A comparison of $text{H}_2$ formation models at high redshift. (arXiv:2003.04329v2 [astro-ph.GA] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Schabe_A/0/1/0/all/0/1">Alexander Schäbe</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Romano_Diaz_E/0/1/0/all/0/1">Emilio Romano-Díaz</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Porciani_C/0/1/0/all/0/1">Cristiano Porciani</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ludlow_A/0/1/0/all/0/1">Aaron D. Ludlow</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tomassetti_M/0/1/0/all/0/1">Matteo Tomassetti</a>
Modelling the molecular gas that is routinely detected through CO
observations of high-redshift galaxies constitutes a major challenge for ab
initio simulations of galaxy formation. We carry out a suite of cosmological
hydrodynamic simulations to compare three approximate methods that have been
used in the literature to track the formation and evolution of the simplest and
most abundant molecule, H$_2$. Namely, we consider: i) a semi-empirical
procedure that associates H$_2$ to dark-matter haloes based on a series of
scaling relations inferred from observations, ii) a model that assumes chemical
equilibrium between the H$_2$ formation and destruction rates, and iii) a model
that fully solves the out-of-equilibrium rate equations and accounts for the
unresolved structure of molecular clouds. We study the impact of finite spatial
resolution and show that robust H$_2$ masses at redshift $zapprox 4$ can only
be obtained for galaxies that are sufficiently metal enriched in which H$_2$
formation is fast. This corresponds to H$_2$ reservoirs with masses
$M_{mathrm{H_2}}gtrsim 6times 10^9 mathrm{M}_odot$. In this range,
equilibrium and non-equilibrium models predict similar molecular masses (but
different galaxy morphologies) while the semi-empirical method produces less
H$_2$. The star formation rates as well as the stellar and H$_2$ masses of the
simulated galaxies are in line with those observed in actual galaxies at
similar redshifts that are not massive starbursts. The H$_2$ mass functions
extracted from the simulations at $zapprox 4$ agree well with recent
observations that only sample the high-mass end. However, our results indicate
that most molecular material at high $z$ lies yet undetected in reservoirs with
$10^9<M_{mathrm H_2}<10^{10} mathrm{M}_odot$.
Modelling the molecular gas that is routinely detected through CO
observations of high-redshift galaxies constitutes a major challenge for ab
initio simulations of galaxy formation. We carry out a suite of cosmological
hydrodynamic simulations to compare three approximate methods that have been
used in the literature to track the formation and evolution of the simplest and
most abundant molecule, H$_2$. Namely, we consider: i) a semi-empirical
procedure that associates H$_2$ to dark-matter haloes based on a series of
scaling relations inferred from observations, ii) a model that assumes chemical
equilibrium between the H$_2$ formation and destruction rates, and iii) a model
that fully solves the out-of-equilibrium rate equations and accounts for the
unresolved structure of molecular clouds. We study the impact of finite spatial
resolution and show that robust H$_2$ masses at redshift $zapprox 4$ can only
be obtained for galaxies that are sufficiently metal enriched in which H$_2$
formation is fast. This corresponds to H$_2$ reservoirs with masses
$M_{mathrm{H_2}}gtrsim 6times 10^9 mathrm{M}_odot$. In this range,
equilibrium and non-equilibrium models predict similar molecular masses (but
different galaxy morphologies) while the semi-empirical method produces less
H$_2$. The star formation rates as well as the stellar and H$_2$ masses of the
simulated galaxies are in line with those observed in actual galaxies at
similar redshifts that are not massive starbursts. The H$_2$ mass functions
extracted from the simulations at $zapprox 4$ agree well with recent
observations that only sample the high-mass end. However, our results indicate
that most molecular material at high $z$ lies yet undetected in reservoirs with
$10^9<M_{mathrm H_2}<10^{10} mathrm{M}_odot$.
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