The (galaxy-wide) IMF in giant elliptical galaxies: From top to bottom

TL;DR

This paper proposes a two-stage, time-dependent IMF model for giant elliptical galaxies, reconciling spectral and dynamical evidence for a non-universal IMF with observed stellar populations and remnants.

Contribution

It introduces a novel two-stage formation scenario with a top-heavy early IMF followed by a bottom-heavy phase, explaining various observational constraints.

Findings

A time-dependent IMF model fits observed properties of elliptical galaxies.

A top-heavy early phase explains metal-rich populations and remnants.

A bottom-heavy later phase accounts for low-mass star excess.

Abstract

Recent evidence based independently on spectral line strengths and dynamical modelling point towards a non-universal stellar Initial Mass Function (IMF), probably implying an excess of low-mass stars in elliptical galaxies with a high velocity dispersion. Here we show that a time-independent bottom-heavy IMF is compatible neither with the observed metal-rich populations found in giant ellipticals nor with the number of stellar remnants observed within these systems. We suggest a two-stage formation scenario involving a time-dependent IMF to reconcile these observational constraints. In this model, an early strong star-bursting stage with a top-heavy IMF is followed by a more prolonged stage with a bottom-heavy IMF. Such model is physically motivated by the fact that a sustained high star formation will bring the interstellar medium to a state of pressure, temperature and turbulence that can drastically alter the fragmentation of the gaseous component into small clumps, promoting the formation of low-mass stars. This toy model is in good agreement with the different observational constrains on massive elliptical galaxies, such as age, metallicity, alpha-enhancement, M/L, or the mass fraction of the stellar component in low-mass stars.