SCYON Abstract

Received on February 19 2007

The presence of intermediate-mass black holes in globular clusters and its connection with extreme horizontal branch stars

AuthorsP. Miocchi
INAF - Osservatorio Astronomico di Teramo, via M. Maggini, I- 64100 Teramo (Italy)
Submitted toMonthly Notices of the Royal Astronomical Society


By means of a multimass isotropic and spherical model including self-consistently a central intermediate-mass black hole (IMBH), the influence of this object on the morphological and physical properties of globular clusters is investigated in this paper. Confirming recent numerical studies, it is found that a cluster (with mass M) hosting an IMBH (with mass MBH) shows, outside the black hole gravitational influence region, a core-like profile resembling a King profile with concentration c < 2, though with a slightly steeper behaviour in the core region. In particular, the core logarithmic slope is s < 0.25 for reasonably low IMBH masses (MBH < 10-2M), while c decreases monotonically with MBH. Completely power-law density profiles (similar to, e.g., that of collapsed clusters) are admitted only in the presence of a black hole with an unrealistic MBH > M. The mass range estimate 12s - 4.8 < log (MBH/M) < -1.1c - 0.69, depending on morphological parameters, is deduced considering a wide grid of models. Applying this estimate to a set of 39 globular clusters (including G1, in M31), it is found that NGC 2808, NGC 6388, M80, M13, M62 and M54 probably host an IMBH. For them, the scaling laws MBH ~ 0.02(M/M(sun))0.8 M(sun) and MBH ~ 100(σobs/km s-1)0.9 M(sun), are identified from weighted least-squares fit. An important result of this "collective" study is that a strong correlation exists between the presence of an extreme blue horizontal branch (HB) and the presence of an IMBH, at a statistically significant level of confidence (>90 percent). In particular, the presence of a central IMBH could explain why extreme HB stars are observed in M13 and NGC 6388, but not in M3 and 47 Tuc where this object is likely absent according to our analysis.