Until now, there have been two known types of black holes: stellar-mass black holes that are several times more massive than our sun and are created when really big stars die out, and supermassive black holes that are millions to billions of times the mass of the sun and which sit in the center of most, maybe all, galaxies, including our own Milky Way. While astrophysicists have been fairly certain of how the smaller black holes are created, the creation of the larger ones has been largely a mystery. The main hypothesis is that they are formed from the merger of multiple medium-size black holes. But no one had ever confirmed the existence of black holes of this size. Until this week.
A team led by scientists at the Centre d'Etude Spatiale des Rayonnements in France reports in today's issue of Nature that they have found a black hole that is more than 500 times more massive than the sun. They found an X-ray source, now named Hyper-Luminous X-ray source 1 (HLX-1), on the edge of the galaxy ESO 243-49 that has a maximum X-ray brightness about 260 million times that of the sun. As gas falls into a black hole, energy is released, much of it in the form of X-rays. Only a medium-size black hole could create an X-ray signature that bright, the scientists say.
"This is the best detection to date of such long sought after intermediate mass black holes," lead author Sean Farrell said in a statement. "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 supermassive black holes that exist at the center of the Milky Way and other galaxies."