Red Halos of Galaxies - Reservoirs of Baryonic Dark Matter?

TL;DR

Deep optical and near-IR observations reveal faint, red halos around various galaxies, potentially indicating a form of baryonic dark matter composed of bottom-heavy stellar populations, challenging previous explanations.

Contribution

This paper reviews current evidence for red halos, discusses alternative explanations, and explores tests for the bottom-heavy IMF hypothesis as a form of baryonic dark matter.

Findings

Red halos are detected around diverse galaxy types.

Alternative explanations like dust or metallicity are disfavored.

A bottom-heavy IMF could explain the red halos as baryonic dark matter.

Abstract

Deep optical/near-IR surface photometry of galaxies outside the Local Group have revealed faint and very red halos around objects as diverse as disk galaxies and starbursting dwarf galaxies. The colours of these structures are too extreme to be reconciled with stellar populations similar to those seen in the stellar halos of the Milky Way or M31, and alternative explanations like dust reddening, high metallicities or nebular emission are also disfavoured. A stellar population obeying an extremely bottom-heavy initial mass function (IMF), is on the other hand consistent with all available data. Because of its high mass-to-light ratio, such a population would effectively behave as baryonic dark matter and could account for some of the baryons still missing in the low-redshift Universe. Here, we give an overview of current red halo detections, alternative explanations for the origin of the red colours and ongoing searches for red halos around types of galaxies for which this phenomenon has not yet been reported. A number of potential tests of the bottom-heavy IMF hypothesis are also discussed.