Radio Loudness of Active Galactic Nuclei: Observational Facts and Theoretical Implications

Abstract

We investigate how the total radio luminosity of AGN-powered radio sources depends on their accretion luminosity and the central black hole mass. Our studies cover about 7 orders of magnitude in accretion luminosity (expressed in Eddington units, i.e., as Eddington ratios) and the full range of AGN black hole masses. We find that AGNs form two distinct and well-separated sequences on the radio-loudness-Eddington-ratio plane. The upper sequence is formed by radio-selected AGNs, and the lower sequence contains mainly optically selected objects. Whereas an apparent gap between the two sequences may be an artifact of selection effects, the sequences themselves mark the real upper bounds of radio loudness of two distinct populations of AGNs: those hosted respectively by elliptical and disk galaxies. Both sequences show the same dependence of the radio loudness on the Eddington ratio (an increase with decreasing Eddington ratio), which suggests that the normalization of this dependence is determined by the black hole spin. This implies that central black holes in giant elliptical galaxies have (on average) much larger spins than black holes in spiral/disk galaxies. This galaxy-morphology-related radio dichotomy breaks down at high accretion rates where the dominant fraction of luminous quasars hosted by elliptical galaxies is radio quiet. This led to speculations in the literature that formation of powerful jets at high accretion rates is intermittent and related to switches between two disk accretion modes, as directly observed in some black hole X-ray binaries. We argue that such intermittency can be reconciled with the spin paradigm, provided that successful formation of relativistic jets by rotating black holes requires collimation by MHD outflows from accretion disks.