Black Hole Spin and the Radio Loud/Quiet Dichotomy of Active Galactic Nuclei

Abstract

Radio loud active galactic nuclei (AGN) are on average 1000 times brighter in the radio band compared to radio quiet AGN. We investigate whether this radio loud/quiet dichotomy can be due to differences in the spin of the central black holes that power the radio-emitting jets. Using general relativistic magnetohydrodynamic simulations, we construct steady state axisymmetric numerical models for a wide range of black hole spins (dimensionless spin parameter 0.1 <= a <= 0.9999 and a variety of jet geometries. We assume that the total magnetic flux through the black hole horizon at radius r_H(a) is held constant. If the black hole is surrounded by a thin accretion disk, we find that the total black hole power output depends approximately quadratically on the angular frequency of the hole, P \propto \Omega_H^2 \propto (a/r_H)^2. We conclude that, in this scenario, differences in the black hole spin can produce power variations of only a few tens at most. However, if the disk is thick such that the jet subtends a narrow solid angle around the polar axis, then the power dependence becomes much steeper, P \propto \Omega_H^4 or even \propto \Omega_H^6. Power variations of 1000 are then possible for realistic black hole spin distributions. We derive an analytic solution that accurately reproduces the steeper scaling of jet power with \Omega_H, and we provide a numerical fitting formula that reproduces all our simulation results. We discuss other physical effects that might contribute to the observed radio loud/quiet dichotomy of AGN.