Smallest black hole ever found may be in a class of its own

Black holes come in a few different sizes, from modest ones a few times heftier than the Sun, right up to supermassive monsters at the heart of galaxies. But now researchers from Ohio State University claim to have discovered the smallest black hole ever detected, which may represent a brand new class of the objects.

For a long time, black holes were only thought to come in two size ranges. There are stellar mass black holes, which have a mass of between five and about 30 times that of the Sun. And way up the other end of the scale sits supermassive black holes, lurking at the centers of galaxies and having masses of a few million Suns.

But that doesn’t seem to paint a full picture. The most obvious gap lies in the middle – jumping from 30 to a million is quite a leap. Intermediate-mass black holes are believed to fill that, with masses between 100 and 10,000 Suns. Evidence is beginning to mount that these middleweights exist.

At the top of the scale, it’s been argued that a new class of “ultramassive black holes” should be coined, to describe those with masses of over 10 billion Suns.

But there’s another gap at the low end too. When stars collapse under their own gravity, they either form black holes or dense, energetic neutron stars. The limit seems to be around 2.1 solar masses – if the star is smaller than that, it collapses into a neutron star, but over that limit it becomes a black hole. The question then is: where are all the black holes between about 2.1 and 5 solar masses?

The researchers on the new study set out to investigate whether black holes do exist in this range. To do so, they combed through data gathered by Apache Point Observatory Galactic Evolution Experiment (APOGEE), which measures the light spectra of stars.

The team was looking for changes in light from those stars that might indicate they’re orbiting an unseen object. Of the 100,000 stars in the catalog, the researchers focused in on 200 that were particularly interesting. Next they compiled thousands of images of each star from the All-Sky Automated Survey for Supernovae.

From all of that, the team isolated one potential candidate. A giant red star, known as 2MASS J05215658+4359220, appears to be orbiting something. With further examination, the team calculated a probable black hole of 3.3 solar masses.

“What we’ve done here is come up with a new way to search for black holes, but we’ve also potentially identified one of the first of a new class of low-mass black holes that astronomers hadn’t previously known about.” says Todd Thompson, lead author of the study. “The masses of things tell us about their formation and evolution, and they tell us about their nature.”

The research was published in the journal Science.

Source: Ohio State University