In a universe filled with black holes small and gargantuan, astronomers have puzzled over why they don't see anything in between -- a kind of middleweight black hole.
After all, if you got really big, say millions to billions of times the mass of the sun, you surely had to go through some sort of growth stages to get there.
Now, a team of European astronomers says it has found the best evidence yet for the existence of these in-between objects. Their candidate for an intermediate-mass black hole tips the scales at more than 500 times the sun's mass. It calls a galaxy 290 million light-years away home.
A formal report of the team's find appears in the latest issue of the journal Nature.
The object, dubbed HLX-1, appears as an extraordinarily bright object at x-ray wavelengths -- some 260 million times brighter than the sun. That's in a range where you'd expect a middleweight black hole to shine, writes Sean Farrel in an email. He's a University of Leicester astronomer who led the effort.
Those x-ray emissions don't really come from the black hole itself, of course. A black hole's gravity is so intense that even if you could travel at the speed of light, you won't escape its grip. Instead, the x-rays are generated by the gas that's being drawn into the black hole. It's like a final shout-out before oblivion.
HLX-1's brightness falls between that of black holes that form from individual stars and the brightness of supermassive black holes found at the centers of galaxies. This trait, combined with the way HLX-1's brightness varies over time, left the team with no other conclusion.
"I think our case is pretty solid," Dr. Farrell says.
For a bit more on the discovery, you can go here for the backstory on detecting a possible middleweight black hole.
And you can read about future plans for confirming the observation -- and discovering new candidates -- here.