A team of astronomers using Hubble to hunt for an elusive type of black hole have stumbled onto an even weirder scene. In the center of a nearby globular cluster, Hubble discovered what looks like a whole gang of small black holes being uncharacteristically chummy.
Most of the black holes we know about fall into two size groups – there are the stellar mass black holes, with masses a few times that of the Sun, and then there are the supermassive black holes, which contain millions or even billions of solar masses.
Obviously that leaves quite a large gap in the middle, so astronomers have naturally wondered whether other black holes exist between the two extremes. These hypothetical intermediate-mass black holes (IMBHs) are estimated to have masses of a few hundred to a few thousand Suns, and evidence for them has already been discovered in the “stirring” motions of stars in a cluster or flares given off as they snack on unsuspecting stars.
The astronomers on the new study suspected that an IMBH was lurking in the center of a globular cluster called NGC 6397, located about 7,800 light-years from Earth. The team studied the velocities of the stars and other objects in the cluster to figure out the distribution of its mass. Basically, faster motions suggest larger masses concentrated in one spot.
But the results weren’t what the team expected. While they did detect “invisible mass” like that of a black hole, it wasn’t just one – the motions bore the signature of a group of smaller, stellar black holes spread further apart.
"We found very strong evidence for an invisible mass in the dense core of the globular cluster, but we were surprised to find that this extra mass is not 'point-like' (that would be expected for a solitary massive black hole) but extended to a few percent of the size of the cluster,” says Eduardo Vitral, an author of the study.
From their studies, the researchers concluded that there’s an invisible mass of about 1,000 to 2,000 solar masses, and it would be contained inside a swarm of mostly black holes, but possibly also some white dwarfs or neutron stars. These dense objects appear to have sunk towards the center of the cluster.
"Ours is the first study to provide both the mass and the extent of what appears to be a collection of mostly black holes in the center of a core-collapsed globular cluster," says Vitral.
The study could shed new light on the evolution of black holes of all sizes, and suggests that these tightly packed black holes could be a common source of gravitational waves as they collide.
The research was published in the journal Astronomy & Astrophysics.
Source: Hubble
