Implications of galaxy buildup for putative IMF variations in massive galaxies

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 $\approx 10^6{\mathrm{M}}_{\odot }$) 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.

22 pages, 10 figures, 2 tables. Key figures are 3, 6, and 7. Accepted for publication in ApJ

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• The Dawes Review 8: Measuring the Stellar Initial Mass Function
• An evolving and mass dependent ${\sigma }_{\text{sSFR}}$-${\mathrm{M}}_{\star }$ relation for galaxies
• Simulating a metallicity-dependent initial mass function: Consequences for feedback and chemical abundances
• Comparing Models for IMF Variation Across Cosmological Time in Milky Way-like Galaxies
• On the Optimal Choice of Nucleosynthetic Yields, IMF and Number of SNe Ia for Chemical Evolution Modelling
• Stellar velocity dispersion and initial mass function gradients in dissipationless galaxy mergers
• Hierarchical Bayesian inference of the Initial Mass Function in Composite Stellar Populations
• Stellar initial mass function variation in massive early-type galaxies: the potential role of the deuterium abundance