Black holes are remnants of stars

Unsuccessful search for black holes with several thousand solar masses in the heart of a globular cluster

Globular clusters

Pasadena (USA) - Astronomers are familiar with two different types of black holes: the supermassive giants in the centers of galaxies and stellar black holes, the remains of burned-out stars. So far, it is controversial whether there are also medium-weight black holes with a mass several thousand times the mass of the sun. But the search for such a middleweight in a globular cluster 50 million light years away has so far been unsuccessful, reports an American-British research team in the "Astrophysical Journal".

"Some theories predict that small black holes fall into the center of globular clusters and merge there to form a medium-sized black hole," explains Daniel Stern from NASA's Jet Propulsion Laboratory in Pasadena, California. That is why he and his colleagues used several instruments to take a closer look at the globular cluster RZ2109, which belongs to a galaxy 50 million light years away.

During their observations with the European X-ray satellite XMM-Newton, the researchers actually encountered the characteristic X-rays of a black hole in the heart of RZ2109. This radiation arises when matter from the environment falls into the black hole. However, the observations did not yet reveal how big this black hole is. To do this, Stern and his colleagues had to carry out further measurements with the ten-meter Keck telescope in Hawaii. This showed that the black hole is rather modest in size: it weighs only about ten times the mass of the sun.

"If there were a medium-weight black hole in the center of RZ2109, it would either swallow such small black holes or throw them out of the globular cluster," says Stern. "So our observations suggest that there are no medium-sized black holes in the center of globular clusters." The researcher admits that the existence of the middle weights cannot yet be completely ruled out. For example, they could hide in dwarf galaxies - and would be very difficult to detect there.