Black holes vary greatly in size, from relatively small ones several times the mass of the sun, which are born of collapsed stars, to supermassive lurkers like the one at the center of our Milky Way Galaxy, with the mass of about four million suns.

But medium-size black holes—those with hundreds or thousands of times the mass of the sun—have proved an elusive quarry. A study in this week's Nature identifies a new candidate for this seemingly rare third class about 300 million light-years away in the spiral galaxy ESO 243-49. (Scientific American is part of the Nature Publishing Group.)

If confirmed, the midsize black hole and others like it could provide clues to how their supermassive brethren form, which could, in turn, inform how galaxies like ours developed.

The proposed intermediate-mass black hole appears to reside outside the main bulge of its host galaxy, which likely also harbors an ordinary supermassive black hole at its center, says astrophysicist Sean Farrell of the University of Leicester in England, the lead author of the study.

Farrell says that the extreme luminosity of the object, dubbed HLX-1, makes it the most compelling such candidate yet. (Even though a black hole's gravitational pull precludes the escape of radiation from its grasp, emissions, in this case x-rays, from the material falling rapidly into a black hole can reveal its existence.)

"Where this one stands out is it's approximately 10 times brighter than the most bright candidate to date," Farrell says, which largely rules out a smaller, stellar-mass black hole that merely appears bright from our vantage point. At the very least, the study's authors conclude, the black hole should have the mass of at least 500 suns. Such an object might arise from mergers between dense stellar remnants in collections of stars known as globular clusters, Farrell says.

Jon Miller, an astronomy professor at the University of Michigan at Ann Arbor, calls the proposed black hole "a credible claim." Farrell and his colleagues, Miller says, "have done a thorough job in their analysis."

Cole Miller, a professor of astronomy at the University of Maryland, College Park, who wrote a commentary for Nature Physics on the Farrell group's research, says that HLX-1 "is the single-best candidate for an [intermediate-mass black hole] that has yet been discovered."

"Although the most hardened skeptics will wait until we get direct dynamical evidence" such as observations of objects in orbit around such a black hole, he says, "I think this has strongly enhanced the case for intermediate-mass black holes."