Star formation and molecular hydrogen in dwarf galaxies: a non-equilibrium view

We study the connection of star formation to atomic (H I) and molecular hydrogen (H₂) in isolated, low-metallicity dwarf galaxies with high-resolution (m_gas = 4 M_⊙, N_ngb = 100) smoothed particle hydrodynamics simulations. The model includes self-gravity, non-equilibrium cooling, shielding from a uniform and constant interstellar radiation field, the chemistry of H₂ formation, H₂-independent star formation, supernova feedback and metal enrichment. We find that the H₂ mass fraction is sensitive to the adopted dust-to-gas ratio and the strength of the interstellar radiation field, while the star formation rate is not. Star formation is regulated by stellar feedback, keeping the gas out of thermal equilibrium for densities n < 1 cm^-3. Because of the long chemical time-scales, the H₂ mass remains out of chemical equilibrium throughout the simulation. Star formation is well correlated with cold (T ≤ 100 K) gas, but this dense and cold gas - the reservoir for star formation - is dominated by H I, not H₂. In addition, a significant fraction of H₂ resides in a diffuse, warm phase, which is not star-forming. The interstellar medium is dominated by warm gas (100 K < T ≤ 3 × 10⁴ K) both in mass and in volume. The scaleheight of the gaseous disc increases with radius while the cold gas is always confined to a thin layer in the mid-plane. The cold gas fraction is regulated by feedback at small radii and by the assumed radiation field at large radii. The decreasing cold gas fractions result in a rapid increase in depletion time (up to 100 Gyr) for total gas surface densities Σ _{H I+H_2} ≲ 10 M_⊙ pc^-2, in agreement with observations of dwarf galaxies in the Kennicutt-Schmidt plane.