There are two types of black holes: small ones that form from the remnants of collapsed stars, and their supermassive counterparts at the center of galaxies. But astronomers believe there's a third, intermediate-sized type that we haven't found yet...until now.
Intermediate black holes would be anywhere from 100 to 1,000 times the mass of our Sun, and debate has long raged in the astronomical literature as to whether they actually exist. A new source of X-ray radiation located some 300 million light-years away, designated HLX-1, might well be the long-awaited intermediate black hole.
HLX-1 is the most brilliant example ever seen of an ultra-luminous X-ray source. Such sources are almost always black holes, but at such a tremendous distance it can't be a regular, stellar mass black hole. And yet, its placement with respect to its neighboring galaxy ESO 243-49 doesn't fit the bill for a supermassive black hole, as it's outside the galaxy instead of at its center. So if it's not a small black hole and it's not a huge black hole, then that surely leaves only the middle option.
Well, that would be the case, as long as HLX-1 is actually where we think it is. Astronomers realized that they might be looking at a cosmic optical illusion, in which the X-ray source is actually many millions of light-years further away and it just appears to line up with ESO 243-49. If that were true, then HLX-1 might just be the supermassive black hole at the center of a far more distant galaxy.
To test this possibility, astronomers at the UK's University of Leicester tested the redshift of the galaxy and HLX-1. Redshift is, in essence, the amount light is stretched by the expansion of the universe as it travels between two points, and so it's a useful indicator of how far away something is. The redshifts of the two objects matched up, indicating the black hole really is right next to ESO 243-49, and, in all likelihood, it really must be intermediate-sized.
It's unclear what the visible light source is that surrounds HLX-1, but the two leading candidates only bolster the argument that this really is a mid-sized black hole. Astronomers think the light is either coming from a dwarf galaxy or a star cluster, and those are also the two most likely locations such a black hole would be found.