Missing Link Uncovered: Hubble Unveils Hidden Black Hole in Omega Centauri

Black holes are the universe’s trap doors leading into an abyss. They normally come in two sizes. The first type are stars that are massive enough that when they burn out they collapse down to a singularity that’s equivalent to five to ten times our sun’s mass crushed into something smaller than the period at the end of this sentence.

The second known type is monster black holes that are millions or even billions of times our sun’s mass. They dwell in the dark hearts of galaxies, gobbling up any stars that pass nearby. They must be intimately tied to the early days of galaxy formation.

What’s elusive are so-called intermediate-mass black holes weighing hundreds or a few thousand solar masses. Are they a “missing link” in black hole evolution?

One of the best candidates is lurking in the center of the huge globular star cluster Omega Centauri. Because black holes cannot be directly observed, astronomers must use “stellar forensics” by measuring how a black hole’s gravitational pull affects the motions of nearby stars. Stellar velocities can be used to estimate the black hole’s mass. The faster a star moves, the more powerful the tug from the black hole.

This is a meticulous process that only the Hubble Space Telescope can do for something as far away as Omega Centauri. Astronomers used more than 500 images from Hubble — spanning two decades of observations — to follow the motion of seven fast-moving stars in the innermost region of the star cluster.

The black hole in Omega Centauri is estimated to be 8,200 times that of our sun. If confirmed, it is closer to Earth than the 4.3-million-solar-mass black hole in the center of the Milky Way. That black hole is 26,000 light-years away, while Omega Centauri is 17,700 light-years from us.

Hubble Space Telescope Finds Strong Evidence for Intermediate-Mass Black Hole in Omega Centauri

Most known black holes are either extremely massive, like the supermassive black holes that lie at the cores of large galaxies, or relatively lightweight, with a mass of under 100 times that of the Sun. Intermediate-mass black holes (IMBHs) are scarce, however, and are considered rare “missing links” in black hole evolution.

Now, an international team of astronomers has used more than 500 images from NASA’s Hubble Space Telescope — spanning two decades of observations — to search for evidence of an intermediate-mass black hole by following the motion of seven fast-moving stars in the innermost region of the globular star cluster Omega Centauri.

These stars provide new compelling evidence for the presence of the gravitational pull from an intermediate-mass black hole tugging on them. Only a few other IMBH candidates have been found to date.

Omega Centauri consists of roughly 10 million stars that are gravitationally bound. The cluster is about 10 times as massive as other big globular clusters — almost as massive as a small galaxy.

Research and Discoveries on IMBH

Among the many questions scientists want to answer: Are there any IMBHs, and if so, how common are they? Does a supermassive black hole grow from an IMBH? How do IMBHs themselves form? Are dense star clusters their favored home?

The astronomers have now created an enormous catalog for the motions of these stars, measuring the velocities for 1.4 million stars gleaned from the Hubble images of the cluster. Most of these observations were intended to calibrate Hubble’s instruments rather than for scientific use, but they turned out to be an ideal database for the team’s research efforts.

“We discovered seven stars that should not be there,” explained Maximilian Häberle of the Max Planck Institute for Astronomy in Germany, who led this investigation. “They are moving so fast that they would escape the cluster and never come back. The most likely explanation is that a very massive object is gravitationally pulling on these stars and keeping them close to the center. The only object that can be so massive is a black hole, with a mass at least 8,200 times that of our sun.”

Implications and Historical Significance of the Discovery

Several studies have suggested the presence of an IMBH in Omega Centauri. However, other studies proposed the mass could be contributed by a central cluster of stellar-mass black holes, and had suggested the lack of fast-moving stars above the necessary escape velocity made an IMBH less likely in comparison.

“This discovery is the most direct evidence so far of an IMBH in Omega Centauri,” added team lead Nadine Neumayer of the Max Planck Institute for Astronomy in Germany, who initiated the study, together with Anil Seth from the University of Utah, Salt Lake City. “This is exciting because there are only very few other black holes known with a similar mass. The black hole in Omega Centauri may be the best example of an IMBH in our cosmic neighborhood.”

If confirmed, at a distance of 17,700 light-years the candidate black hole resides closer to Earth than the 4.3-million-solar-mass black hole in the center of the Milky Way, located 26,000 light-years away.

Omega Centauri is visible from Earth with the naked eye and is one of the favorite celestial objects for stargazers living in the southern hemisphere. Located just above the plane of the Milky Way, the cluster appears almost as large as the full Moon when seen from a dark rural area. It was first listed in Ptolemy’s catalog nearly 2,000 years ago as a single star. Edmond Halley reported it as a nebula in 1677. In the 1830s the English astronomer John Herschel was the first to recognize it as a globular cluster.

The discovery paper led by Häberle et al. was published online on July 10 in the journal Nature.

Reference: “Fast-moving stars around an intermediate-mass black hole in ω Centauri” by Maximilian Häberle, Nadine Neumayer, Anil Seth, Andrea Bellini, Mattia Libralato, Holger Baumgardt, Matthew Whitaker, Antoine Dumont, Mayte Alfaro-Cuello, Jay Anderson, Callie Clontz, Nikolay Kacharov, Sebastian Kamann, Anja Feldmeier-Krause, Antonino Milone, Maria Selina Nitschai, Renuka Pechetti and Glenn van de Ven, 10 July 2024, Nature.
DOI: 10.1038/s41586-024-07511-z

The Hubble Space Telescope has been operating for over three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations. Lockheed Martin Space, based in Denver, Colorado, also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, Maryland, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.