Accretion and Outflow in Active Galaxies

TL;DR

This paper reviews how accretion and outflows in active galactic nuclei influence black hole growth and galaxy evolution, highlighting the chaotic nature of accretion, wind properties, and the role of outflows in regulating black hole mass.

Contribution

It introduces a model linking small-scale chaotic accretion events, wind-driven outflows, and the regulation of supermassive black hole growth through the $M-\sigma$ relation.

Findings

AGN winds have velocities ~0.1c and produce observable resonance lines.

Shocked wind creates cooling regions observable in emission lines.

Black hole growth stalls at the $M-\sigma$ relation mass.

Abstract

I review accretion and outflow in active galactic nuclei. Accretion appears to occur in a series of very small--scale, chaotic events, whose gas flows have no correlation with the large--scale structure of the galaxy or with each other. The accreting gas has extremely low specific angular momentum and probably represents only a small fraction of the gas involved in a galaxy merger, which may be the underlying driver. Eddington accretion episodes in AGN must be common in order for the supermassive black holes to grow. I show that they produce winds with velocities $v \sim 0.1c$ and ionization parameters implying the presence of resonance lines of helium-- and hydrogenlike iron. The wind creates a strong cooling shock as it interacts with the interstellar medium of the host galaxy, and this cooling region may be observable in an inverse Compton continuum and lower--excitation emission lines associated with lower velocities. The shell of matter swept up by the shocked wind stalls unless the black hole mass has reached the value $M_{\sigma}$ implied by the $M - \sigma$ relation. Once this mass is reached, further black hole growth is prevented. If the shocked gas did not cool as asserted above, the resulting (`energy-driven') outflow would imply a far smaller SMBH mass than actually observed. Minor accretion events with small gas fractions can produce galaxy-wide outflows, including fossil outflows in galaxies where there is little current AGN activity.