The Milky Way鈥檚 near-twin galaxy, Andromeda, harbours a supermassive black hole at its core that is surrounded by an unexpected and unexplained disc of young stars.
These new observations by the Hubble Space Telescope answer one longstanding mystery: the source of bright blue light very close to the spiral galaxy鈥檚 central black hole, first spotted using Hubble a decade ago. Yet solving this mystery has immediately created another in its place.
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The newly discovered disc is composed of over 400 very hot, young blue stars, orbiting like a planetary system very close to the black hole. That puzzles astronomers because the black hole鈥檚 intense gravitational field should have torn apart any clouds of matter long before they could coalesce to form new stars.
The stars form a very flat disc that is only one light year across. An elliptical disc of older red stars surrounds it, spanning about five light years. Since the two discs appear to be in the same plane, they are probably related, but no one yet understands how either disc came into being. View an animation of how the two discs of stars might look, in , or format.
It鈥檚 not unusual?
Spectroscopic observations made with Hubble鈥檚 Space Telescope Imaging Spectrograph (STIS) show that the disc of blue stars is only about 200 million years old, while the galaxy itself has been around for about 12 billion years. Intriguingly, there are signs of young stars very close to the core of our own galaxy as well.
So this puzzling phenomenon may not be unusual, says Hubble team member Tod Lauer of the National Optical Astronomy Observatory in Tucson, Arizona, US.
Since it would be surprising if astronomers happened to catch the only example of such a young disc, it may be that such rings are constantly being formed, he says.
鈥淭his behaviour may not be the exception but the rule,鈥� he adds. 鈥淲e still don鈥檛 know, however, how such a disc could form in the first place. It鈥檚 an enigma.鈥�
Supermassive black hole
Measurements by the STIS instrument also allowed astronomers to determine the movement of the blue stars and therefore pinpoint the black hole鈥檚 mass.
This proved conclusively that it really is a supermassive black hole, with a mass 140 million times that of our Sun 鈥� three times more massive than previous estimates.
Until these observations, it had been impossible to rule out some highly unlikely alternatives for the object in the centre of the galaxy 鈥� including an extremely dense star cluster, says team member John Kormendy at the University of Texas, US.
鈥淣ailing the black hole in Andromeda鈥� will be remembered as one of Hubble鈥檚 most important findings, Kormendy says. 鈥淚t makes us much more confident that the other central dark objects detected in galaxies are black holes too.鈥�
