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  Current Research Highlight :: February 2005 all highlights

Colliding galaxies light up dormant black holes

In the early universe, many galaxies exhibit extremely bright sources at their nuclei, so-called quasars. It is thought that the luminosity of the quasars is produced by supermassive black holes in the centres of galaxies. The masses of these black holes are tightly correlated with the velocity dispersion of the stars in the central bulge of their host galaxies, which suggests a common formation mechanism. Scientists at the Max-Planck Institute for Astrophysics and at Harvard University, USA, have now for the first time been able to directly follow star formation as well as the growth of black holes in simulations of colliding galaxies. They show that the quasar activity releases enough energy to expel large amounts of gas out of the galaxy center, which limits both the star formation as well as further growth of the black hole. This provides an explanation for the origin of the lifetime of quasars and the close connection between the mass of a supermassive black hole and the velocity dispersion of the stars in the center of a galaxy.

Different phases of the merger of two galaxies with central supermassive black holes. From top to bottom, the individual images of the sequence show the gas of two colliding spiral galaxies. After the first encounter, they first separate again from each other, but then come together for a second encounter and subsequent coalescence. Gravity is driving gas into the centers of the galaxies and leads to the formation of extended tidal arms. As a result of the nuclear inflow, the black holes grow strongly in mass during a quasar phase. This phase lasts up to 100 million years and releases enough energy to heat the gas and to expel it into extragalactic space. At the end an elliptical galaxy remains (its stars are not shown in the image sequence) which contains almost no residual gas and hosts at its center the merged pair of supermassive black holes.

Please click on the picture to start the movie. Time evolution of a galaxy merger over a period of 2 billion years. Only the gas distribution is shown, where brightness increases with gas density while the color hue encodes the gas temperature, from blue/cold to yellow/hot. The movie has high resolution (20 MB, 1024x768 pixels) and may require the divx/MPEG-4 codec for playback; it can be downloaded for free for Windows, Apple, and Linux platforms at, if needed.

Collisions and mergers of galaxies play an important role in the modern picture of galaxy formation. Over time, they lead to the build up of ever larger galaxies with modified morphology. For example, merging spiral galaxies are transformed into elliptical galaxies. However, the recent discovery of supermassive black holes at the centres of most galaxies poses new puzzles for astronomers. It is unclear why the mass of black holes is so well correlated with the size of the stellar bulge of galaxies. Are black holes only an interesting side phaenomenon of the formation of galaxies, or are they possibly influencing the process in a decisive way?

Answers to these questions can be provided by complex computer simulations which account for the gravitational dynamics of individual galaxies as well as for the most important aspects of the physics of star formation and the growth of black holes. Tiziana Di Matteo and Volker Springel from the Max-Planck Institute for Astrophysics, and Lars Hernquist from Harvard University, have devised new ways to study this problem numerically: They represented the supermassive black hole with a simulation particle which can accrete gas from its surroundings, with a rate estimated based on a simple theoretical model. In this way, they could simulate whole galaxies together with the growing black holes embedded at their centres.

Due to fricting in the gas flow around the black holes, the infalling gas is heated to enormous temperatures and emits energetic radiation. In fact, about 10 per cent of the total rest mass enery (E = mc2) of the gas is released before it vanishes in the event horizon of the black hole. These are huge amounts of energy, turning the black holes into real monsters, the strongest known sources of energy in the Universe. While most of the energy escapes from the center of the galaxy, a small part of it heats the gas in the larger environment of the black hole. The team around Dr. Di Matteo assumed that about 5 per cent of the radiation contributes to this heating.

The simulations have now shown that this energy has a crucial influence on mergers of spiral galaxies. When the galaxies collide, gravitational tidal forces drive diffuse gas into their centers. There it is compressed until an intense burst of star formation is triggered. At the same time, the infalling gas is also feeding a nuclear supermassive black hole which therefore quickly grows in mass. The energy released by this growth is strongly heating the surrounding gas. Figure 1 shows the distribution and temperature of the gas in different phases of the merger of two galaxies of size comparable to the Milky Way. The more massive the black hole becomes the faster it grows further, so that the rate of energy release quickly becomes larger. The center of the galaxy is shining as a luminous quasar in this phase. However, eventually the pressure in the heated gas becomes too large - a powerful wind is formed which expels the remaining gas in the galaxy out of the center, thereby terminating the quasar-phase and the starburst on a short timescale.

Tiziana Di Matteo, Volker Springel and Lars Hernquist have studied a series of simulations with collisions of galaxies of different size. They show that in larger galaxies more gas is available to feed a black hole and that the gravitational potential that binds this gas is deeper, so that the black holes have to grow to ever larger masses before their released energy is sufficient to stop the quasar activity by expelling the gas. The growth is therefore a self-limiting process which is at the same time linked to the formation of the stellar bulge in the center of galaxies: A direct connection between the size of this stellar population and the mass of the black hole in the merger remnant is established. A comparison with astronomical data shows that these first self-consistent simulations for the growth of black holes are already in remarkable agreement with the most important observational data.

These results have far reaching implications for the standard model of hierarchical galaxy formation: The acitivity of a black hole has large effects on its host galaxy because it can reduce the strength of the starburst during a merger of galaxies and also limits later star formation due to the heating of the gas. As a result, the formed elliptical galaxies are comparatively gas poor and show little residual star formation; their stellar populations are therefore aging quickly and develop those red spectral colors that are observed in many massive elliptical galaxies today. Without the influence of the black holes, the colors of these "dead" elliptical galaxies are difficult to understand.

Galaxy formation and the growth of supermassive black holes appear as a tightly linked process which should be treated as a unity in future theoretical models. Hydrodynamical numerical simulations are among of the most promising tools to gain further insights into this fascinating conncetion.

Tiziana Di Matteo, Volker Springel, Lars Hernquist

Original publication:

Tiziana Di Matteo, Volker Springel, Lars Hernquist:
Energy input from quasars regulates the growth and activity of black holes and their host galaxies, Nature, 10 February 2005

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