Imladris: a detailed and flexible model for galaxy simulations with individual stars

Coupling stellar feedback to the evolution of individual stars, as opposed to averaging over the initial mass function (IMF), substantially improves the fidelity of galaxy formation simulations by capturing stochastic population effects. Existing treatments can typically only operate at a narrow mass resolution range, limiting their applicability. We present Imladris, a detailed model for star formation and stellar feedback with individual stars. At high resolution, each star can be represented by its own particle ("star-by-star"). At coarser resolution, star particles represent specific realisations of stellar populations sampled from the IMF. Both methods share a unified implementation of stellar feedback tied to the individually tracked stars, including supernovae, stellar winds and radiation. Imladris has been optimised for both computational efficiency and memory footprint. We demonstrate the model with idealised galaxy simulations ($M_\mathrm{vir}\sim10^{10}-10^{11}\,\mathrm{M_\odot}$) spanning a baryonic mass resolution range of $2.5-1000\,\mathrm{M_\odot}$. Without re-calibration, the time-averaged star formation rate (SFR), galactic wind mass and energy loadings close to the disc are converged up to a resolution of $20\,\mathrm{M_\odot}$ within a factor of 1.1, 1.1 and 1.3, respectively, and 1.4, 1.6 and 2.5 up to $100\,\mathrm{M_\odot}$. Above this, SFRs become more bursty, while loading factors increase substantially. This is linked to resolution-dependent supernova clustering, which represents a fundamental barrier to convergence for any scheme attempting to model a self-consistent stellar feedback-regulated interstellar medium. Regardless, the ability to deploy the scheme across a wide range of resolutions (and to carry out in-depth resolution convergence studies) makes Imladris a powerful tool for numerical investigations of galaxy formation.