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

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