Dynamical evolution of intermediate mass black holes and their observable signatures in the nearby Universe

Abstract

We investigate the consequences of a model of the assembly and growth of massive black holes from primordial seeds, remnants of the first generation of stars in a hierarchical structure formation scenario. Our model traces the build-up of MBHs from an early epoch, and follows the merger history of dark matter halos and their associated holes via Monte Carlo realizations of the merger hierarchy from early times to the present time. The sequence of minor and major mergers experienced by galactic halos in their hierarchical growth affects the merger history of MBHs embedded in their nuclei. So, if the formation route for the assembly of SMBHs dates back to the early universe, a large number of BH interactions is inevitable. Binary black holes coalescence timescales can be long enough for a third BH to fall in and interact with the central binary. These BH triple interactions lead typically to the final expulsion of one of the three bodies and to the recoil of the binary. Also, asymmetric emission of gravitational waves in the last stages of the black hole merging can give a recoil velocity to the centre of mass of the coalescing binary. This scenario leads to the prediction of a population of intermediate mass BHs (IMBHs) wandering in galaxy halos at the present epoch. We compute the luminosity distribution produced by these IMBHs accreting from their circumstellar medium. We find that in a Milky Way-sized galaxy they are unable to account for sources with luminosities > 1e39 erg/s unless they carry a baryonic remnant from which they are able to accrete for a long time. We also find that, for typical spiral galaxies, the bright end of the point source distribution correlates with the mass of the galaxy, and the most luminous sources are expected to be found in the disk.