Implementing molecular hydrogen in hydrodynamic simulations of galaxy formation

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 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. We use simulations of isolated disc galaxies to verify that the transition from atomic to molecular hydrogen occurs at realistic densities and surface densities. Using these same isolated galaxies, we establish that gas particles of 104 M⊙ or less are necessary to follow the molecular gas in this implementation. 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 supernova feedback. We analyse the amount and distribution of star formation in the galaxy using simulated observations of the H i gas and in various optical bands. From these simulated observations, 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 interstellar medium (ISM) result in an increased amount of cold, dense gas in the disc of the galaxy and the formation of a clumpier 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.

Influence of baryonic physics in galaxy simulations:

“…This is due to the variation in both the structure of the gas and the amount of nearby recent star formation. The transition density is low compared to similar work by Gnedin et al (2009) or Christensen et al (2012b, because of our calibration to obtain a realistic global H 2 mass fraction and of our generally lower densities compared to Gnedin et al (2009) and Christensen et al (2012b). These lower densities arise partly from our choice of a lower threshold density for star formation, which prevents the gas from becoming as dense.…”

In-N-Out: The Gas Cycle From Dwarfs to Spiral Galaxies

“…The parameters used to simulate the galaxies and their final properties at z = 0 are listed in Table 1. We integrate over the H and He chemical networks to produce non-equilibrium ion abundances and H 2 abundance (Christensen et al 2012). H 2 forms both on dust grains, assuming a fixed dustto-metallicity ratio and a clumping factor of 10 (Wolfire et al 2008;Gnedin et al 2009), and via H − , following the minimal model of Abel et al (1997).…”

“…Photoionization and heating rates of H and He are calculated assuming a set redshift-dependent cosmic ultraviolet (UV) background (Haardt & Madau 2005). 10 The Lyman-Werner radiation, which is responsible for H 2 photodissociation, is calculated based on emission from nearby stellar particles (Christensen et al 2012). H 2 is shielded from dissociating radiation through both self-shielding and dust shielding (Draine & Bertoldi 1996;Glover & Mac Low 2007;Gnedin et al 2009), using the smoothing lengths of particles for the column lengths.…”

“…c Appears in Christensen et al (2014b). d Appears in Christensen et al (2012). e Appears in Zolotov et al (2012).…”

Redshift evolution of stellar mass versus gas fraction relation in 0 <z< 2 regime: observational constraint for galaxy formation models

“…Schaye et al 2010Schaye et al , 2015Okamoto et al 2014;Thompson et al 2014;Vogelsberger et al 2014). In addition, some of these theoretical studies have implemented the equilibrium or non-equilibrium formation of H2, as well as star formation from such H2 gas, in cosmological simulations (Gnedin et al 2009;Christensen et al 2012;Thompson et al 2014;Tomassetti et al 2015) and SAMs (Lagos et al 2011b;Fu et al 2012;Popping et al 2014;Somerville et al 2015).…”