The physics of galactic winds driven by active galactic nuclei

By C. Faucher-Giguére, E. Quataert. 2012

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Active galactic nuclei (AGN) drive fast winds in the interstellar medium of their host galaxies. It is commonly assumed that the high ambient densities and intense radiation fields in galactic nuclei imply short cooling times, thus making the outflows momentum conserving. We show that cooling of high-velocity shocked winds in AGN is in fact inefficient in a wide range of circumstances, including conditions relevant to ultraluminous infrared galaxies (ULIRGs), resulting in energy-conserving outflows. We further show that fast energy-conserving outflows can tolerate a large amount of mixing with cooler gas before radiative losses become important. For winds with initial velocity vin ≳ 10 000 km s−1, as observed in ultraviolet and X-ray absorption, the shocked wind develops a two-temperature structure. While most of the thermal pressure support is provided by the protons, the cooling processes operate directly only on the electrons. This significantly slows down inverse Compton cooling, while free–free cooling is negligible. Slower winds with vin ∼ 1000 km s−1, such as may be driven by radiation pressure on dust, can also experience energy-conserving phases but under more restrictive conditions. During the energy-conserving phase, the momentum flux of an outflow is boosted by a factor ∼vin/2vs by work done by the hot post-shock gas, where vs is the velocity of the swept-up material. Energy-conserving outflows driven by fast AGN winds (vin ∼ 0.1c) may therefore explain the momentum fluxes of galaxy-scale outflows recently measured in luminous quasars and ULIRGs. Shocked wind bubbles expanding normal to galactic discs may also explain the large-scale bipolar structures observed in some systems, including around the Galactic Centre, and can produce significant radio, X-ray and γ-ray emission. The analytic solutions presented here will inform implementations of AGN feedback in numerical simulations, which typically do not include all the important physics.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/c9d8ab88472d13a7403bb6e8af39127305306b68

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Citations in the corpus

Quasar feedback and the origin of radio emission in radio-quiet quasars (2014, background): “…active galaxies, where the radiativelydriven wind would get quenched by the interaction with the interstellar medium, but could be typical of quasardriven winds (Moe et al. 2009; Zubovas & King 2012; Faucher-Giguère & Quataert 2012) even on galaxy-wide scales (Arav et al. 2013; Liu et al. 2013b).”
Cold Molecular Outflows in the Local Universe and Their Feedback Effect on Galaxies (2018, background): “Radiation pressure-driven outflows can reach (voutfṀoutf )/(LAGN/c) ∼ 5:1.”
Gravitational torque-driven black hole growth and feedback in cosmological simulations (2016, background): “The appropriate momentum and energy loading of winds depend on the physical scale (e.g. Faucher-Giguère & Quataert 2012).”
Massive molecular outflows and evidence for AGN feedback from CO observations (2013, background): “…nature of these outflows and the AGN feedback models that have been proposed so far, in which a fast and highly ionised wind, arising from the nuclear regions of the AGN, creates a shock wave that propagates into the ISM of the galaxy (Zubovas & King 2012; Faucher-Giguère & Quataert 2012).”
Kiloparsec-scale outflows are prevalent among luminous AGN: outflows and feedback in the context of the overall AGN population (2014, background): “However, it has been predicted that for high-accretion rate AGN, an accretion-disc wind could sweep up material and become a galaxy-wide energy-driven outflow (e.g., King et al. 2011; Faucher-Giguère & Quataert 2012; Debuhr et al. 2012; Zubovas & King 2012).”
FAST MOLECULAR OUTFLOWS IN LUMINOUS GALAXY MERGERS: EVIDENCE FOR QUASAR FEEDBACK FROM HERSCHEL (2013, background): “In-situ formation of the accelerated clouds (Faucher-Giguère, Quataert, & Murray 2012) and/or energy-conserving acceleration due to inefficient cooling in the shocked winds of AGN (Faucher-Giguère & Quataert 2012) may also effectively reduce the value of LM .”
Very extended cold gas, star formation and outflows in the halo of a bright QSO at z>6 (2014, background): “…fast winds originating from the accretion disk (e.g. King 2003, 2005, 2010; Lapi et al. 2005; Menci et al. 2006, 2008; Zubovas & King 2012, 2014; Faucher-Giguère & Quataert 2012; Costa et al. 2014b and references therein), and the models invoking radiation pressure on dusty clouds as the main…”
Observations of feedback from radio-quiet quasars - I. Extents and morphologies of ionized gas nebulae (2013, background): “The radiation pressure of the quasar (Murray et al. 1995; Proga et al. 2000) drives a low-density high-temperature wind (Faucher-Giguère & Quataert 2012; Zubovas & King 2012) which slams into dense clumps in the host galaxy, resulting in shocked emission.”
AGN wind scaling relations and the co-evolution of black holes and galaxies (2017, background): “Models implying shocks expanding into an isothermal sphere (density ∝R−2) predict ṀOF ∝ L1/3bol (Faucher-Giguere & Quataert 2012; King & Pounds 2015).”
Selection bias in dynamically measured supermassive black hole samples: its consequences and the quest for the most fundamental relation (2016, background): “…weaker trend:Mbh ∝ σ4, though the exact normalization and slopes predicted by AGN-feedback models as a function of time, mass, and host morphology is still a matter of intense debate (e.g., Fabian 1999; King 2003; Fabian 2012; Faucher-Giguère & Quataert 2012; Gabor & Bournaud 2014; King 2014).”
Physical Models of Galaxy Formation in a Cosmological Framework (2014, background): “Faucher-Giguère & Quataert (2012) argued recently based on analytic calculations that AGN-driven outflows are likely to be largely energy-conserving in many situations relevant to observed systems, particularly for “fast” (vw ∼ 10, 000-30,000 km/s) winds.”
Stellar and Quasar Feedback in Concert: Effects on AGN Accretion, Obscuration, and Outflows (2015, methodology): “Also, many observations of galaxy-scale, AGN-driven winds suggest large momentum-loading in the winds, with ṗ up to ∼ 10L/c (see Sturm et al. 2011; Faucher-Giguere & Quataert 2012; Cicone et al. 2014).”
Unification of X-ray winds in Seyfert galaxies: from ultra-fast outflows to warm absorbers (2012, background): “…all of them rely on a two step process in which an initial ∼sub-pc scale fast (vout > 1, 000 km s −1) AGN accretion disc outflow perturbs/shocks the interstellar medium, sweeping it up on its way and then decelerates/cools (e.g. King 2010a; Zubovas & King 2012; Faucher-Giguère & Quataert 2012).”
Observations of feedback from radio-quiet quasars – II. Kinematics of ionized gas nebulae (2013, background): “…but when its size reaches the typical scaleheight of the galactic disc, the directions perpendicular to the disc become much less obstructed by the interstellar medium, and the wind breaks out in these directions (Faucher-Giguère & Quataert 2012) producing the ‘bubbles’ discussed in Section 5.”