What survives in a black hole

Black hole survived collision of galaxies

Cambridge, USA - The only known medium-sized black hole to date is surrounded by a large cluster of young stars. This is shown by observations made by an international team of researchers with the Hubble space telescope. This is an indication that the black hole was originally located in the center of a small galaxy, according to the astronomers in the specialist journal “Astrophysical Journal Letters”. The dwarf galaxy was believed to have been torn apart and swallowed up by a larger star system, but the black hole survived this catastrophe.

Medium weight black hole

"For the first time we can observe the area around the medium-weight black hole," explains Mathieu Servillat from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. “And that gives us clues about its origin and formation.” Together with his colleagues, Servillat observed the black hole ESO 243-49 HLX-1, 290 million light-years away, which has a mass of 20,000 suns and is in the vicinity of a larger galaxy is located.

On the one hand, astronomers know about stellar black holes, the remains of exploded stars with several times the mass of the sun. And on the other hand, there are supermassive black holes in the centers of galaxies, with millions or even billions of times the mass of the sun. There is still no satisfactory explanation of how these giants came into being. One theory is that they emerged from the merging of medium-weight black holes with 10,000 to 100,000 times the mass of the sun. ESO 243-49 HLX-1 is the only known black hole in this mass range to date.

The observations by Servillat and his colleagues now show that this black hole is surrounded by around one million mostly young stars. The star cluster is about 250 light years in diameter and the researchers estimate the age of the stars to be 200 million years. Astronomically speaking, that is no more than a moment. According to the team, the only plausible explanation is that a dwarf galaxy was engulfed by the neighboring large star system 200 million years ago. The black hole in the core of this dwarf galaxy survived the catastrophe and left the large galaxy again. The stars, in turn, were formed from gas in the vicinity of the black hole when the two galaxies collided. The further fate of the black hole is still unclear. It may fall on a spiral path into the large galaxy and merge with the supermassive black hole at its center in several hundred million years.