Evidence Against Dark Matter Halos Surrounding the Globular Clusters MGC1 and NGC 2419

TL;DR

This study uses stellar density profiles of globular clusters NGC 2419 and MGC1 to test the hypothesis that they formed within dark matter halos, finding strong evidence against significant dark matter presence.

Contribution

The paper provides analytic constraints on dark matter halos around GCs, challenging the idea that these clusters formed within their own dark halos.

Findings

NGC 2419 and MGC1 cannot be embedded in dark halos with mass > 10^6 Msun.

Dark matter halo mass-to-stellar mass ratio must be less than 1.

Globular clusters likely did not form within their own dark halos.

Abstract

The conjecture that the ancient globular clusters (GCs) formed at the center of their own dark matter halos was first proposed by Peebles (1984), and has recently been revived to explain the puzzling abundance patterns observed within many GCs. In this paper we demonstrate that the outer stellar density profile of isolated GCs is very sensitive to the presence of an extended dark halo. The GCs NGC 2419, located at 90 kpc from the center of our Galaxy, and MGC1, located at ~200 kpc from the center of M31, are ideal laboratories for testing the scenario that GCs formed at the centers of massive dark halos. Comparing analytic models to observations of these GCs, we conclude that these GCs cannot be embedded within dark halos with a virial mass greater than 10^6 Msun, or, equivalently, the dark matter halo mass-to-stellar mass ratio must be Mdm/M_*<1. If these GCs have indeed orbited within weak tidal fields throughout their lifetimes, then these limits imply that these GCs did not form within their own dark halos. Recent observations of an extended stellar halo in the GC NGC 1851 are also interpreted in the context of our analytic models. Implications of these results for the formation of GCs are briefly discussed.