Yes, galaxies do expel black holes!

A team of scientists from the Max Planck Institute for Extraterrestrial Physics (MPE) has observed a new black hole being expelled from its parent galaxy, following the merging of two black
holes, this startling observation, which confirms a key prediction in Einstein’s theory of relativity, is expected to improve our understanding of how galaxies form and evolve.

The researchers observed that the merging of two black holes triggered a gravitational radiation ripple outward through the galaxy at the speed of light. The waves moved in one direction, while
the black hole was pushed in the opposite direction. This resulted in the black hole breaking away from its location in the nucleus of the galaxy. A strong kick velocity ensured the black hole
fully escaping from the galaxy.

Using broad emission lines of gas around the black hole, the researchers were able to calculate that the recoiling black hole hit a high speed of 2,650 kilometres per second. The power of the
recoil, which had a mass of several 100 million solar masses, effectively forced the black hole to be hurled from its parent galaxy, the scientists explained. They calculated that the velocity
of this recoil is equal to someone being able to travel from New York to Los Angeles in just under two seconds.

The Max Planck team also found a narrow set of emission lines generated from gas that remained in the galaxy. The radiation from the recoiling black hole helped energise the gas. It is
estimated that this gas, known as ‘accretion disk gas’, will continue to ‘feed’ the recoiling black hole for millions of years. During the accretion process, the gas shines in X-ray
wavelengths. The researchers also found that the X-ray emission emerged around the black hole at a distance of 10 billion light years after the ROSAT satellite scanned the area.

Such extreme events, which are a key prediction in Einstein’s theory of relativity, have been the subject of much debate over the years. Up until now, scientists needed supercomputers to
simulate these extreme events. The observations made by the Max Planck team now show that these events really do occur. They in fact prove that black holes can merge, and that these mergers are
accompanied by gravitational ripples that can kick start other events. The observations also indicate that galaxies without black holes in their nuclei must exist, as well as black holes that
endlessly float in space between the galaxies.

The team will now look at uncovering whether galaxies and black holes formed and evolved together in the ‘early Universe’, or if a ‘population’ of galaxies had been denied their central black
holes. The team’s discovery will encourage astrophysicists to develop more detailed simulations of the gravitational ‘superkicks’ and asses their impact on the evolution of black holes and
galaxies.

For further information:
https://www.journals.uchicago.edu/toc/apj/2008/678/2
https://www.mpe.mpg.de/main.html

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