Supermassive Black Holes Power Galactic Outflows
Alexei Kritsuk, Tomasz Plewa & Ewald Müller

Researchers at the Max-Planck-Institut für Astrophysik have recently simulated large scale outflows from giant elliptical galaxies. Alexei Kritsuk (a visitor from the University of St. Petersburg, Russia), Tomasz Plewa (a visitor from the Nicolaus Copernicus Astronomical Center, Warsaw, Poland) and Ewald Müller have assumed that in the central regions of the galaxy interstellar gas is heated up beyond its equilibrium temperature of about 10 million degrees. The heating of the gas, for example, could be caused by the activity of the central supermassive black hole, which is supposed to exist in the very center of most galaxies and to power the observed extragalactic jets, or by stellar winds from bright massive stars, or/and by supernova explosions. Independent of the exact nature of the heat source a giant bubble of gas forms which has a size of several thousand light years and which is somewhat hotter and less dense than the surrounding gas. As it is lighter than its environment the bubble is buoyant and starts to rise in the gravitational field of the galaxy.
Buoyant structures in astrophysical (radio map of galaxy M87) and in terrestrial (nuclear test) environments
In the simulations the rising bubble triggers a large scale outflow which becomes convective and which eventually extends out to distances of a few hundreds of thousands of light years. Such a large scale outflow may recently have been observed in the giant elliptical galaxy M87 in the center of the Virgo galaxy cluster. A radio map of M87 (see figure) suggests an outflow from the nucleus of M87 which appears to inflate two radio ``bubbles'' stretching about 100 000 light years away from the center of M87. Although of very different size the mushroom-shaped clouds observed in nuclear tests in the Earth atmosphere (see figure) are caused by the same physical phenomenon, namely by rising buoyant hot gas in the gravitational field of the Earth.
Simulation of the development of the convectively unstable outflow of buoyant hot interstellar gas from the center of an elliptical galaxy
The simulations of Alexei Kritsuk, Tomasz Plewa and Ewald Müller exploit a numerical code for adaptively refining the computational grid which was recently developed by Tomasz Plewa and Ewald Müller at the Max-Planck-Institut für Astrophysik. The new program can efficiently handle the vastly different length scales encountered in simulations of galactic flows ranging from a few hundred light years up to a million light years. The simulations cover a period of time of up to 300 million years. The flow pattern is characterized during the early evolution (see figure) by ascending and descending mushroom-like structures.

An MPEG movie illustrating the onset of the outflow in case of excessive central heating in the innermost 3000 light years of the model galaxy up to 45 million years is shown here . (0.6 MByte)

An MPEG movie of the evolution of the outflow in case of excessive central heating up to 100 million years covering a much larger region of about 10 000 light years can be found here . (1.0 MByte)