Feeding the Milky Way Central Black Hole with Stellar Winds

Using novel computer simulations, scientists at the Max Planck Institute for Astrophysics have investigated the gas flows in the centre of the Milky Way. The models show that the black hole there, Sgr A*, swallows only a small fraction of the gas, and that the rate at which this happens is highly variable with time. Most of the gas, however, escapes away from the black hole. This explains partially the current low luminosity of Sgr A*, but the scientists' models also lead to the prediction that periods of much higher activity can be expected in the future.

Fig. 1: Movie showing the gas expelled from the stars, the "stellar winds", in a simulation of the Galactic Centre. The stars are shown as black dots in the movie, and the black hole (not shown) is in the centre. Part of the gas cools and form clumps (shown in yellow) and, in this simulation, a disc is formed by these clumps. However, most of the gas escapes from the inner region, while a small quantity is consumed by the black hole. [To play you may need to install the free DivX codecs.]

Fig. 2:"Accretion rate", that is the quantity of gas consumed by the black hole, as a function of time for the simulation shown in Fig. 1. When one of the cool clumps goes to the proximity of the black hole, it is eaten and a peak in the accretion rate is produced.

It is now believed that there is a super-massive black hole at the centre of every galaxy. Our own Galaxy, the Milky Way, is no exception. At its centre, 26,000 light years away from us, there is a very massive black hole called Sgr A*. It is 3 million times more massive than the Sun. Astronomers cannot see the black hole directly, but they have used the largest telescopes to measure the motion of stars close to it. The orbits of stars around Sgr A*, like the Moon orbit around the Earth, permit astronomers to calculate the mass of the central object, and in this case show beyond any doubt the presence of the giant monster.

Even though black holes cannot be directly seen (they are black), the gas that falls onto them usually emits a lot of radiation, because the matter is compressed by the strong gravitational force, making it very hot. This is how quasars are created, very bright objects in the centres of far away galaxies, which are powered by accreting black holes. But in Sgr A* almost nothing is seen. This is a puzzle, because there are many stars close to the black hole. The stars are at a safe distance from the black hole, but the gas they expel, the "stellar winds", should be falling onto the black hole and producing much more light. To understand the reason of this low luminosity, it is very important to know how much gas gets captured by the black hole. Moreover, Sgr A* is the closest massive black hole from us, and studying its dimness will help to understand black holes in general.

In the Galactic centre, there are dozens of stars, each producing gas that flows in all directions. The gas is attracted to the black hole by gravity, and moreover gas streams coming from different stars collide with each other. All this produces very complicated gas motions. One cannot study this complexity with analytical equations, so it is necessary to run sophisticated simulations in super-computers to get a clear picture of what is happening.

Using Gadget, a computational code developed at MPA, scientists at this institute have started to study this problem. They have found that the stellar winds indeed produce a complicated gas morphology in the inner region (see Fig. 1). Part of the gas forms cool clumps, and some of these clumps have a trajectory that takes them to the black hole. Each time this happens, maybe every 100 years, the amount of material that falls onto the black hole, the so-called "accretion rate" (see Fig. 2), increases, and the region could become very luminous. It is interesting to note that observations show that just 350 year ago Sgr A* was indeed much brighter. This higher luminosity was perhaps caused by the kind of variability found in this simulation, and we can expect similar activity in the future.

The new simulations also showed that only a fraction of the winds go to the inner region, close to the black hole. This can be understood as a consequence of the stellar motions. Since the stars are orbiting the black hole, they give already some important initial velocity to the stellar winds, making it easier for the gas to escape from the black hole. However, this effect is not enough to explain the very low luminosity of the gas around Sgr A*, and it seems that the complicated processes of radiative cooling and hydrodynamic shock waves in the close proximity of the black hole play an important role. With the results of the detailed simulations being ran at MPA, physicists develop a better understanding of the physics of this inner region and how the "feeding" of supermassive black holes works.

J. Cuadra, S. Nayakshin, V. Springel, T. Di Matteo

Further information

linkPfeil.gif Accretion of Stellar Winds in the Galactic Centre

Original publications

Accretion of cool stellar winds on Sgr A*: another puzzle of the Galactic Centre?, MNRAS 360 (2005) L55

Galactic Centre stellar winds and Sgr A* accretion, MNRAS, in press

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