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

We investigate the consequences of a model of the assembly and growth of massive black holes (MBHs) 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 haloes and their associated holes via Monte Carlo realizations of the merger hierarchy, from early times to the present time, in a Lambda cold dark matter cosmology. The sequence of minor and major mergers experienced by galactic haloes in their hierarchical growth affects the merger history of MBHs embedded in their nuclei. So, if the formation route for the assembly of supermassive black holes dates back to the early universe, a large number of black hole (BH) interactions is inevitable. The coalescence time-scales of binary black holes 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 BH 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 black holes (IMBHs) wandering in galaxy haloes 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 greater than or similar to 10(39) erg s(-1), 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 disc.