Momentum feedback from marginally resolved H <scp>ii</scp> regions in isolated disc galaxies

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