Formation of Massive Black Holes in Globular Clusters

Abstract

The hydrodynamic formation of massive black holes (BHs) in globular clusters is considered. In particular, we examine the possibility of the BH formation induced by the radiation drag that is exerted on the interstellar matter by stellar radiation in globular clusters. The radiation drag extracts angular momentum from interstellar gas and thus allows the gas to accrete on to the cluster center.By incorporating the realistic chemical evolution of globular clusters, we scrutinize the efficiency of the radiation drag to assess the total accreted mass. As a result, we find that if a globular cluster is more massive than $\approx 6\times 10^{6}M_{\odot}$ in the present-day stellar component, a massive BH with $>260M_{\odot}$ can form within it. But, the BH-to-cluster mass ratio s considerably smaller than the BH-to-bulge mass ratio ($\approx 10^{-3}$) found in galactic bulges. The results are not sensitive to the assumed stellar initial mass function and star formation rate in the cluster, as long as the resultant color-magnitude relation, metallicity, and mass-to-luminosity ratio satisfy those observed in globular clusters. Hence, the putative linear relation between BH mass and bulge mass($M_{\rm BH}-M_{\rm bulge}$ relation) cannot be extrapolated to globular cluster systems. In the present regime, we discuss the BH formation in M15, G1, $\omega$ Cen, the M33 nucleus, and the compact X-ray sources in M82. Finally, we argue observational indications of the formation process of massive BHs in globular clusters. We find that the final phase of BH growth due to the radiation drag can be observed as ultraluminous X-ray sources (ULXs) with $\sim 10^{41}{\rm erg} {\rm s}^{-1}$.