Astronomers using the newly upgraded Karl G. Jansky Very Large Array radio telescope in New Mexico, USA, have discovered two black holes inside an ancient cluster of stars in our galaxy.
These are optical images of M22 and the candidate companion stars to the radio sources M22-VLA1 and M22-VLA2: the globular cluster M22, on the left, and the location of the radio sources on archival Hubble images (Doug Matthews / Adam Block / NOAO / AURA / NSF / Hubble team / NASA / ESA)
A new study, published in the journal Nature (arXiv.org version), describes the detection of two black holes in the globular cluster named M22, a group of stars more than 10,000 light-years from Earth.
The astronomers hoped to find evidence of a rare type of black hole in the cluster’s center called an intermediate-mass black hole, which is more massive than those larger than the Sun’s mass, but smaller than the supermassive black holes found at the cores of galaxies. However, they found something very surprising – two smaller black holes, which is unusual because most theorists say there should be at most one black hole in the cluster.
These objects, dubbed M22-VLA1 and M22-VLA2, are about 10 to 20 times heavier than our Sun.
This radio continuum image shows the core of the globular cluster M22: the two bright circled objects are the black holes M22-VLA1 and M22-VLA2, the faint circled object is a millisecond pulsar, a red cross marks the photometric cluster center (Jay Strader et al / Michigan State University / Harvard-Smithsonian Center for Astrophysics)
“The discovery of two black holes in the same cluster was a complete surprise. All the theory up to now says that should not happen in a globular cluster that is 12 billion years old,” said co-author Dr James Miller-Jones of the Curtin University node of the International Center for Radio Astronomy Research.
“The study was originally searching for just one larger black hole within the cluster of hundreds of thousands of stars which, when viewed from the naked eye, resembles a hazy round ‘puff’ of light.”
“Simulations of how globular clusters evolve show many black holes are created early in a cluster’s history. The many black holes then sink towards the middle of the cluster where they begin a chaotic dance leading to most being thrown out of the cluster until only one surviving black hole remains. We were searching for one large black hole in the middle of the cluster, but instead found two smaller black holes a little way out from the center, which means all the theory and simulations need refinement.”
“M22 may contain as many as 100 black holes but we can’t detect them unless they’re actively feeding on nearby stars,” he said.
“We plan to do further study to pin down the properties of the two we’ve already found,” Dr Miller-Jones concluded.
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Bibliographic information: Jay Strader et al. 2012. Two stellar-mass black holes in the globular cluster M22. Nature 490, 71–73; doi: 10.1038/nature11490
