Implementing molecular hydrogen in hydrodynamic simulations of galaxy formation
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abstract
Motivated by the observed connection between molecular hydrogen (H2) and star
formation, we present a method for tracking the non-equilibrium abundance and
cooling processes of H2 and H2-based star formation in Smoothed Particle
Hydrodynamic simulations. The local abundances of H2 are calculated by
integrating over the hydrogen chemical network. This calculation includes the
gas-phase and dust grain formation of H2, shielding of HI and H2, and
photodissociation of H2 by Lyman-Werner radiation from nearby stellar
populations. Because this model does not assume equilibrium abundances, it is
particularly well suited for simulations that model low-metallicity
environments, such as dwarf galaxies and the early Universe. We further
introduce an explicit link between star formation and local H2 abundance. This
link limits star formation to "star-forming regions," represented by areas with
abundant H2.
With this implementation, we determine the effect of H2 on star formation in
a cosmological simulation of a dwarf galaxy. This simulation is the first
cosmological simulation with non-equilibrium H2 abundances to be integrated to
a redshift of zero or to include efficient SN feedback. We find that our
simulations are consistent with the observed Tully-Fisher, global
Kennicutt-Schmidt, and resolved Kennicutt-Schmidt relations. We find that the
inclusion of shielding of both the atomic and molecular hydrogen and, to a
lesser extent, the additional cooling from H2 at temperatures between 200 and
5000 K increases the amount of cold gas in the galaxies. The changes to the ISM
result in an increased amount of cold, dense gas in the disk of the galaxy and
the formation of a clumpier interstellar media (ISM). The explicit link between
star formation and H2 and the clumpier ISM results in a bluer galaxy with a
greater spatial distribution of star formation at a redshift of zero.
(abridged)
