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 M_⊙ incorporate non-equilibrium heating, cooling, and chemistry processes, and realize individual massive stars. The simulations follow feedback channels of massive stars that include the interstellar-radiation field variable in space and time, the radiation input by photo-ionization and supernova explosions. Varying the star formation efficiency per free-fall time in the range ϵ_ff = 0.2-50${{\ \rm per\ cent}}$ neither changes the star formation rates nor the outflow rates. While the environmental densities at star formation change significantly with ϵ_ff, the ambient densities of supernovae are independent of ϵ_ff indicating a decoupling of the two processes. At low ϵ_ff, gas is allowed to collapse more before star formation, resulting in more massive, and increasingly more bound star clusters are formed, which are typically not destroyed. With increasing ϵ_ff, there is a trend for shallower cluster mass functions and the cluster formation efficiency Γ for young bound clusters decreases from $50 {{\ \rm per\ cent}}$ to $\sim 1 {{\ \rm per\ cent}}$ showing evidence for cluster disruption. However, none of our simulations form low mass (<10³ M_⊙) 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 be unable to capture star cluster properties without significant fine tuning.