In this paper, the author tests whether the Milky Way's Fermi bubbles could have been caused by an AGN-type explosion, or by a 'wind'. He comes to the conclusion that the wind may be "the same as active galactic nucleus outflows", and that it picks up interstellar gas on the way out of the GC. This paper also talks about SMBH winds. He rules out stellar wind from stars forming near the Galactic Centre.

What causes AGN outflows and these 'winds'? Is it purely mass and energy outflows from the accretion disk of the SMBH?


1 Answer 1


Like outflows from stellar feedback, AGN outflows can be driven by different mechanisms:

Momentum-driven winds

Momentum-driven winds are (mostly, for AGN, see below) caused by pure radiation pressure, and are hence also called radiation-driven. If the central SMBH accretes mass sufficiently fast, the luminosity $L$ of the released radiation will exceed the so-called Eddington limit $L_\mathrm{Edd}$, pushing gas away from the SMBH. This mechanism can also sometimes drive strong winds in the sub-Eddington regime (Proga & Kallman 2004).

For stellar outflows, momentum transfer by cosmic rays may dominate over photons (see e.g. Hopkins et al. (2021) and Huang & Davis (2022) for some recent papers on this). AGN also emit lost of cosmic rays, and are probably the main source of extragalactic cosmic rays (Berezhko 2008), but I don't think that (or I should probably say "don't know if") they can drive outflows from AGN.

Thermally-driven winds

Outflows can also be caused simply by X-rays heating the gas in the inner region, causing it to expand with the local sound velocity (Begelman et al. 1983; Woods et al. 1996). The buoyancy of the resulting bubbles of hot gas cause it to rise, possibly exceeding the escape velocity.

Which of these two processes dominate depends on various factors such as geometry, accretion rate, gas composition, and, in particular, the spectral shape of the AGN, i.e. how "hard" the radiation is.

Magnetic fields

In addition to the two main mechanisms above, there is a third that I don't really know anything about, but will mention anyway: Magnetic fields can be "frozen" in the disk due to the ionized gas. This can drive outflows by the centrifugal force, but I don't think much is known about how strong an effect this is. The mechanism is discussed in Blandford & Payne (1982) and Konigl & Kartje (1994).


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