Radio-Quiet Versus Radio-Loud AGN: What Makes the Difference?

We investigate how the total radio luminosity and the jet production efficiency of active galactic nuclei (AGN) depend on the accretion power and the mass of a central black hole. We argue that another parameter in addition to the accretion rate must play a role in determining the observed radio properties of AGN, and that this parameter is related to the properties of their host galaxies. In particular, we propose that this is the spin of the supermassive black hole which determines the relative power of relativistic outflows (for a given accretion rate) in all active galaxies, and that the supermassive black holes can reach large spins only in early type galaxies but not (in a statistical sense) in spiral galaxies. In this context, we comment on the cosmological evolution of the spin parameter for supermassive black holes.