Are the red halos of galaxies made of low-mass stars? Constraints from subdwarf star counts in the Milky Way halo

TL;DR

This study investigates whether faint red halos around galaxies could be composed of low-mass stars, using star counts in the Milky Way to constrain such models and find they are largely inconsistent with observations.

Contribution

The paper provides observational constraints on the hypothesis that red galaxy halos are made of low-mass stars, ruling out smooth halo models with bottom-heavy initial mass functions.

Findings

Smooth red halo models with bottom-heavy IMFs are ruled out by star count data.

Such halos contain too little mass to explain missing baryons.

Star clusters could hide red halo stars, warranting further investigation.

Abstract

Surface photometry detections of red and exceedingly faint halos around galaxies have resurrected the old question of whether some non-negligible fraction of the missing baryons of the Universe could be hiding in the form of faint, hydrogen-burning stars. The optical/near-infrared colours of these red halos have proved very difficult to reconcile with any normal type of stellar population, but can in principle be explained by advocating a bottom-heavy stellar initial mass function. This implies a high stellar mass-to-light ratio and hence a substantial baryonic mass locked up in such halos. Here, we explore the constraints imposed by current observations of ordinary stellar halo subdwarfs on a putative red halo of low-mass stars around the Milky Way. Assuming structural parameters similar to those of the red halo recently detected in stacked images of external disk galaxies, we find that a smooth halo component with a bottom-heavy initial mass function is completely ruled out by current star count data for the Milky Way. All viable smooth red halo models with a density slope even remotely similar to that of the stacked halo moreover contain far too little mass to have any bearing on the missing-baryon problem. However, we note that these constraints can be sidestepped if the red halo stars are locked up in star clusters, and discuss potential observations of other nearby galaxies that may be able to put such scenarios to the test.