Radio-loudness of Active Galaxies and the Black Hole Evolution

TL;DR

This paper explores the relationship between radio-loudness, galaxy morphology, and black hole spin in active galactic nuclei, proposing that black hole spin influences jet production efficiency and varies with galaxy type.

Contribution

It introduces a dual-sequence framework on the radio-loudness versus Eddington ratio plane, linking galaxy morphology, black hole spin, and jet activity, and discusses evolutionary scenarios for spin dichotomy.

Findings

Two distinct AGN sequences on the radio-loudness--Eddington ratio plane.

Radio-loudness correlates with galaxy morphology and black hole spin.

Jet production is suppressed at high accretion rates in luminous quasars.

Abstract

Active galactic nuclei (AGNs) form two distinct sequences on the radio-loudness -- Eddington-ratio plane. The `upper' sequence contains radio selected AGNs, the `lower' sequence is composed mainly of optically selected AGNs. The sequences mark the upper bounds for the radio-loudness of two distinct populations of AGNs, 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 another parameter in addition to the accretion rate must play a role in determining the efficiency of jet production in AGNs. We speculate that this additional parameter is the spin of the black hole, assuming that black holes in giant elliptical galaxies have (on average) much larger spins than black holes in disc galaxies. Possible evolutionary scenarios leading to such a spin dichotomy are discussed. The galaxy-morphology related radio-dichotomy breaks down at high accretion rates where the dominant fraction of luminous quasars being hosted by giant ellipticals is radio quiet. This indicates that the production of powerful jets at high accretion rates is in most cases suppressed and, in analogy to X-ray binary systems (XRB) during high and very high states, may be intermittent. Such intermittency can be caused by switches between two different accretion modes, assuming that only during one of them an outflow from the central engine is sufficiently collimated to form a relativistic jet.