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_bullet /M = 5.8-1.2+0.9 × 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 Msol, thus giving an IMBH mass in the range 1.3×104 < M_bullet < 2.3×104 Msol (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.
A mass estimate of an intermediate-mass black hole in omega Centauri
MIOCCHI, PAOLO
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_bullet /M = 5.8-1.2+0.9 × 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 Msol, thus giving an IMBH mass in the range 1.3×104 < M_bullet < 2.3×104 Msol (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.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.