A systematic variation of the stellar initial mass function in early-type galaxies

TL;DR

This study provides clear evidence that the stellar initial mass function varies systematically with galaxy mass in early-type galaxies, impacting our understanding of galaxy formation and evolution.

Contribution

It demonstrates a strong correlation between the IMF and stellar mass-to-light ratio in early-type galaxies, revealing non-universality of the IMF.

Findings

IMF varies up to a factor of three in mass across galaxies.

The variation correlates with stellar mass-to-light ratio.

Implications for galaxy formation history.

Abstract

Much of our knowledge of galaxies comes from analysing the radiation emitted by their stars. It depends on the stellar initial mass function (IMF) describing the distribution of stellar masses when the population formed. Consequently knowledge of the IMF is critical to virtually every aspect of galaxy evolution. More than half a century after the first IMF determination, no consensus has emerged on whether it is universal in different galaxies. Previous studies indicated that the IMF and the dark matter fraction in galaxy centres cannot be both universal, but they could not break the degeneracy between the two effects. Only recently indications were found that massive elliptical galaxies may not have the same IMF as our Milky Way. Here we report unambiguous evidence for a strong systematic variation of the IMF in early-type galaxies as a function of their stellar mass-to-light ratio, producing differences up to a factor of three in mass. This was inferred from detailed dynamical models of the two-dimensional stellar kinematics for the large Atlas3D representative sample of nearby early-type galaxies spanning two orders of magnitude in stellar mass. Our finding indicates that the IMF depends intimately on a galaxy's formation history.