Density and kinematic cusps in M54 at the heart of the Sagittarius dwarf galaxy: evidence for a 10^4 M_sun Black Hole?

TL;DR

This study detects a stellar density cusp and increased velocity dispersion in M54's center, suggesting a possible ~9400 solar mass black hole or alternative anisotropic stellar motions, shedding light on the galaxy's core dynamics.

Contribution

It provides evidence for a potential intermediate-mass black hole in M54 and explores alternative explanations involving stellar anisotropy.

Findings

Velocity dispersion peaks at 20.2 km/s in M54's center.

Black hole mass estimated at approximately 9400 solar masses.

Alternative explanation involves moderate radial anisotropy of cusp stars.

Abstract

We report the detection of a stellar density cusp and a velocity dispersion increase in the center of the globular cluster M54, located at the center of the Sagittarius dwarf galaxy (Sgr). The central line of sight velocity dispersion is 20.2 +/- 0.7 km/s, decreasing to 16.4 +/- 0.4 km/s at 2.5" (0.3 pc). Modeling the kinematics and surface density profiles as the sum of a King model and a point-mass yields a black hole (BH) mass of ~ 9400 M_sun. However, the observations can alternatively be explained if the cusp stars possess moderate radial anisotropy. A Jeans analysis of the Sgr nucleus reveals a strong tangential anisotropy, probably a relic from the formation of the system.