A new twist on black holes: the role of black hole spin in galaxy formation.

Abstract

We study the coevolution of black holes (BHs) and their host galaxies through cosmic time. The calculation is embedded in the GALFORM semi-analytic model which simulates the formation and evolution of galaxies in a cold dark matter (CDM) universe. The BH and galaxy formation models are coupled: during the evolution of the host galaxy, hot and cold gas are added to the BH by flows triggered by halo gas cooling, disc instabilities and galaxy mergers. This builds up the mass and spin of the BH, and the resulting accretion power regulates gas cooling and subsequent star formation. Using the Blandford–Znajek mechanism for jet production to calculate the jet power, our model reproduces the radio loudness of radio galaxies, LINERS and Seyferts, suggesting that the jet properties of active galactic nuclei (AGN) are a natural consequence of both the accretion rate onto and the spin of the central BH. The model also reproduces the observed luminosity functions (LF) of AGN (optical, soft and hard X-ray, and bolometric) for a wide range of redshifts (0 < z < 6). We find downsizing in the AGN population, in terms of the differential growth with redshift of the space density of faint and bright AGN. This arises naturally from the interplay between the different accretion channels that drive the growth of BHs. The predictions of our model are extended to 6 < z < 20 to study the early growth of BHs. Our model predicts that the first 10^ M⊙ BHs appear at z = 14, along with the first luminous quasars. Finally, we explore the dependence of AGN activity and luminosity on environment. We find that quasars inhabit haloes with masses 10^ − 10^ M⊙. Quasar activity in more massive haloes in suppressed due to AGN feedback. In contrast, radio galaxies occupy the centres of the most massive haloes. Our model represents the first consistent demonstration that the phenomenology and evolution of AGN can be naturally explained by the coeval evolution of galaxies and BHs, coupled by AGN feedback, in a CDM universe.