A Search for Mid-Scale Black Holes in the Hubble Space Telescope Archives: The Motion of Stars in a New Cluster of Galaxies

The search for midsize black holes has a long history of claims that are later disproven. Astrophysicists long suspected that some sources of ‘ultraluminous’ X-rays could be black holes in this size range. Most of the candidates have been found to be neutron stars, which shine more brightly when they become overheated from a companion star. “These are most likely associated with ‘normal’ young binary systems,” says Giuseppina Fabbiano, an astrophysicist at the Harvard–Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

By perusing two decades’ worth of pictures from the Hubble Space Telescope archives, astrophysicists have found what could be evidence of a nearby black hole at least 8,200 times as massive as the Sun.

The team patched the images together to reconstruct the movement of more than 150,000 stars in the cluster. Hberle says that most stars move in a way that models predict. Some of them were moving faster. The seven stars were too fast to be held by the gravity of the cluster.

It was suggested that the stars had been accelerated by the pull of a black hole. It needs at least 8,200 solar masses to be visible from the stars, but it could weigh as much as 50,000 suns. Hberle says they didn’t know if they would find it or not. “It was a little bit of a risk, and we might have found nothing.”