Powerful Outflows and Feedback from Active Galactic Nuclei

TL;DR

This work argues that active galactic nuclei influence their host galaxies predominantly through fast, highly ionized winds (UFOs) that inject energy and momentum into the bulge gas. A two-regime feedback model is developed: momentum-driven shocks near the black hole regulate growth and establish the M–σ relation, while once the black hole mass reaches Mσ, cooling becomes inefficient and energy-driven, galaxy-scale outflows clear the bulge gas and drive large-scale molecular outflows. Observational evidence from UFO detections (e.g., PG1211+143, NGC 4051) supports the ubiquity and energetics of these winds, while theoretical treatment links wind properties to the SMBH–bulge scaling relations and to disc-affected feedback morphologies. The study also evaluates alternative driving mechanisms (radiation pressure on electrons and dust) and discusses implications for cosmological simulations, star formation in discs, and the overall coevolution of black holes and galaxies, emphasizing that the strongest feedback is episodic and governed by cooling physics.

Abstract

Active Galactic Nuclei (AGN) represent the growth phases of the supermassive black holes in the center of almost every galaxy. Powerful, highly ionized winds, with velocities $\sim 0.1- 0.2c$ are a common feature in X--ray spectra of luminous AGN, offering a plausible physical origin for the well known connections between the hole and properties of its host. Observability constraints suggest that the winds must be episodic, and detectable only for a few percent of their lifetimes. The most powerful wind feedback, establishing the $M -σ$ relation, is probably not directly observable at all. The $M - σ$ relation signals a global change in the nature of AGN feedback. At black hole masses below $M-σ$ feedback is confined to the immediate vicinity of the hole. At the $M-σ$ mass it becomes much more energetic and widespread, and can drive away much of the bulge gas as a fast molecular outflow.

Figures (11)

• Figure 1: Ratio of EPIC pn data to a simple power law continuum for the 2001 XMM-Newton observation of PG1211+143 showing a deep absorption line near 7 keV and additional structure between $\sim$1 and 4 keV. Deriving an outflow velocity requires the correct identification of the individual absorption lines, which ideally requires spectral modelling with a photoionised absorber
• Figure 2: (top) A Gaussian fit to the $\sim$7 keV absorption feature finds a line energy of 7.06$\pm$0.02 keV with (1$\sigma$) width 100$\pm$30 eV. Identification with the Fe XXV 1s-2p resonance line (6.70 keV rest energy) gives an outflow velocity $v\sim 0.12\pm 0.01c$. (lower) Alternative modelling with a photoionised gas over the wider 1--10 keV spectral band yields a good fit with a relatively high column density $N_{H}$$\sim$3.2$\pm$0.7$\times 10^{23}$ cm$^{-2}$, moderate ionisation parameter $\log\xi=2.7\pm 0.1$ erg cm s$^{-1}$, and outflow velocity of v$\sim$0.15$\pm$0.01c. The Fe XXV absorption line profile is seen to include lower energy components due to the addition of one or more L-shell electrons, showing why the simple Gaussian fit gives too low a velocity
• Figure 3: The PCygni profile of Fe XXV from stacked XMM-Newton pn observations of PG1211+143 is characteristic of a wide angle outflow. The comparable equivalent width of emission and blue-shifted absorption components indicates the highly ionized outflow has a large covering factor. From Pounds and Reeves 2009
• Figure 4: Distribution of outflow velocities, ionization parameter (erg cm s$^{-1}$) and column density (cm$^{-2}$) obtained from modelling the individual spectra from extended observations of type 1 AGN in the XMM-Newton and Suzaku data archives (Tombesi et al. 2011, Gofford et al. 2013). The red lined histogram refers to lower limits in column density
• Figure 5: The outflow velocity and ionization parameters for 6 XSTAR photoionised absorbers used to fit the RGS and EPIC spectra of NGC 4051, together with a high point representative of the pre-shock wind, show the linear correlation expected for a mass-conserved cooling flow (see Pounds and King 2013)