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 »  M 10 6) with individual IMFs that depend on various physical conditions, such as velocity dispersion, metallicity, or star formation rate, at the time and place in which 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 composed 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.

The Dawes Review 8: Measuring the Stellar Initial Mass Function

“…He finds that the "bottom heavy" models lead to variations in stellar mass and SFR functions similar to the uncertainty in the determination of those quantities, while the "top heavy" models lead to an underestimate in the high mass end of the galaxy stellar mass function, compared to a fixed Kroupa (2001) IMF. Blancato et al (2017) also explore the impact of observed IMF variations on models. They implement various IMF dependencies by tagging stellar particles in their simulation with individual IMFs using observationally derived dependencies on velocity dispersion, metallicity or star formation rate.…”

“…It also means that analyses can be clear about the spatial and temporal scales for the stellar mass functions they are using, or making predictions for. For example, the investigation of Blancato et al (2017) adopts an observed gIMF, which is then implemented as an sIMF in simulations. They find, perhaps not surprisingly given the discussion above, that this does not lead to the observed gIMF being reproduced in the simulated galaxies, concluding that sIMFs need to be more extreme than adopted in order to replicate the observed gIMF.…”

On the Optimal Choice of Nucleosynthetic Yields, Initial Mass Function, and Number of SNe Ia for Chemical Evolution Modeling

“…This outcome could be biased because only a single galaxy was considered and it is known that the chemical enrichment of galaxies of a certain mass depends on their evolutionary environment (Blancato et al 2017;Vogelsberger et al 2013, fig. 5, fig.…”

Calibrated, cosmological hydrodynamical simulations with variable IMFs I: Method and effect on global galaxy scaling relations

“…To address this question, recent studies have begun investigating the effect of IMF variations by post-processing cosmological simulations and semi-analytic models, and by conducting self-consistent, small-scale, numerical simulations. In a post-processing analysis of the Illustris simulations, Blancato et al (2017) study how variations in the IMF of individual star particles manifests as global IMF trends between galaxies, finding that the IMF of individual particles must vary much more strongly than the global trends imply in order to obtain the observed MLE-σ trends. Sonnenfeld et al (2017) use an evolutionary model based on dark matter-only numerical simulations to predict the evolution of the IMF in early-type galaxies due to dry mergers from z = 2 to 0, finding that dry mergers tend to decrease the MLE of individual galaxies over time, while the correlation between the IMF and σ should remain time-invariant.…”