Simulating Globular Clusters in Dark Matter Sub-Halos

TL;DR

This study uses cosmological simulations to analyze the evolution and dark matter content of star clusters within Milky Way-like halos, revealing how initial placement affects their dynamics and dark matter retention.

Contribution

It introduces a novel simulation approach with star clusters placed at different initial radii in dark matter sub-halos, examining their evolution and dark matter retention.

Findings

Clusters at small radii experience strong tidal forces and are often out of equilibrium.

Clusters at larger radii show a smooth decline in velocity dispersion with radius.

About 25% of centrally placed clusters retain local dark matter.

Abstract

A cosmological zoom-in simulation which develops into a Milky Way-like halo is started at redshift 7. The initial dark matter distribution is seeded with dense star clusters, median mass $5\times 10^5 M_\sun$, placed in the largest sub-halos present, which have a median peak circular velocity of 25 \kms. Three simulations are initialized using the same dark matter distribution, with the star clusters started on approximately circular orbits having initial median radii 6.8 kpc, 0.14 kpc, and, at the exact center of the sub-halos. The simulations are evolved to the current epoch at which time the median galactic orbital radii of the three sets of clusters are 30, 5 and 16 kpc, with the clusters losing about 2, 50 and 15\% of their mass, respectively. Clusters started at small orbital radii have so much tidal forcing that they are often not in equilibrium. Clusters started at larger sub-halo radii have a velocity dispersion that declines smoothly to $\simeq$20\% of the central value at $\simeq$20 half mass radii. The clusters started at the sub-halo centers can show a rise in velocity dispersion beyond 3-5 half mass radii. That is, the clusters formed without local dark matter always have stellar mass dominated kinematics at all radii, whereas about 25\% of the clusters started at sub-halo centers have remnant local dark matter.