The Hubble Space Telescope may have just found a rare "missing link" black hole hiding in Earth's cosmic backyard.
Located roughly 6,000 light-years away at the core of the nearby star cluster Messier 4, the intermediate-mass black hole candidate is an ultradense region of space packed with the mass of 800 suns, causing nearby stars to orbit it like "bees swarming around a hive," according to the researchers who detected it.
"It's too tiny for us to be able to explain other than it being a single black hole," study lead-author Eduardo Vitral, an astrophysicist at the Space Telescope Science Institute in Maryland, said in a statement. "Alternatively, there might be a stellar mechanism we simply don't know about, at least within current physics."
Black holes are born from the collapse of giant stars and grow by gorging on gas, dust, stars and other black holes. Currently, known black holes tend to fall into two general categories: stellar-mass black holes, which range from a few to a few dozen times the sun's mass, and supermassive black holes, cosmic monsters that can be anywhere from a few million to 50 billion times as massive as the sun.
Intermediate-mass black holes — which, theoretically, range from 100 to 100,000 times the sun's mass — are the most elusive black holes in the universe. While there have been several promising candidates, no intermediate-mass black holes have been definitively confirmed to exist.
This poses a puzzle for astronomers. If black holes grow from stellar to supermassive size by gorging themselves in an endless feeding frenzy, the lack of confirmed sightings of black holes in their awkward teenage phases points to an even bigger hole in our understanding of the cosmic monsters.
To look for signs of a lurking intermediate-mass black hole, the authors of the new study pointed the Hubble Space Telescope toward the globular star cluster Messier 4. Globular clusters are clumps of tens of thousands to millions of tightly packed stars, many of which are among the most ancient to have ever formed in our universe. Roughly 180 globular clusters dot our Milky Way galaxy and, because they have a high concentration of mass in their centers, are ideal stomping grounds for adolescent black holes.
Messier 4 is the closest globular star cluster to Earth. By using the Hubble and Gaia space telescopes, the researchers used 12 years of data to pinpoint the stars in the cluster and study their movements around its center. By applying physical models to how these stars moved, the researchers discovered that the stars were moving around something massive and were not directly detectable in the cluster's center.
"We have good confidence that we have a very tiny region with a lot of concentrated mass," Vitral said. "It's about three times smaller than the densest dark mass that we had found before in other globular clusters."
The region the researchers found was more compact than they would expect if its intense gravity had been produced by other dense star corpses, like neutron stars and white dwarfs, and it would take 40 stellar-mass black holes packed into a space one-tenth of a light-year across to make the stars orbit them so intensely.
"The consequences are that they would merge and/or be ejected in a game of interstellar pinball," the researchers wrote in the statement.
To confirm that they have spotted an intermediate-mass black hole and not accidentally discovered some new physics, the researchers said further observations, possibly using the James Webb Space Telescope alongside Hubble, should be made.
"Science is rarely about discovering something new in a single moment. It’s about becoming more certain of a conclusion step by step, and this could be one step towards being sure that intermediate-mass black holes exist," Timo Prusti, a project scientist at the Gaia telescope, said in the statement.
The researchers published their findings May 23 in the journal Monthly Notices of the Royal Astronomical Society.
