The Initial Mass Function of Early-type Galaxies: no correlation with [Mg/Fe]

TL;DR

This study analyzes a large sample of early-type galaxy spectra and finds that the initial mass function correlates strongly with velocity dispersion but shows no significant dependence on the [Mg/Fe] abundance ratio.

Contribution

It provides the first large-scale spectroscopic evidence that the IMF variation in ETGs is not driven by [Mg/Fe], challenging previous hypotheses about chemical drivers.

Findings

IMF correlates with velocity dispersion

No significant correlation between IMF and [Mg/Fe]

Results are consistent with dynamical studies

Abstract

The Initial Mass Function (IMF) of early-type galaxies (ETGs) has been found to feature systematic variations by both dynamical and spectroscopic studies. In particular, spectral line strengths, based on gravity-sensitive features, suggest an excess of low-mass stars in massive ETGs, i.e. a bottom-heavy IMF. The physical drivers of IMF variations are currently unknown. The abundance ratio of alpha elements, such as [Mg/Fe], has been suggested as a possible driver of the IMF changes, although dynamical constraints do not support this claim. In this letter, we take advantage of the large SDSS database. Our sample comprises 24,781 high-quality spectra, covering a large range in velocity dispersion (100<sigma0<320 km/s) and abundance ratio (-0.1<[Mg/Fe]<+0.4). The large volume of data allows us to stack the spectra at fixed values of sigma0 and [Mg/Fe]. Our analysis -- based on gravity-sensitive line strengths -- gives a strong correlation with central velocity dispersion and a negligible variation with [Mg/Fe] at fixed sigma0. This result is robust against individual elemental abundance variations, and seems not to raise any apparent inconsistency with the alternative method based on galaxy dynamics.