Most known black holes are either extremely massive, such as the supermassive black holes found in the centers of large galaxies, or relatively light, with masses less than 100 times that of the Sun. Intermediate-mass black holes (IMBHs), however, are rare 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 tracking the motion of seven fast-moving stars in the inner region of the globular cluster Omega Centauri.
These stars provide new compelling evidence for the presence of the gravity of an intermediate-mass black hole pulling on them. Only a few other IMBH candidates have been found so far.
Omega Centauri consists of about 10 million stars that are gravitationally bound. The cluster is about 10 times more massive than other large globular clusters — almost as massive as a small galaxy.
Among the many questions scientists want to answer: Are IMBHs real, 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 preferred home?
The astronomers have now created a massive catalog of these stars’ motions, measuring the velocities of 1.4 million stars they collected from Hubble images of the cluster. Most of these observations were intended to calibrate Hubble’s instruments rather than for scientific use, but they proved to be an ideal database for the team’s research efforts.
“We found seven stars that shouldn’t be there,” explained Maximilian Häberle of the Max Planck Institute for Astronomy in Germany, who led the research. “They’re moving so fast that they would escape the cluster and never come back. The most likely explanation is that a very massive object is pulling on these stars and keeping them close to the center. The only object that could be that massive is a black hole, with a mass at least 8,200 times that of our sun.”
Several studies have suggested the presence of an IMBH in Omega Centauri. However, other studies have suggested that the mass could be contributed by a central cluster of stellar-mass black holes, and have suggested that the lack of fast-moving stars above the necessary escape velocity made an IMBH less likely by comparison.
“This discovery is the most direct evidence of an IMBH in Omega Centauri to date,” added team leader Nadine Neumayer of the Max Planck Institute for Astronomy in Germany, who initiated the study with Anil Seth of the University of Utah, Salt Lake City. “This is exciting because very few other black holes of comparable mass are known. The black hole in Omega Centauri is perhaps the best example of an IMBH in our cosmic neighborhood.”
If the suspicion is confirmed, the candidate black hole would be 17,700 light-years closer to Earth than the 4.3 million solar-mass black hole at the center of the Milky Way, which is 26,000 light-years away.
Omega Centauri is visible to the naked eye from Earth and is a favorite of stargazers living in the Southern Hemisphere. The cluster lies just above the plane of the Milky Way and appears almost as large as the full moon when viewed from a dark rural area. It was first noted in Ptolemy’s catalogue as a single star almost 2,000 years ago. Edmond Halley reported it as a nebula in 1677. In the 1830s, English astronomer John Herschel was the first to recognize it as a globular cluster.
The research report by Häberle et al. is published online today in the journal Nature.
The Hubble Space Telescope has been in operation for more than three decades and continues to make breakthrough discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and the European Space Agency (ESA). 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 managed by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.