A mass estimate of an intermediate-mass black hole in ω Centauri

INAF, Osservatorio Astronomico di Bologna, and Dipartimento di Astronomia, Universitá di Bologna, via Ranzani 1, Bologna 40127, Italy e-mail: paolo.miocchi@unibo.it

Received: 4 July 2009
Accepted: 8 February 2010

Abstract

Context. The problem of the existence of intermediate-mass black holes (IMBHs) at the centre of globular clusters is a hot and controversial topic in current astrophysical research with important implications in stellar and galaxy formation.

Aims. The purpose of this paper is to provide further evidence on the presence of an IMBH in ω Centauri and to give an independent estimate of its mass.

Methods. We employed a self-consistent spherical model with anisotropic velocity distribution. It consists in a generalisation of the King model by including the Bahcall-Wolf distribution function in the IMBH vicinity.

Results. By the parametric fitting of the model to recent HST/ACS data for the surface brightness profile, we found an IMBH to cluster total mass ratio of /M = × 10^-3. It is also found that the model yields a fit of the line-of-sight velocity dispersion profile that is better without mass segregation than in the segregated case. This confirms the current thought of a non-relaxed status for this peculiar cluster. The best fit model to the kinematic data leads, moreover, to a cluster total mass estimate of M = (3.1 ± 0.3) × 10⁶ , thus giving an IMBH mass in the range 1.3×10⁴ < < 2.3×10⁴ (at 1σ confidence level). A slight degree of radial velocity anisotropy in the outer region (r 12') is required to match the outer surface brightness profile.