The impact of a non-universal Initial Mass Function on the Star Formation Histories of Early-Type Galaxies

TL;DR

This study investigates how variations in the Initial Mass Function (IMF) influence the derived star formation histories of early-type galaxies, showing that non-universal IMFs aligned with galaxy velocity dispersion yield more consistent evolutionary insights.

Contribution

It demonstrates that adopting a non-universal, velocity dispersion-dependent IMF significantly alters star formation history estimates for early-type galaxies.

Findings

Steepening IMF slopes reduces old stellar population contributions by up to four times.

Adjusting IMF slopes to galaxy velocity dispersion aligns star formation histories across different galaxy masses.

Using a non-universal IMF results in more consistent and plausible galaxy evolution interpretations.

Abstract

Recent results on the non-universality of the Initial Mass Function (IMF) have shown strong evidence of IMF variations with galaxy velocity dispersion, with a corresponding impact on other stellar population parameters, line indices and colours. Using a set of stellar population models with varying IMF slopes, we provide additional caveats on the assumption of a universal IMF. The present study shows that the derived star formation histories of early-type galaxies vary significantly with the IMF slope. For instance, a steepening in the slope of a single power-law IMF decreases substantially, by a factor of up to four, the contribution of the old stellar populations to the total light/mass. This trend is milder for a segmented-like IMF shape, where the contribution of the very low mass stars is decreased. It is also shown that, by tuning each IMF slope to its prescribed value according to each galaxy velocity dispersion, a sample of early-type galaxies covering a range of masses yield comparable star formation histories. On the one hand, a small contribution from relatively young stellar populations appear in the star formation histories of most massive elliptical galaxies when adopting a steep IMF. On the other hand, we find that low mass early-type galaxies that look like genuinely young objects with a standard IMF (i.e. ``baby elliptical galaxies``), turn out to be older when a slightly flatter IMF is employed. In summary, the use of a non-universal IMF, tuned according to the velocity dispersion of the galaxy, seems to provide more consistent results.