Astronomers detect new type of black hole
Astronomy has so far identified only two types of black holes: supermassive and stellar. Researchers have long suspected that there must be something in between, and now an international research team led by Maximilian Häberle from the Max-Planck-Institut für Astronomie in Heidelberg has found reliable evidence for just such an intermediate black hole.
For a long time, it has been suspected in astronomy that there must also be intermediate mass black holes. However, reliable observations have not been achieved so far. Now, however, this team has discovered the best evidence for exactly such a black hole.
In the archival data of the Hubble Space Telescope, the team detected seven stars moving with extremely high velocities in the center of the globular cluster Omega Centauri. Only the gravitational force of a black hole can explain the motion of these stars, according to the researchers, who wrote about their findings in the journal "Nature". From their data, they conclude that the black hole in the center of Omega Centauri has a mass of 8200 times that of the Sun.
The newly described black hole is - in cosmic terms - relatively close to Earth. It is approximately 18,000 light-years away, explains co-author Nadine Neumayer. This makes it the nearest known example of a massive black hole. The supermassive black hole in the center of the Milky Way is located about 27,000 light-years away.
A Thousand Solar Masses
Astronomers had long searched for new black holes by looking for such rapidly moving stars - unsuccessfully. Häberle took up the search again and turned to data from the Hubble Telescope, which had not been used for this purpose before. A total of 500 archive images from a 20-year period were available to Häberle. The researcher meticulously measured the motion of about 150,000 stars on these images.
"The search for fast stars and the documentation of their motion was like the proverbial search for a needle in a haystack," explains the researcher. In the end, Häberle not only created the most comprehensive catalog of star movements in Omega Centauri, but also found seven needles in the haystack: seven stars that move with high velocities. With this velocity, these stars would have to fly out of the star cluster, according to Häberle. Only the gravitational force of a black hole with a mass of 8200 times that of the Sun can hold the stars in place, as his calculations show.
Discovering a black hole with such a mass is of great significance for astronomers. To date, astronomers were only familiar with two types of black holes. Stellar black holes, with a mass of up to 150 solar masses, form when large stars have exhausted their nuclear fuel and collapse. And then there are the supermassive black holes in the centers of galaxies with the million- or even billion-fold mass of the Sun.
These super-massive black holes are believed to have formed, according to hypothesis, through the merger of smaller black holes with several thousand solar masses. Some of such intermediate-mass black holes should still exist in the cosmos today. Indeed, astronomers have discovered a whole series of candidates for such objects in smaller galaxies and globular clusters. However, direct evidence for this has not been obtained yet: The movement of stars in such distant objects is too difficult to observe.
A Galaxy Swallowed by the Milky Way
Omega Centauri comes into play here: It is the largest globular cluster in the Milky Way, with ten million stars, and is even visible to the naked eye in the southern hemisphere. It is believed that Omega Centauri was once the central region of a small galaxy that collided with the Milky Way and lost its outer regions billions of years ago.
The idea was: If such a collision occurred, then a previously existing intermediate-mass black hole in the center of the small galaxy should still be present in Omega Centauri today. And because of the proximity of the globular cluster to Earth, the movement of stars there can be observed.
The stars recently discovered by Häberle and his team support this assumption and provide the best evidence so far for the existence of intermediate-mass black holes. However, the Hubble images only show the movement of the stars in the sky and not the movement towards or away from us. The radial movement of the seven rapidly moving stars, which the researchers now want to measure with the James Webb Space Telescope, will help to finally lay the last doubts about the existence of the black hole in Omega Centauri to rest.
This new discovery of an intermediate-mass black hole with a mass of 8200 times that of the Sun in Omega Centauri is significant for astronomy, as it expands our understanding of black hole types beyond stellar and supermassive. This finding could also support the hypothesis that such black holes form through the merger of smaller black holes.
The proximity of Omega Centauri, which is approximately 18,000 light-years away, makes it an ideal target for observing the movement of stars, providing crucial evidence for the existence of intermediate-mass black holes. The movement of the seven rapidly moving stars discovered by Häberle's team will be further studied using the James Webb Space Telescope, aiming to shed light on the black hole's radial movement and provide final confirmation.
This research, conducted using archived data from the Hubble Space Telescope, highlights the importance of revisiting and reanalyzing existing data in astronomy. By meticulously measuring the motion of 150,000 stars in over 500 images, Häberle was able to identify the seven stars moving with extremely high velocities, leading to the groundbreaking discovery of this intermediate-mass black hole.
