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 seven 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, 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 radioloudness 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/disc 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 BH 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.

The power of relativistic jets is larger than the luminosity of their accretion disks

“…It will be interesting to explore less luminous jetted sources, to get hints on the possible dependencies of the jet power on the black hole spin and the possible existence of a minimum spin value for the very existence of the jet. In turn, this should shed light on the long standing problem of the radio-loud/radio-quiet quasar dichotomy 30 . Figure 1: Radiative jet power vs disk luminosity.…”

Physics of Accretion Flows around Compact Objects

“…This conclusion was extended to AGN and quasars whose luminosities were supposed to follow the same type of single relation. However, as found recently by Sikora, Stawarz & Lasota [59] in the case of AGN there are two such relations (Fig. 3).…”

“…However, the condition for launching a Poynting-flux dominated outflow, which later becomes converted to the matter dominated relativistic jet, is to supply a high magnetic-to-rest-mass energy ratio (≫ 1) at the base of the outflow. This is obviously satisfied in the case of the magnetic field anchored on a (rotating) neutron star (see [59] for more details).…”

“…do not eject powerful jets). It is not clear if the distribution of radio-loudness is bi-modal, as had been believed for a long time (see [59] and references therein), but there are certainly quasars with very similar optical properties whose radio luminosity differs by several orders of magnitude (see Fig. 3).…”

“…[58]). Accreting compact objects produce relativistic jets, [59] i.e. well-collimated outflows with bulk motion corresponding to Γ = 1/1 − (v bulk /c) 2 > 1.…”