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.Comment: Accepted for publication by The Astrophysical Journal (Letters).
Latex. 5 pages, 4 color figures, 2 with reduced resolution, one in greyscale.
A full-resolution color version of the paper can be retrieved from
http://www.bo.astro.it/SGR/Sgr_BH.pd
