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

A new type of compact stellar systems, labeled ultracompact 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 2.3 mu m - 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 should be up to 30% weaker in UCDs compared to sources in the same metallicity range 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). In the Fornax and Virgo Clusters, all known UCDs can be mapped within a single square degree pointing. Therefore, the best observational approach is wide-field imaging with narrowband CO filters, complemented by precise measurements of [Fe/H] from optical multi-object spectroscopy. We conclude that measuring the CO index of UCDs is a promising tool to test whether their high M/L ratios are caused by a bottom-heavy IMF.