Why Did This Black Hole Mysteriously Lose a Ton of Weight?

Astronomers are fascinated by a weirdly skinny black hole.

Approximately one billion light-years from Earth, caught in the midst of a galactic collision, sits an enigmatic black hole in the galaxy SDSS J1126+2944.

Typically, black holes are surrounded by a massive clump of stars. This black hole surprisingly has very few, however, giving it an oddly "skinny" appearance, in the words of one researcher. This suggests that at some point, it lost a huge amount of mass.

A black hole is a place in space where the pull of gravity is so strong, nothing can escape it, not even light. This one may turn out to be the first observable example of what's known as an intermediate black hole, which is basically a medium-sized black hole.

The odd black hole was discovered by Julie Comerford, an astrophysicist at the University of Colorado Boulder.

Comerford was scanning the cosmos for multi-black hole galaxies, which are incredibly rare, when she noticed something strange. While looking at the edge of the galaxy SDSS J1126+2944, she spotted a small sphere of stars resembling a cosmic beauty mark.

By using both an optical telescope and an X-ray telescope, Comerford was able to determine the location of two black holes in the galaxy: one in the center, as she expected, but also a second one on the edge of the galaxy, right where the "beauty mark" was.

The outer edge black hole, or the southern black hole, as Comerford calls it, was surprisingly bereft of stars.

"The southern black hole has 500 times fewer stars associated with it than the northern black hole," Comerford said. "We are trying to find out why there is such a discrepancy."

There are a few possible explanations for the unusual black hole.

SDSS J1126+2944 is caught in the middle of a galactic merger, meaning two galaxies have slammed into each other and are in the process of combining. Galactic mergers, like the one seen here usually happen over the course of 100 million years, which is incredibly short on the universal time scale, making it nearly impossible to observe. Eventually, as the two galaxies merge, the black holes will combine into one even larger supermassive black hole.

One possibility for the lack of stars around the southern black hole could be that they were sucked away by gravitational and tidal forces produced by the merger. If this is the case, the southern black hole would have started out as a supermassive black hole just like its northern counterpart.

"One possibility is that its stars were ripped away by gravitational forces," Comerford said. "In other words: it went on a major crash diet."

However, it's also possible that Comerford has discovered an intermediate black hole.

Technically, there have been no confirmed cases of intermediate black holes, she said. (In August 2014, a team measured another black hole at 428 suns, which would make it an intermediate-size black hole.)

In order to determine that's what we see here, Comerford and her team plan to observe the galaxy simultaneously with the Chandra X-ray Observatory and the Very Large Array—a giant radio telescope—in order to determine its mass.

If the mass is in the designated range, then we could have hard evidence for an intermediate black hole. However, if the mass lands is over 1 million solar masses, then we know the black hole's surrounding stars have been ripped away by gravitational forces.

Being able to identify and study an intermediate black hole is crucial to our understanding, as they are thought to be a crucial step in black hole evolution.