Astronomers Discover Medium-Sized Class of Black Holes

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It’s the Goldilocks variety of black holes: not too big and not too small.

The new source HLX-1,  the light blue object to the top left of the galactic bulge, is the ambassador for a new class of black holes, more than 500 times the mass of the Sun. It lies on the periphery of the edge-on spiral galaxy ESO 243-49, about 290 million light years from Earth.

The discovery, led by Sean Farrell at Britain’s University of Leicester, appears today in the journal Nature.

Until now, identified black holes have been either super-massive (several million to several billion times the mass of the Sun) in the center of galaxies, or about the size of a typical star (between three and 20 solar masses).

The new discovery is the first solid evidence of a new class of medium-sized black holes and was made using the European Space Agency’s XMM-Newton X-ray space telescope. At the time of the discovery, Farrell and his team were working at the Centre d’Etude Spatiale des Rayonnements in France.

A black hole is a remnant of a collapsed star with such a powerful gravitational field that it absorbs all the light that passes near it and reflects nothing.

“While it is widely accepted that stellar mass black holes are created during the death throes of massive stars, it is still unknown how super-massive black holes are formed,” Farrell said.

It had been long believed by astrophysicists that there might be a third, intermediate class of black holes, with masses between a hundred and several hundred thousand times that of the Sun. However, such black holes had not been reliably detected until now.

One theory suggests that super-massive black holes may be formed by the merger of a number of intermediate mass black holes, Farrell said.

“To ratify such a theory, however, you must first prove the existence of intermediate black holes. This is the best detection to date of such long sought after intermediate mass black holes. ”

Using XMM-Newton observations carried out in 2004 and 2008, the team showed that HLX-1 displayed a variation in its X-ray signature. This indicated that it must be a single object and not a group of many fainter sources. The huge radiance observed can only be explained if HLX-1 contains a black hole more than 500 times the mass of the Sun. The authors say that no other physical explanation can account for the data.

Lead image caption: Artist’s impression of HLX-1 in the periphery of the edge-on spiral galaxy ESO 243-49. Credit: Heidi Sagerud.

Sources: Nature and the University of Leicester