Searching in the dark: the dark mass content of the Milky Way globular clusters NGC288 and NGC6218

TL;DR

This study estimates that over 60% of the mass in the inner regions of two Milky Way globular clusters is dark matter, likely dark remnants, based on spectroscopic and photometric data analysis.

Contribution

It provides the first detailed observational estimate of dark mass fraction and distribution in the core regions of NGC 288 and NGC 6218 globular clusters.

Findings

Dark matter constitutes over 60% of the mass in the clusters' inner regions.

Dark component is more concentrated than luminous stars.

Results are robust against binaries and tidal heating effects.

Abstract

We present an observational estimate of the fraction and distribution of dark mass in the innermost region of the two Galactic globular clusters NGC 6218 (M12) and NGC 288. Such an assessment has been made by comparing the dynamical and luminous mass profiles derived from an accurate analysis of the most extensive spectroscopic and photometric surveys performed on these stellar systems. We find that non-luminous matter constitutes more than 60% of the total mass in the region probed by our data (R<1.6 arcmin~r_h) in both clusters. We have carefully analyzed the effects of binaries and tidal heating on our estimate and ruled out the possibility that our result is a spurious consequence of these effects. The dark component appears to be more concentrated than the most massive stars suggesting that it is likely composed of dark remnants segregated in the cluster core.