  We investigate the conjecture by Sikora, Stawarz & Lasota (2007) that the
observed AGN--radio-loudness 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.

Astrophysics