July 18, 2024
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Space

A black hole for Omega Centauri – Sky & Telescope

It has at least 8,200 times the mass of our Sun and lies less than 18,000 light-years away. A giant black hole is hiding at the very center of the enormous Omega Centauri globular cluster, a prominent naked-eye object in the southern sky, according to a new study.

Expand Omega Centauri
A three-panel enlargement of a new Hubble color image of Omega Centauri, with the likely position of the intermediate-mass black hole marked with a dotted circle.

ESA/Hubble and NASA, M. Häberle (MPIA)

A team led by Maximilian Häberle (Max Planck Institute for Astronomy, Germany) found seven stars in the core of the globular cluster that are moving so fast that they would have escaped the cluster altogether if it were not for the additional gravity of a massive central object. “It’s a beautiful measurement,” says facilitator Simon Portegies Zwart (Leiden Observatory, the Netherlands), who was not involved in the study.

If confirmed, the Omega Centauri black hole would be one of the first convincing examples of a intermediate mass black hole (IMBH). These black holes are much more massive than the stellar variety left behind by supernova explosions, but they are not nearly as big as their supermassive brethren at the centers of galaxies.

In the past, spectroscopic measurements of the velocities of stars in the cores of globular clusters had already suggested the presence of IMBHs in Omega Centauri, 47 Tucanae and M15, among others. “What is spectacular here,” says Portegies Zwart, “is that they have measured the velocities of individual stars.”

Häberle and his colleagues meticulously measured the motions of 1.4 million individual stars in more than 500 archival Hubble Space Telescope images of Omega Centauri taken over the past 20 years. Near its center, they found seven stars moving more than 2.41 milliarcseconds per year across the sky. At the cluster's distance of 17,700 light-years, that own motion corresponds to a projected speed of more than 62 kilometers per second (140,000 mph), the estimated escape velocity from the globe's core if it does not contain a black hole. The fastest star in the group has a proper motion of 4.41 milliarcseconds per year, corresponding to a projected speed of 113 kilometers per second.

After zooming in on the Omega Centauri globular cluster, an animation shows the motion of stars near the cluster's center.

The discovery of these seven fast stars requires the presence of something massive: a compact object that would increase the speed needed to escape the core, explains Häberle's team in this week's issue. NatureFrom the observed motions, the researchers deduce that the compact object's mass must be at least 8,200 times that of the Sun. (Since motions can only be measured in the plane of the sky, the stars' actual velocities through space are likely greater, which would also imply a greater mass for the black hole.)

This is only the second time that researchers have derived the existence of a massive black hole from the motions of individual surrounding stars; the first was the Nobel Prize-winning measurements of massive stars orbiting Sagittarius A*, the supermassive black hole at the center of our Milky Way galaxy.

In this video, Häberle's team visually shows how Hubble data was collected over two decades.

Portegies Zwart says that Häberle and his colleagues are “absolute experts” in this field; however, he is a bit cautious about the team’s conclusions. “We don’t know the cluster’s escape velocity very precisely,” he says. “Let’s assume it’s 80 kilometers per second instead of 62; that would leave only two stars moving faster, one of which could be an unrelated foreground object. In that case, we wouldn’t have a strong case.”

The authors agree that it would be great to also determine the radial velocities – the speeds of stars that are moving towards or away from us rather than across the sky – and they have already secured observing time on the James Webb Space Telescope for this. More precise positional measurements of the stars in the fast-moving cluster could also reveal the expected curvature of their trajectories, due to their orbital motion around the black hole. “I would love to see a real orbit,” says Portegies Zwart.

The presence of an intermediate-mass black hole in Omega Centauri would reinforce the theory that the globular cluster is actually the stripped-down core of a small galaxy that was devoured by our Milky Way in the distant past. That merger would have halted the growth of that galaxy's central black hole. The discovery is therefore not only interesting in itself, but could also shed light on the evolution of black holes in low-mass galaxies.

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