How black holes power galactic super-winds

August 01, 2020

When interstellar gas falls towards a supermassive black hole, it liberates vast amounts of energy - so vast as to be capable of ejecting much of a galaxy’s gaseous reservoir. Ultimately, supermassive black holes may thus deprive themselves of further fuel and bring about the end of their own growth and that of their host galaxies. A new model developed at MPA now makes it possible to simulate winds accelerated by accreting black holes in galaxy evolution simulations in a physically accurate and validated way. By blowing dense gas from the galactic nucleus, and by halting inward flows from the galactic halo, the winds play a vital role in shaping the evolution of the black hole host galaxy.

By arranging AREPO cells in rigid, spherical layers, it is possible to represent accreting black holes as spherical boundaries (central panel). The boundary surfaces are then used to inject a wind with properties in line with those of observed small-scale outflows. When applied to homogeneous media, black hole winds power complex, spherical outflows (left-hand panel). One of the crucial tests to the new model was to verify that the properties of the large-scale, spherical outflows closely agree with theoretical expectations.

Every massive galaxy is thought to harbour a supermassive black hole in its nucleus. The more massive the galaxy, the greater the mass of the supermassive black hole. Thus, while the Milky Way hosts a black hole with a mass of about four million solar masses, giant galaxies such as M87 contain black holes with masses exceeding a billion times the mass of the Sun.

Supermassive black holes grow by accreting gas from their vicinity. As interstellar gas clouds spiral inwards under the black hole’s gravitational pull, they accelerate to prodigious speeds. If their kinetic energy is dissipated via friction, it can be liberated in the form of intense radiation or emanate from the galactic nucleus in the form of a wind.

Over the lifetime of a typical supermassive black hole, the total energy liberated via accretion exceeds the binding energy of its host galaxy by a factor of about 100. It takes only 1% of the liberated energy to expel the bulk of the galaxy’s gaseous reservoir. Without a source of gas, the galaxy is unable either to form stars or to feed its central black hole. Ultimately, supermassive black holes may bring thus about the end of their own growth together with that of their host galaxies.

A small team of scientists based at the Max-Planck-Institute for Astrophysics (Garching, Germany) led by Tiago Costa has created a new method to accurately model winds launched from the vicinity of accreting supermassive black holes in realistic simulations of galaxy evolution. The new model exploits the irregular geometry of the mesh, on which flows of interstellar gas are represented in the state-of-the-art code AREPO developed by Volker Springel. Unlike in traditional hydrodynamic codes, where fluids are discretised onto Cartesian grids, AREPO solves the hydrodynamic equations on an irregular mesh (Fig. 1) that is free to move with the gas flow. By arranging AREPO cells in rigid, spherical layers, it is possible to represent accreting black holes as spherical boundaries. The boundary surfaces are then used to inject a wind with properties in line with those of observed small-scale outflows.

After carefully testing the model (see figure), the research team went on to probe the impact of winds driven by supermassive black holes on the evolution of a massive disc galaxy such as the Milky Way. They performed a number of simulations of galactic discs containing, at the centre, a spherical boundary to represent an accreting, supermassive black hole.

Simulation of winds accelerated by accreting black holes

Density and temperature of gas around the accreting black hole, modelled with a spherical boundary. The black hole, located in the nucleus of the edge-on disc, liberates a spherical, small-scale wind. As it collides with the gaseous environment, the wind powers complex, bi-conical outflows that remove gas from the galactic nucleus but also from the extended gaseous halo that continuously feeds the central galaxy.

While the small-scale winds powered by the black hole are initially spherical, they are influenced by the geometry of the gaseous environment, having an easier time propagating towards the poles than along the disc. As a result, the winds power large-scale, conical outflows that remove gas from the galactic nucleus as well as from the extended gaseous halo surrounding the disc (see video).

The winds impact the host galaxy through multiple channels. On the one hand, they collide against gas in the galactic nucleus, ejecting it from the galaxy. By clearing out gas in the halo, on the other hand, the winds also prevent new material from replenishing the disc galaxy, slowly starving it off star formation fuel.

The new model for physical, black hole winds makes it possible to shed new light on the multiple mechanisms whereby supermassive black holes impact galaxy evolution. It also permits a completely new treatment for black hole accretion, which can be computed by measuring inflow rates across the boundary, rather than through assumptions about what this rate should look like. Applying the model to large-scale cosmological simulations will prove vital to test whether supermassive black holes can after all end star formation in the most massive galaxies.

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