Black-hole Spin and Galactic Morphology

We investigate the conjecture by Sikora, Stawarz & Lasota (2007) that the observed AGN–radioloudness bimodality can be explained by the morphology related bimodality of black-hole spin distribution in the centers of galaxies: central black holes in giant elliptical galaxies may have (on average) much larger spins than black holes in spiral/disc galaxies. We study how accretion from a warped disc influences the evolution of black hole spins and conclude that within the cosmological framework, where the most massive BHs have grown in mass via merger driven accretion, one indeed expects most supermassive black holes in elliptical galaxies to have on average higher spin than black holes in spiral galaxies, where random, small accretion episodes (e.g. tidally disrupted stars, accretion of molecular clouds) might have played a more important role. Subject headings: cosmology: theory – black holes – galaxies: evolution – quasars: general