Speaker: Dr. Tabassum Tanvir (Iowa State University)

Title: Environmental variation of the low-mass IMF

Abstract: The stellar initial mass function is arguably the most important distribution in astrophysics since it is at least partly determining everything from chemical evolution to the strength of stellar feedback during galaxy formation. While the IMF seems to be nearly universal in nearby, Milky Way-like galaxies, in recent years tentative evidence has emerged that the IMF varies slightly in the most extreme star-forming environments (e.g.: early-type galaxies). In a series of magnetohydrodynamic simulations including radiative and protostellar outflow feedback, we study the environmental variation of the initial mass function. The simulations represent a carefully controlled experiment whereby we keep all dimensionless parameters of the flow constant except for feedback-related ones. We show that radiation feedback suppresses the formation of lower-mass objects more effectively as the surface density increases, while protostellar outflows grow more effective at suppressing the formation of massive stars. The combined effect of these two trends is towards an IMF with a lower characteristic mass and a narrower overall mass range in high surface density environments. We further explore the effects of varying the metallicity, and thus the dust properties, of the star-forming gas. Dust matters because it is the crucial component of the gas that couples it to stellar radiation fields. We show that over a range in metallicity from 1% of Solar to 3x Solar, corresponding to the lowest metallicity dwarf galaxies in the local universe to the most metal-rich early-types, metallicity variations are much less important than variations in surface density in driving changes in the IMF. We therefore conclude that the higher surface densities and pressure found in early-type galaxies are most likely responsible for their anomalous IMFs, while metallicity effects are negligible in comparison.