Black hole given a 'superkick' from its parent galaxy

Berlin, April 30 : Astronomers have observed a black hole being expelled from its parent galaxy, which is the first recorded instance of this extreme ejection event, that had been predicted by theorists earlier.

The team that detected this event was led by Stefanie Komossa from the Max Planck Institute for extraterrestrial Physics (MPE).

When two black holes merge, waves of gravitational radiation ripple outward through the galaxy at the speed of light. Because the waves are emitted mainly in one direction, the black hole itself is pushed in the opposite direction, much like the recoil that accompanies the firing of a rifle or the launching of a rocket.

The black hole is booted from its normal location in the nucleus of the galaxy. If the kick velocity is high enough, the black hole can escape the galaxy completely.

Though these extreme events had only been simulated in supercomputers till now, the MPE team's discovery verifies, for the first time, that they actually occur.

The recoiling black hole caught the astrophysicists' attention by its high speed - 2650 km/s - which was measured via the broad emission lines of gas around the black hole.

Because of the tremendous power of the recoil the black hole, which has a mass of several 100 millions solar masses, was catapulted from the core of its parent galaxy.

In addition to the emission lines from gas bound to the recoiling black hole, the astronomers were also struck by a remarkably narrow set of emission lines originating from gas left behind in the galaxy.

This gas has been excited by radiation from the recoiling black hole.

Gas that moves with the black hole - the so-called accretion disk gas - continues to "feed" the recoiling black hole for millions of years. In the process of being accreted, this gas shines in X-rays.

In fact, the team around Komossa also detected this X-ray emission from the disk around the black hole at a distance of 10 billion light years.

The new discovery is important because it indirectly proves that black holes do in fact merge and that the mergers are sometimes accompanied by large kicks.

Though this process had been postulated by theory, it has never before confirmed via direct observation.

Another implication of the discovery is that there must be galaxies without black holes in their nuclei - as well as black holes which float forever in space between the galaxies.

The discovery of the MPE team will provide new impetus for theorists to develop more detailed models of the superkicks and their consequences for the evolution of black holes and galaxies.

ANI