The [alpha/Fe] Ratios in Dwarf Galaxies: Evidence for a Non-universal Stellar Initial Mass Function?

TL;DR

This study uses semi-analytical models to show that variations in the initial mass function and star formation timescales can explain the observed alpha/Fe ratios in dwarf and elliptical galaxies, suggesting a non-universal IMF.

Contribution

It demonstrates that the combined effects of a variable IMF and galaxy downsizing can reproduce observed [alpha/Fe] trends across galaxy masses.

Findings

IGIMF theory predicts flatter IMFs at higher star formation rates.

Models reproduce the steepening of [alpha/Fe] in dwarf galaxies.

Results support a non-universal, variable IMF in galaxy evolution.

Abstract

It is well established that the [alpha/Fe] ratios in elliptical galaxies increase with galaxy mass. This relation holds also for early-type dwarf galaxies, although it seems to steepen at low masses. The [alpha/Fe] vs. mass relation can be explained assuming that smaller galaxies form over longer timescales (downsizing), allowing a larger amount of Fe (mostly produced by long-living Type Ia Supernovae) to be released and incorporated into newly forming stars. Another way to obtain the same result is by using a flatter initial mass function (IMF) in large galaxies, increasing in this way the number of Type II Supernovae and therefore the production rate of alpha-elements. The integrated galactic initial mass function (IGIMF) theory predicts that the higher the star formation rate, the flatter the IMF. We have checked, by means of semi-analytical calculations, that the IGIMF theory, combined with the downsizing effect (i.e. the shorter duration of the star formation in larger galaxies), well reproduces the observed [alpha/Fe] vs. mass relation. In particular, we show a steepening of this relation in dwarf galaxies, in accordance with the available observations.