Black holes, remnants of collapsed stars with spectacular gravitational fields that allow not even light to escape, have previously come in two known categories: super massive or stellar size. But a recent discovery has bridged that gap.
Discovery of Black Hole HLX-1
The black hole was discovered using the European Space Agency's XMM-Newton X-ray space telescope. At more than 500 times the mass of the sun, the black hole is the first to be found that falls in the intermediate category. Super-massive black holes are several million to several billion times the mass of the sun and lie at the center of galaxies, whereas stellar-sized black holes are about the size of a typical star between three and 20 solar masses.
The black hole was named HLX-1 for Hyper-Luminous X-ray source 1. It lies toward the edge of galaxy ESO 243-49, which is 290 million light-years from Earth. As a source that is ultraluminous in x-rays, HLX-1 has a maximum x-ray brightness of approximately 260 million times that of the sun.
HLX-1 lacks a counterpart in optical images, confirming that it is neither a foreground star nor background galaxy. Its position on the periphery also indicates that it is not the core of its host galaxy.
New Black Hole Class Aids Understanding of Formation
Dr. Sean Farrell at the University of Leicester is the lead author of the paper that describes this discovery and establishes the new class of black holes.
"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 says. "One theory is that super-massive black holes may be formed by the merger of a number of intermediate mass black holes. To ratify such a theory, however, you must first prove the existence of intermediate black holes."
The discovery of HLX-1 became the needed stepping stone. "This is the best detection to date of such long sought after intermediate black holes," Farrell says. "Such a detection is essential. While it is already known that stellar mass black holes are the remnants of massive stars, the formation mechanisms of supermassive black holes are still unknown. The identification of HLX-1 is therefore an important step towards a better understanding of the formation of the super-massive black holes that exist at the center of the Milky Way and other galaxies."
Image of HLX-1
This artist's impression of the new class of black hole represented by HLX-1 shows the source as the light blue object to the top left of the galactic bulge. Galaxy ESO 243-49 is a spiral galaxy as seen edge-on.
Source: University of Leicester
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