Annu. Rev. Astron. Astrophys. 2015. 53:115-154 Copyright © 2015 by Annual Reviews. All rights reserved

8. SMALL vs LARGE SCALE FEEDBACK

We have shown that UFO winds are very common in AGN, despite quite restrictive conditions on their observability. They provide an obvious way for the central supermassive black hole to communicate its presence to its host. This in turn suggests ways of understanding both the SMBH – galaxy scaling relations, and the need to expel gas from the galaxy spheroid to terminate star formation. It is clear that this AGN feedback must operate at times on small scales, and at others on large scales. Our discussion of feedback points to a natural association between momentum–driving and small scales, and between energy–driving and large scales. Small–scale phenomena naturally explained by wind momentum–driving include

1. Super–solar elemental abundances in AGN spectra. Wind momentum–driving automatically sweeps up and compresses the same gas many times before the black hole mass reaches M_σ. Generations of massive stars forming out of the same swept–up gas can repeatedly enrich the gas close to the SMBH with nuclear–processed material before the M_σ mass is reached, and momentum–driving changes to energy–driving.
2. Dark matter cusp removal. The same repeated sweeping–up of a gas mass comparable to the SMBH mass, followed by fallback, has a strong tendency to weaken dark matter cusps. Because the baryonic mass involved is much larger, this is a more powerful version of the mechanism invoked by Pontzen & Governato (2012) (see also Garrison–Kimmel et al. 2013), who considered supernovae near the SMBH.
3. Quiescence of AGN hosts. Most AGN hosts do not show dramatically elevated star formation in the central regions of their galaxy discs, or so far much evidence for high–speed ( ∼ 1000 km s⁻¹ and massive ( ∼ few 100M_⊙ yr⁻¹) outflows on large scales. This is compatible with wind driving by momentum but not energy.

Large–scale phenomena suggesting the action of energy–driving include

4. Metals in the circumgalactic medium. These must be made in galaxies and only later expelled to make the CGM. This suggests that expulsion through energy–driving acts only after stellar evolution has had time to enrich a significant fraction of the galaxy bulge gas.
5. Mechanical luminosities of galaxy–scale molecular outflows. These are observed to be close to 5% of their central AGN luminosities L, just as expected for energy–driving, with momenta close to the predicted 20L / c.
6. Suppression of cosmological infall. Energy–driven outflows at large radii probably prevent galaxies accreting indefinitely (Costa et al. 2014).

This list seems to favor a combination of momentum and energy driving, with some kind of switch between them. The suppression of inverse Compton shock cooling at the point when the black hole mass reaches M_σ appears promising, but requires further work on how observable the cooling is, and the possibility of two–fluid effects. Radiation driving on dust could produce similar behavior, although the physics controlling the required switch between momentum and energy driving is so far unexplained.

It is worth stressing that even a detailed understanding of the dual role of AGN feedback in establishing both the SMBH – host scaling relations and the quenching of star formation would solve only half of the problem. For a full picture of how black holes and galaxies influence each other we need to know what physical mechanism can produce a supply of gas with so little angular momentum that much of it can accrete on to the central supermassive black hole within a few Salpeter times (see Section 3.1, and equation 20). We saw in Section 1.2 that the hole's gravity is far too weak to influence the galaxy on the mass scale needed for this. Only feedback can do this, perhaps suggesting that SMBH feedback may ultimately cause SMBH feeding (cf Dehnen & King 2013).

ACKNOWLEDGMENTS

We thank Walter Dehnen, Dean McLaughlin, Sergei Nayakshin, Chris Nixon, Chris Power, Jim Pringle, James Reeves, Simon Vaughan, Mark Wilkinson and Kastytis Zubovas for help, collaboration and advice on the subjects reviewed here. We have benefitted hugely from discussions with many people over the years, including Mitch Begelman, Martin Elvis, Andy Fabian, Claude–André Faucher-Giguère, Reinhard Genzel, Martin Haehnelt, Luis Ho, Knud Jahnke, Roberto Maiolino, David Merritt, Ramesh Narayan, Ken Ohsuga, Brad Peterson, Eliot Quataert, Martin Rees, Joop Schaye, Joe Silk, Francesco Tombesi, and Sylvain Veilleux.