An elusive dark central mass in the globular cluster M4

Abstract

Recent studies of nearby globular clusters have discovered excess dark mass in their cores, apparently in an extended distribution, and simulations indicate that this mass is composed mostly of white dwarfs (respectively stellar-mass black holes) in clusters that are core collapsed (respectively with a flatter core). We perform mass-anisotropy modelling of the closest globular cluster, M4, with intermediate slope for the inner stellar density. We use proper motion data from Gaia Early Data Release 3 (EDR3) and from observations by the Hubble Space Telescope. We extract the mass profile employing Bayesian Jeans modelling, and check our fits with realistic mock data. Our analyses return isotropic motions in the cluster core and tangential motions (β ≈ −0.4 ± 0.1) in the outskirts. We also robustly measure a dark central mass of roughly $800\pm 300 \, \rm M_\odot$ , but it is not possible to distinguish between a point-like source, such as an intermediate-mass black hole (IMBH), and a dark population of stellar remnants of extent ${\approx} 0.016\, {\rm pc} \simeq 3300\, {\rm au}$ . However, when removing a high-velocity star from the cluster centre, the same mass excess is found, but more extended (${\sim} 0.034\, {\rm pc} \approx 7000\, {\rm au}$ ). We use Monte Carlo N-body models of M4 to interpret the second outcome, and find that our excess mass is not sufficiently extended to be confidently associated with a dark population of remnants. Finally, we discuss the feasibility of these two scenarios (i.e. IMBH versus remnants), and propose new observations that could help to better grasp the complex dynamics in M4’s core.

HSTPROMO Internal Proper-motion Kinematics of Dwarf Spheroidal Galaxies. I. Velocity Anisotropy and Dark Matter Cusp Slope of Draco

“…Having constructed the PM catalog, we proceed to construct velocity dispersion profiles that will be used throughout the next sections. As in Section 2.2.3, all our computations of the dispersion σ of a given random variable follow the recipe presented in van der Marel & Anderson (2010), also recently employed in Vitral et al (2023a). This consists of a maximum likelihood fit of a Gaussian distribution to the data, aiming to recover the respective standard deviation of the fit.…”

Fueling Processes on (Sub-)kpc Scales

“…An promising way to detect IMBH is to search for AGN signatures, and candidates have been observed in several low-mass galaxies [41]. Candidates for IMBH exist in several dwarf active galaxy nuclei (e.g., [42]), however, in the core of globular clusters, where they were expected, only a collection of stellar-mass black holes have been detected instead [43,44].…”

Discovery of Astrometric Accelerations by Dark Companions in the Globular Cluster ω Centauri

“…As of this writing, no accelerating source in M 4 has been reported. The combined analysis of proper motions from HST and Gaia DR3 claim a central dark mass of 800 M ☉ in M 4 (Vitral et al 2023). , 12714, 13100, 13570, 14031, 14393, 15000, 15593, 15733, 16413, and 16588 (all by PI: V. Kozhurina-Platais) are addressing a variety of specific aspects of the WFC3 instrumental calibration, such as photometric stability, geometric distortions, and stability (for the latter aspect, see also Sahlmann et al 2019).…”