Unique mid-sized black hole is the lonely survivor of a shredded galaxy

This is the Hubble Telescope's latest and clearest yet image of the galaxy ESO 243-49. We've been watching this galaxy for a while now, because it's likely home to the only known medium-sized black hole. And now we might know why that is.

The black hole at the center of this galaxy is known as HLX-1. Located 290 million light-years away, this black hole appears to fit somewhere right between the small black stars that form when a giant star collapses and the supermassive black holes, which can be millions of times more massive, that are found at the centers of galaxies. We don't yet have a firm grasp on how these supermassive black holes come to exist. The best explanation is that they are formed by the repeated collisions of galaxies, which smashes black holes together to form larger and larger mergers.

But for that to work, we should be seeing black holes that are still in the middle of the transition from small to supermassive. That's why HLX-1, which is about 20,000 times the mass of our Sun and is found right on the edge of ESO 243-49, is so important.

Astronomers are eager to learn more about its particular origins. Now, thanks to Hubble, we know that it's surrounded by a bunch of young massive blue stars that form a cluster some 250 light-years across. As Hubble scientist Sean Farrell explains, that unusual combination of black hole and stars could give us a very clear picture of where the black hole came from:

"Before this latest discovery, we suspected that intermediate-mass black holes could exist, but now we understand where they may have come from. The fact that there seems to be a very young cluster of stars indicates that the intermediate-mass black hole may have originated as the central black hole in a very-low-mass dwarf galaxy. The dwarf galaxy might then have been swallowed by the more massive galaxy, just as happens in our Milky Way."

The cluster is such a crucial clue because there's no way it could have formed under normal circumstances so far from the heart of the galaxy. This suggests that they were likely formed when HLX-1 was captured by its current galaxy in a collision with its original home, most likely a dwarf galaxy. The black hole was the central black hole in this tiny galaxy, but its stars were ripped away in the collision, which also would have compressed the surround gas and kicked off the star formation that created these young blue giants.

This particular collision was fairly recent — the stars are only about 200 million years old — and HLX-1 is still just beginning the process of feeding and growing. Given another few eons and some more collisions like this, it could get to be a supermassive black hole, with a galaxy all its own. For more images and information, check out the Hubble site.