The Dark Matter Halo of M54

TL;DR

This paper investigates M54, a globular cluster embedded in a dark matter halo, using Gaia data to analyze its stellar populations, velocity dispersion, and dark matter profile, revealing its transitional nature and evolutionary history.

Contribution

It provides the first detailed analysis of M54's dark matter halo structure and its evolutionary implications within the galaxy, combining Gaia data with Jeans modeling.

Findings

Velocity dispersion profile shows a minimum around 30 pc.

Mass profile suggests a small core or cuspy dark matter halo.

M54 is a transitional system between globular clusters with and without dark halos.

Abstract

M54 is a prototype of a globular cluster embedded in a dark matter halo. Gaia EDR3 photometry and proper motions separate the old, metal-poor stars from the more metal rich and younger dwarf galaxy stars. The metal poor stars dominate the inner 50 pc, with a velocity dispersion profile that declines to a minimum around 30 pc then rises back to nearly the central velocity dispersion, as expected for a globular cluster at the center of a dark matter halo. The Jeans analysis of the three separate stellar populations give consistent masses that rise approximately linearly with radius to 1 kpc, implying a small core or cuspy halo. These data are compatible with an infalling CDM dark matter halo reduced to 3x10^8 M_sun at the 50 kpc apocenter 2.3 Gyr ago, with a central globular cluster surrounded by the remnant of a dwarf galaxy. Tides gradually remove material beyond 1 kpc but have little effect on the stars and dark matter within 300 pc of the center. M54 appears to be a transitional system between globular clusters with and without local dark halos, whose evolution within the galaxy depends sensitively on the time of accretion and orbital pericenter.