The evolution of massive black holes and their spins in their galactic hosts

Future space-based gravitational-wave detectors, such as the Laser Interferometer Space Antenna (LISA/SGO) or a similar European mission (eLISA/NGO), will measure the masses and spins of massive black holes up to very high redshift, and in principle discriminate among different models for their evolution. Because the masses and spins change as a result of both accretion from the interstellar medium and the black hole mergers that are expected to naturally occur in the hierarchical formation of galaxies, their evolution is inextricably entangled with that of their galactic hosts. On the one hand, the amount of gas present in galactic nuclei regulates the changes in the black hole masses and spins through accretion, and affects the mutual orientation of the spins before mergers by exerting gravitomagnetic torques on them. On the other hand, massive black holes play a central role in galaxy formation because of the feedback exerted by active galactic nuclei on the growth of structures. In this paper, we study the mass and spin evolution of massive black holes within a semi-analytical galaxy-formation model that follows the evolution of dark-matter haloes along merger trees, as well as that of the baryonic components (hot gas, stellar and gaseous bulges, and stellar and gaseous galactic discs). This allows us to study the mass and spin evolution in a self-consistent way, by taking into account the effect of the gas present in galactic nuclei both during the accretion phases and during mergers. Also, we present predictions, as a function of redshift, for the fraction of gas-rich black hole mergers – in which the spins prior to the merger are aligned due to the gravitomagnetic torques exerted by the circumbinary disc – as opposed to gas-poor mergers, in which the orientation of the spins before the merger is roughly isotropic. These predictions may be tested by LISA or similar spaced-based gravitational-wave detectors such as eLISA/NGO or SGO.