Is the IMF in ellipticals bottom-heavy? Clues from their chemical abundances

TL;DR

This study investigates whether the initial mass function (IMF) in elliptical galaxies is bottom-heavy by comparing chemical abundance models with observations, finding that a variable, time-dependent IMF best explains the data.

Contribution

The paper introduces and tests a time-dependent IMF model that switches from top-heavy to bottom-heavy, providing a better fit to observed chemical abundances in ellipticals.

Findings

Constant Salpeter IMF reproduces observations.

Time-dependent IMF switching from top to bottom fits data.

Bottom-heavy, time-independent IMFs do not match observed abundance trends.

Abstract

We tested the implementation of different IMFs in our model for the chemical evolution of ellipticals, with the aim of reproducing the observed relations of [Fe/H] and [Mg/Fe] abundances with galaxy mass in a sample of early-type galaxies selected from the SPIDER-SDSS catalog. Abundances in the catalog were derived from averaged spectra, obtained by stacking individual spectra according to central velocity dispersion, as a proxy of galaxy mass. We tested initial mass functions already used in a previous work, as well as two new models, based on low-mass tapered ("bimodal") IMFs, where the IMF becomes either (1) bottom-heavy in more massive galaxies, or (2) is time-dependent, switching from top-heavy to bottom-heavy in the course of galactic evolution. We found that observations could only be reproduced by models assuming either a constant, Salpeter IMF, or a time-dependent distribution, as other IMFs failed. We further tested the models by calculating their M/L ratios. We conclude that a constant, time-independent bottom-heavy IMF does not reproduce the data, especially the increase of the $[\alpha/Fe]$ ratio with galactic stellar mass, whereas a variable IMF, switching from top to bottom-heavy, can match observations. For the latter models, the IMF switch always occurs at the earliest possible considered time, i.e. $t_{\text{switch}}= 0.1$ Gyr.