Black Holes in Globular Star Clusters May Reveal Ripples in Spacetime

For 40 years, astrophysicists have believed that there was no way there could be black holes in the globular star clusters in our own galaxy. Now, though, a team has identified these black holes, upsetting previous research and revealing that their existence is not only possible, but definite.

Globular star clusters are large groupings of stars thought to contain some of the oldest stars in the universe. In the same distance from our sun to the nearest neighbor, Proxima Centauri, its nearest neighbor, these globular star clusters could have a million to tens of millions of stars.

"The old theory believed that the interaction of stars was thought to kick out any black holes that formed," said Tom Maccarone, one of the researchers, in a news release. "They would interact with each other and slingshot black holes out of the cluster until they were all gone."

Yet it seems that isn't the case. While black holes still get kicked out of globular star clusters, it seems that this happens at a much slower pace than first thought. That's what led to the current discovery of black holes in these star clusters.

The first finding of a black hole in a globular star cluster occurred in 2007. That's when Maccarone spied the black hole by spotting an X-ray emission from the gas falling into the black hole and heating up to a few million degrees. This year, he and his team found two examples of globular clusters in our own galaxy by finding radio emission using the Very Large Array of radio telescopes in New Mexico.

"As the black hole eats a star, these jets of material are coming out," said Maccarone in a news release. "Most of the material falls into the black hole, but some is thrown outwards in a jet. To see that jet of material, we look for a radio emission. We found a few radio emissions coming from this globular star cluster that we couldn't explain any other way."

These black holes in globular clusters may also provide a way for them to get close enough to one another to merge into bigger black holes. These mergers, in turn, may produce the "ripples in spacetime" that are called gravitational waves. Trying to detect these waves is one of the biggest problems in physics, since it would be the strongest test of whether or not Einstein's theory of relativity is correct. This means that in the future, these black holes could provide answers to some longstanding questions.

The findings are published in The Astrophysical Journal.