NASA/MSFC/Chandra

This new Chandra X-ray Observatory image shows several "point sources" of X-rays that have frustrated astronomers. Two galaxies in the image, called NGC 4485 and NGC 4490, together contain five point sources that are "ultra luminous," or very, very bright, astronomers said.

That's fine. Problem is, the really bright sources look just like the other point sources in all other measurable respects. So what are they?

Some researchers had hoped the ultra-luminous sources might turn out to be related to intermediate-mass black holes, an elusive class of black holes expected to weigh thousands of times as much as the Sun. Researchers disagree whether they even exist.

If the really bright sources are intermediate black holes, then these potential objects behave very much like regular, stellar black holes, which weigh no more than a few hundred solar masses.

Possibly, an ultra-bright source is just generated by a regular black hole whose rotational axis is pointed our way, researchers say. This would cause a beam of X-rays to shoot at us. If a black hole's axis were not pointed our way, then the radiation kicked up by the object's feeding frenzy would appear less intense.

The new image, released last month at a meeting of the American Astronomical Society, is part of a survey of 90 galaxies that turned up 120 ultra-bright X-ray sources. The analysis was led by Douglas Swartz of NASA's Marshall Space Flight Center.

Blueish-white spots represent hotter, high-energy emissions while red colors are cooler, low-energy X-rays. NGC 4485 is the smaller of the two galaxies and is near the top of the image.

-- Robert Roy Britt