Testing for Dark Matter in the Outskirts of Globular Clusters

Abstract

The proper motions of stars in the outskirts of globular clusters are used to estimate cluster velocity dispersion profiles as far as possible within their tidal radii. We use individual color–magnitude diagrams to select high-probability cluster stars for 25 metal-poor globular clusters within 20 kpc of the Sun, 19 of which have substantial numbers of stars at large radii. Of the 19, 11 clusters have a falling velocity dispersion in the 3–6 half-mass radii range, 6 are flat, and 2 plausibly have a rising velocity dispersion. The profiles are all in the range expected from simulated clusters that started at high redshift in a zoom-in cosmological simulation. The 11 clusters with falling velocity dispersion profiles are consistent with no dark matter above the Galactic background. The six clusters with approximately flat velocity dispersion profiles could have local dark matter, but are ambiguous. The two clusters with rising velocity dispersion profiles are consistent with a remnant local dark matter halo, but need membership confirmation and detailed orbital modeling to further test these preliminary results.

The Dark Matter Halo of M54

“…Comparisons of the observed profiles to detailed n-body velocity dispersion profiles find that ∼10% of currently measured clusters have velocity dispersion profiles above the values predicted from the stars (Baumgardt & Hilker 2018;Bianchini et al 2019;Wan et al 2021). For a set of 19 halo clusters measured to at least 5 halfmass radii about 10% have a rising velocity dispersion profile and another 30% are approximately flat (Carlberg & Grillmair 2021). The current measurements are still somewhat noisy but will improve significantly with future Gaia releases.…”

“…There are a number of possible explanations for some or all of the excess velocity dispersion, including single-epoch observations of binary stars (Wan et al 2021), the complexity of orbits between the cluster and the tidal surface (Fukushige & Heggie 2000;Claydon et al 2017), particularly for highly elliptical orbits near the galactic center (Carlberg & Grillmair 2021), stripped galactic nuclei (Kuzma et al 2018;Wirth et al 2020), and dark matter halos (Mashchenko & Sills 2005a, 2005bPeñarrubia et al 2017;Boldrini et al 2020;Vitral & Boldrini 2021;Carlberg & Keating 2022). Dynamical measurements of potential progenitor systems can be used to test these ideas.…”

The Dark Matter Halo of M54

“…Most of the Milky Way halo globular clusters have no significant dark matter detected in their outskirts. However, ∼10% of the accessible halo clusters have been tentatively identified as being consistent with a local dark matter (Carlberg & Grillmair 2021). Simulations show that approximately 25-35% of clusters started in the centers of local halos are able to retain them as the galactic halo assembles (Carlberg & Keating 2021).…”

“…Star clusters started at the centers of dark matter sub-halos have about a 50% chance of retaining sufficient dark matter that the outer velocity profile was either flat or rising. Flat or rising velocity dispersion profiles also can develop around dense star clusters that have eccentric orbits taking them within 1-2 kpc of the galactic center, where they experience such strong tidal pumping that the stars in the outer region can have a rising velocity dispersion profile without any dark matter (Carlberg & Grillmair 2021). M54 has a galactic orbital pericenter estimated to be a relative distant 12.6 kpc (Baumgardt et al 2019).…”

“…For clusters measured to at least 3 half mass radii about 10% have a rising velocity dispersion and another 30% are approximately flat (Carlberg & Grillmair 2021), although these measurements are noisy and will improved with future Gaia releases. There are a number of possibilities to explain some or all of the excess velocity dispersion, including single epoch observations of binary stars (Wan et al 2021), the complexity of orbits between the cluster and the tidal surface (Fukushige & Heggie 2000;Claydon et al 2017), particularly for highly elliptical orbits near the galactic center (Carlberg & Grillmair 2021), stripped galactic nuclei (Kuzma et al 2018;Wirth et al 2020), and dark matter halos (Peñarrubia et al 2017;Boldrini et al 2020;Vitral & Boldrini 2021;Carlberg & Keating 2022).…”