Momentum feedback from marginally resolved Hii regions in isolated disc galaxies

Abstract

We present a novel, physically motivated sub-grid model for Hii region feedback within the moving mesh code arepo, accounting for both the radiation pressure-driven and thermal expansion of the ionized gas surrounding young stellar clusters. We apply this framework to isolated disc galaxy simulations with mass resolutions between 10(3) and 10(5)M(circle dot) per gas cell. Each simulation accounts for the self-gravity of the gas, the momentum and thermal energy from supernovae, the injection of mass by stellar winds, and the non-equilibrium chemistry of hydrogen, carbon, and oxygen. We reduce the resolution dependence of our model by grouping those Hii regions with overlapping ionization front radii. The Stromgren radii of the grouped Hii regions are at best marginally resolved, so that the injection of purely thermal energy within these radii has no effect on the interstellar medium. By contrast, the injection of momentum increases the fraction of cold and molecular gas by more than 50 percent at mass resolutions of 10(3)M(circle dot), and decreases its turbulent velocity dispersion by similar to 10 kms(-1). The mass-loading of galactic outflows is decreased by an order of magnitude. The characteristic lifetime of the least-massive molecular clouds (M/M-circle dot less than or similar to 5.6 x 10(4)) is reduced from similar to 18 to less than or similar to 10 Myr, indicating that Hii region feedback is effective in destroying these clouds. Conversely, the lifetimes of intermediate-mass clouds (5.6 x 10(4) less than or similar to M/M-circle dot less than or similar to 5 x 10(5)) are elongated by similar to 7 Myr, likely due to a reduction in supernova clustering. The derived cloud lifetimes span the range from 10 to 40 Myr, in agreement with observations. All results are independent of whether the momentum is injected from a 'spherical' or a 'blister-type' Hii region.