AGN cool feedback and analogy with X-ray binaries: from radiation pressure to cosmic ray driven outflows

TL;DR

This paper explores how cosmic rays and radiation pressure from active galactic nuclei drive cool galactic outflows, highlighting their dependence on accretion rates and flow modes, and drawing analogies with X-ray binary states.

Contribution

It demonstrates the complementary roles of cosmic rays and radiation pressure in AGN feedback and links outflow mechanisms to accretion flow modes and cosmic evolution.

Findings

CR-driven outflows dominate at low accretion rates and large radii.

Radiation pressure-driven outflows dominate at high accretion rates and small radii.

The transition between outflow mechanisms correlates with accretion flow mode changes.

Abstract

Cool outflows are now commonly observed in galaxies, but their physical origin and driving mechanism remain unclear. Active galactic nucleus (AGN) feedback can potentially accelerate cool galactic outflows via cosmic rays (CR) and radiation pressure on dust. Here we investigate the relative importance of CR and radiation feedback in AGNs, and we analyse the physical conditions for outflow launching as a function of the black hole accretion flow mode. We assume CRs from AGN jet origin and consider the analogy with Galactic X-ray binaries, whereby the jet is prominent at low accretion rates (hard state) and quenched at high accretion rates (soft state). We show that CR-driven outflows can be powered at low accretion rates and at large radii, whereas radiation pressure-driven outflows dominate at high accretion rates and small radii. Thus the two AGN feedback mechanisms -- CRs and radiation pressure on dust -- may play complementary roles in driving cool outflows on galactic scales. The transition from radiation pressure-driven outflows at higher accretion rates to CR-driven outflows at lower accretion rates likely corresponds to a transition in the underlying accretion flow modes (from a radiatively efficient accretion disc to a radiatively inefficient jet-dominated flow) over cosmic time.