The challenge of simulating the star cluster population of dwarf galaxies with resolved interstellar medium

We present results on the star cluster properties from a series of high resolution smoothed particles hydrodynamics (SPH) simulations of isolated dwarf galaxies as part of the GRIFFIN project. The simulations at sub-parsec spatial resolution and a minimum particle mass of 4 \(\mathrm{M_\odot}\) incorporate non-equilibrium heating, cooling and chemistry processes, and realise individual massive stars. All the simulations follow feedback channels of massive stars that include the interstellar-radiation field, that is variable in space and time, the radiation input by photo-ionisation and supernova explosions. Varying the star formation efficiency per free-fall time in the range \(\epsilon_\mathrm{ff}\) = 0.2 - 50\(\%\) neither changes the star formation rates nor the outflow rates. While the environmental densities at star formation change significantly with \(\epsilon_\mathrm{ff}\), the ambient densities of supernovae are independent of \(\epsilon_\mathrm{ff}\) indicating a decoupling of the two processes. At low \(\epsilon_\mathrm{ff}\), more massive, and increasingly more bound star clusters are formed, which are typically not destroyed. With increasing \(\epsilon_\mathrm{ff}\) there is a trend for shallower cluster mass functions and the cluster formation efficiency \(\Gamma\) for young bound clusters decreases from \(50 \%\) to \(\sim 1 \%\) showing evidence for cluster disruption. However, none of our simulations form low mass (\(< 10^3\) \(\mathrm{M_\odot}\)) clusters with structural properties in perfect agreement with observations. Traditional star formation models used in galaxy formation simulations based on local free-fall times might therefore not be able to capture low mass star cluster properties without significant fine-tuning.