The metallicity dependence of the stellar initial mass function

Abstract

ABSTRACT During star formation, dust plays the crucial role of coupling gas to stellar radiation fields, allowing radiation feedback to influence gas fragmentation and thus the stellar initial mass function (IMF). Variations in dust abundance therefore provide a potential avenue by which variation in galaxy metallicity might affect the IMF. In this paper, we present a series of radiation-magnetohydrodynamic simulations in which we vary the metallicity and thus the dust abundance from 1 per cent of solar to 3× solar, spanning the range from the lowest metallicity dwarfs to the most metal-rich early-type galaxies (ETGs) found in the local Universe. We design the simulations to keep all dimensionless parameters constant so that the interaction between feedback and star-forming environments of varying surface density and metallicity is the only factor capable of breaking the symmetry between the simulations and modifying the IMF, allowing us to isolate and understand the effects of each environmental parameter cleanly. We find that shifts in the IMF with varying metallicity at a fixed surface density are smaller than the shifts in varying surface density at a fixed surface metallicity. We also find that metallicity-induced IMF variations are too small to explain the mass-to-light ratio shifts seen in the ETGs. We therefore conclude that metallicity variations are much less important than variations in surface density in driving changes in the IMF and that the latter rather than the former are most likely responsible for the IMF variations found in ETGs.