An extreme IMF as an explanation for high M/L ratios in UCDs? The CO index as a tracer of bottom heavy IMFs

TL;DR

This paper proposes using the CO index as a diagnostic tool to determine if bottom-heavy IMFs explain the high mass-to-light ratios observed in ultra-compact dwarf galaxies, challenging dark matter explanations.

Contribution

It introduces an observational method to test for bottom-heavy IMFs in UCDs using the CO index, providing a new approach to understanding their high M/L ratios.

Findings

The CO index is sensitive to low-mass stars and can indicate bottom-heavy IMFs.

A bottom-heavy IMF would weaken the CO index by up to 30% in UCDs.

Current facilities can detect these effects within a few hours to nights.

Abstract

A new type of compact stellar systems, labelled ``ultra-compact dwarf galaxies'' (UCDs), was discovered in the last decade. Recent studies show that their dynamical mass-to-light ratios (M/L) tend to be too high to be explained by canonical stellar populations, being on average about twice as large as those of Galactic globular clusters of comparable metallicity. If this offset is caused by dark matter in UCDs, it would imply dark matter densities as expected for the centers of cuspy dark matter halos, incompatible with cored dark matter profiles. Investigating the nature of the high M/L ratios in UCDs therefore offers important constraints on the phase space properties of dark matter particles. Here we describe an observational method to test whether a bottom-heavy IMF may cause the high M/L ratios of UCDs. We propose to use the CO index at 2.3mu -- which is sensitive to the presence of low-mass stars -- to test for a bottom heavy IMF. In the case that the high M/L ratios are caused by a bottom-heavy IMF, we show that the equivalent width of the CO index will be up to 30% weaker in UCDs compared to sources with similar metallicity that have canonical IMFs. We find that these effects are well detectable with current astronomical facilities in a reasonable amount of time (a few hours to nights). Measuring the CO index of UCDs hence appears a promising tool to investigate the origin of their high M/L ratios.