Why should models of dwarf galaxy evolution care about the initial mass function at low star-formation rates?

Abstract

ABSTRACT When star clusters are formed at low star-formation rates (SFRs), their stellar initial mass function (IMF) can hardly be filled continuously with stars at each mass. This lack holds for massive stars and is verified observationally by the correlation between star-cluster mass and its most massive cluster star. Since galaxy evolution is strongly affected by massive stars, numerical models should account for this lack. Because a filled IMF is mostly applied even when only fractions of massive stars form, here we investigate, by means of 3D chemo-dynamical simulations of isolated dwarf galaxies, how deviations from a standard IMF in star clusters affect the evolution. We compare two different IMF recipes, a filled IMF and one truncated at a maximum mass at which a single complete star forms. Attention is given to energetic and chemical feedback by massive stars. Since their energy release is mass-dependent but steeper than the negative IMF slope, the energetic feedback retains a positive mass dependence, so that a filled IMF regulates star formation (SF) more strongly than truncated IMFs, though only stellar number fractions exist. The higher SFR of the truncated IMF in the simulation leads to more Type II supernovae (SNeII), driving galactic winds. Whether this results from the model-inherent larger SFR is questioned and therefore explored analytically. This shows the expected result for the Lyman continuum, but that the total SNII energy release is equal for both IMF modes, while the power is smaller for the truncated IMF. Reasonably, the different IMFs leave fingerprints in the abundance ratios of massive to intermediate-mass star elements.