The initial mass function of early-type galaxies

TL;DR

This study calibrates the initial mass function (IMF) of early-type galaxies using gravitational lensing, dynamical models, and stellar population synthesis, revealing potential non-universality of the IMF or dark matter halos.

Contribution

It provides an absolute calibration of the IMF in early-type galaxies and explores its possible dependence on galaxy properties, challenging the assumption of universality.

Findings

Salpeter IMF aligns with lensing and dynamical masses

Chabrier IMF underestimates stellar masses

Possible correlation between IMF mismatch and galaxy velocity dispersion

Abstract

We determine an absolute calibration of the initial mass function (IMF) of early-type galaxies, by studying a sample of 56 gravitational lenses identified by the SLACS Survey. Under the assumption of standard Navarro, Frenk & White dark matter halos, a combination of lensing, dynamical, and stellar population synthesis models is used to disentangle the stellar and dark matter contribution for each lens. We define an "IMF mismatch" parameter \alpha=M*(L+D)/M*(SPS) as the ratio of stellar mass inferred by a joint lensing and dynamical models (M*(L+D)) to the current stellar mass inferred from stellar populations synthesis models (M*(SPS)). We find that a Salpeter IMF provides stellar masses in agreement with those inferred by lensing and dynamical models (<\log \alpha>=0.00+-0.03+-0.02), while a Chabrier IMF underestimates them (<\log \alpha>=0.25+-0.03+-0.02). A tentative trend is found, in the sense that \alpha appears to increase with galaxy velocity dispersion. Taken at face value, this result would imply a non universal IMF, perhaps dependent on metallicity, age, or abundance ratios of the stellar populations. Alternatively, the observed trend may imply non-universal dark matter halos with inner density slope increasing with velocity dispersion. While the degeneracy between the two interpretations cannot be broken without additional information, the data imply that massive early-type galaxies cannot have both a universal IMF and universal dark matter halos.