a suite of hydrodynamics simulations of the interstellar medium (ISM)
within a galactic disk, which include radiative transfer, a non-equilibrium
model of molecular hydrogen, and a realistic model for star formation and
feedback, to study the structure of the ISM and H$_2$ abundance as a function
of local ISM properties. We show that the star formation rate and structure of
the ISM are sensitive to the metallicity of the gas with a progressively
smoother density distribution with decreasing metallicity. In addition to the
well-known trend of the HI-H$_2$ transition shifting to higher densities with
decreasing metallicity, the maximum achieved molecular fraction in the
interstellar medium drops drastically at $Z lesssim 0.2 , Z_odot$ as the
formation time of H$_2$ becomes much longer than a typical lifetime of dense
regions of the ISM. We present accurate fitting formulae for both volumetric
and projected $f_mathrm{H_2}$ measured on different scales as a function of
gas metallicity, UV radiation field, and gas density. We show that when the
formulae are applied to the patches in the simulated galaxy the overall
molecular gas mass is reproduced to better than a factor of $lesssim 1.5$
across the entire range of metallicities and scales. We also show that the
presented fit is considerably more accurate than any of the previous
$f_mathrm{H_2}$ models and fitting formulae in the low-metallicity regime. The
fit can thus be used for modeling molecular gas in low-resolution simulations
and semi-analytic models of galaxy formation in the dwarf and high-redshift
regimes.
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