An intermediate black hole in the globular cluster

Omega Centauri

Overview of the globular cluster Omega Centauri (left), a magnified section of the central region of the cluster and the probable position of the intermediate black hole (right). © ESA/Hubble & NASA, M. Häberle (MPIA)

Intermediate black holes are considered an important link between black holes that form from stars and the supermassive gravitational giants at the heart of galaxies. Astronomers may now have discovered one of these long-sought intermediate forms – in the Omega Centauri globular cluster of our Milky Way, around 17,000 light years away. In the center of this remnant of a primeval dwarf galaxy, the researchers discovered seven stars that orbit extremely quickly around an invisible center. Their orbits and speeds suggest that this is a black hole with at least 8200 solar masses – an intermediate black hole. If this is confirmed, it would also be the most massive black hole in our cosmic neighborhood.

Black holes come in different sizes and masses. Stellar black holes, which weigh a maximum of a few dozen solar masses, are the most common and are formed by supernova explosions of massive stars. At the other extreme are supermassive black holes in the centers of galaxies, with masses of millions or even billions of suns. The closest representative of these gravitational giants to us is Sagittarius A*, the central black hole of the Milky Way, which weighs around four million solar masses and is around 25,000 light years away. But according to theory, there should also be an intermediate form of black hole. These intermediate black holes should have between 100 and 100,000 solar masses and could, for example, have been formed by serial mergers of stellar black holes. They are also considered possible precursors to supermassive galaxy centers. So far, however, astronomers have only been able to find a few candidates for this intermediate stage. This means that it remains unclear where these intermediate black holes are hidden and how many of them there are.

Relic of a conquered dwarf galaxy

But now astronomers led by Maximilian Häberle from the Max Planck Institute for Astronomy in Heidelberg may have discovered one of these intermediate black holes – hidden in the center of the globular cluster Omega Centauri. This collection of around ten million stars lies around 17,000 light years away from us and is the brightest and by far the most massive globular cluster in our Milky Way. It can be seen with the naked eye as a small, blurry spot in the southern sky at night. “Omega Centauri is a special case among the globular clusters of the Milky Way,” the astronomers explain. “Because of its large mass, the complex composition of its star population and its dynamics, the star cluster is considered to be the remnant of a dwarf galaxy that was absorbed by the Milky Way.”

This peculiarity of Omega Centauri has long raised suspicions that an intermediate black hole could be hidden in the center of the globular cluster – a precursor to today’s supermassive black holes that has stopped growing due to galaxy merging. However, previous observations have not found any signs of an active black hole in the center of the cluster that devours matter and therefore emits radiation. In addition, the stars in the inner region of the globular cluster are so close together that it is almost impossible to see through them. Häberle and his team therefore chose a different approach: They followed the movements of stars in Omega Centauri, because stars that orbit particularly quickly could reveal whether an invisible large mass is hidden in the center of the cluster.

“Speeding stars” reveal hidden black hole

For their search, Häberle and his colleagues evaluated more than 500 archive images from the Hubble Space Telescope, which it had taken of this globular cluster over the last 20 years – some of them just for calibration purposes. “The search for fast stars and the documentation of their movement was like looking for a needle in a haystack,” says Häberle. Nevertheless, they succeeded: With the help of sophisticated analyses, the astronomers determined the orbits and speeds of a total of 1.4 million stars – this is the most complete catalog of star movement in Omega Centauri to date. Even more importantly, however, the team discovered seven stars in the innermost region of the globular cluster that were moving much faster than they should. “In the area of ​​around three arc minutes around the center, we found seven stars with a speed of more than 2.41 milliarcseconds per year,” the astronomers report. This means that the speed of these stars exceeds the escape velocity at this position – they should have catapulted themselves out of the globular cluster long ago.

“The presence of seven central stars that move faster than the escape velocity of the cluster can only be explained by the fact that they are bound to a massive object near the center of the cluster,” explain Häberle and his colleagues. The gravity of such an object then holds the stars in place despite their high speed. Using a supplementary model, the team reconstructed how heavy the hidden object would have to be at least to hold these “speeding stars.” They arrived at a minimum mass of 8200 solar masses. “This makes an intermediate black hole the only plausible solution,” write the astronomers. Due to the narrow orbits of the fast stars, they were also able to rule out the possibility that this mass is made up of several stellar black holes that are close together – there would simply be too little space for that.

Best evidence to date for an intermediate black hole

According to the team, this provides confirmation that Omega Centauri does indeed contain an intermediate black hole. “In previous studies of the central regions of Omega Centauri, one could always ask critically: Where are the high-speed stars? Now we have the answer, and confirmation that Omega Centauri does indeed contain a medium-sized black hole,” says Häberle’s colleague Nadine Neumayer. At the same time, the study provides the most reliable evidence to date of the existence of such medium-sized black holes in general. To find out how heavy and large this hidden black hole really is, the astronomers are already planning future observations, including with the James Webb Space Telescope. Its high-resolution infrared spectrometers could help to narrow down the movements of the racing stars in the center of Omega Centauri even better and thus reveal the exact position and mass of the black hole.

Source: Maximilian Häberle (Max Planck Institute for Astronomy, Heidelberg) et al., Nature, doi: 10.1038/s41586-024-07511-z

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