Density and Kinematic Cusps in M54 at the Heart of the Sagittarius Dwarf Galaxy: Evidence for A 104 M sun Black Hole?
Dalessandro, E.; Bellazzini, M.; Mackey, A. D.; Chapman, S. C.; Lanzoni, B.; Lewis, G. F.; Ibata, R.; Miocchi, P.; Irwin, M.; Varghese, A.; Ferraro, F.
France, Italy, United Kingdom, Australia
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-1, decreasing to 16.4 ± 0.4 km s-1 at 2farcs5 (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 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.