Implications of galaxy buildup for putative IMF variations in massive galaxies

TL;DR

This study uses cosmological simulations to explore whether variations in the initial mass function (IMF) in massive galaxies can be explained by physical conditions at star formation sites, finding current models insufficient to reproduce observed trends.

Contribution

It demonstrates that applying observed IMF-property relations to simulation conditions does not produce the observed galaxy-to-galaxy IMF variations, highlighting the need for steeper relations or more extreme IMFs.

Findings

Observed IMF relations are insufficient to explain galaxy variations.

Hierarchical galaxy assembly dilutes IMF differences.

More extreme physical conditions may be necessary for IMF variability.

Abstract

Recent observational evidence for initial mass function (IMF) variations in massive quiescent galaxies at $z = 0$ challenges the long-established paradigm of a universal IMF. While a few theoretical models relate the IMF to birth cloud conditions, the physical driver underlying these putative IMF variations is still largely unclear. Here we use post-processing analysis of the Illustris cosmological hydrodynamical simulation to investigate possible physical origins of IMF variability with galactic properties. We do so by tagging stellar particles in the simulation (each representing a stellar population of $\approx10^{6}~\mathrm{M}_{\odot}$) with individual IMFs that depend on various physical conditions, such as velocity dispersion, metallicity, or SFR, at the time and place the stars are formed. We then follow the assembly of these populations throughout cosmic time, and reconstruct the overall IMF of each $z=0$ galaxy from the many distinct IMFs it is comprised of. Our main result is that applying the observed relations between IMF and galactic properties to the conditions at the star-formation sites does not result in strong enough IMF variations between $z = 0$ galaxies. Steeper physical IMF relations are required for reproducing the observed IMF trends, and some stellar populations must form with more extreme IMFs than those observed. The origin of this result is the hierarchical nature of massive galaxy assembly, and it has implications for the reliability of the strong observed trends, for the ability of cosmological simulations to capture certain physical conditions in galaxies, and for theories of star-formation aiming to explain the physical origin of a variable IMF.