Most known black holes are either extremely massive, like the supermassive black holes that lie at the cores of large galaxies, or relatively light, with masses less than 100 times that of the Sun. However, intermediate mass black holes (IMBHs) are rare and are considered rare “missing links” in the evolution of black holes.
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 motions of seven fast-moving stars in the innermost region. globular star cluster Omega Centauri.
These stars provide compelling new evidence for the presence of a gravitational force caused by a medium-mass black hole pulling them along. To date, only a few other IMBH candidates have been found.
Omega Centauri consists of roughly 10 million stars that are gravitationally bound together. 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 there any IMBHs, and if so, how common are they? Is a supermassive black hole growing from the IMBH? How is the IMBH itself formed? Are dense star clusters their favorite home?
Astronomers have now created a huge catalog of the motions of these stars, measuring velocities for 1.4 million stars 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 discovered seven stars that shouldn’t be there,” explained Maximilian Häberle of the Max Planck Institute for Astronomy in Germany, who led the investigation. “They are moving so fast that they would escape the cluster and never return. The most likely explanation is that these stars are being gravitationally pulled by a very massive object and holding them close to the center. The only object that could be that massive is a black hole with a mass of at least 8,200 times larger than our Sun.”
Several studies have suggested the presence of an IMBH in Omega Centauri. However, other studies have suggested that a central cluster of stellar-mass black holes could contribute to the mass, and have suggested that the lack of fast-moving stars above the necessary escape velocity makes IMBHs less likely by comparison.
“This discovery is the most direct evidence yet for an IMBH in Omega Centauri,” 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 similar mass are known. The Omega Centauri black hole may be the best example of an IMBH in our cosmic neighborhood.”
If confirmed, at 17,700 light-years away, the candidate black hole is 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 from Earth with the naked eye and is one of the favorite celestial objects for stargazers living in the Southern Hemisphere. The cluster is located 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 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 work led by Häberle et al. is published online today in the journal Nature.
The Hubble Space Telescope has been operating for more than three decades and continues to make groundbreaking 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 operation of the telescope and the mission. Lockheed Martin Space, based in Denver, Colorado, also supports missions at Goddard. The Space Telescope Science Institute in Baltimore, Maryland, operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.