Astronomers using the NASA/ESA Hubble Space Telescope have identifiedthe source of a mysterious blue light surrounding a supermassive blackhole in our neighbouring Andromeda Galaxy (M31). Though the light haspuzzled astronomers for more than a decade, the new discovery makes thestory even more mysterious.
The blue light is coming from a disk of hot, young stars. Thesestars are whipping around the black hole in much the same way asplanets in our solar system are revolving around the Sun. Astronomersare perplexed about how the pancake-shaped disk of stars could form soclose to a giant black hole. In such a hostile environment, the blackhole's tidal forces should tear matter apart, making it difficult forgas and dust to collapse and form stars. The observations, astronomerssay, may provide clues to the activities in the cores of more distantgalaxies.
By finding the disk of stars, astronomers also have collectedwhat they say is ironclad evidence for the existence of the monsterblack hole. The evidence has helped astronomers rule out allalternative theories for the dark mass in the Andromeda Galaxy's core,which scientists have long suspected was a black hole.
"Seeing these stars is like watching a magician pulling arabbit out of a hat. You know it happened but you don't know how ithappened," said Tod Lauer of the National Optical Astronomy Observatoryin Tucson, Arizona. He and a team of astronomers, led by Ralf Bender ofthe Max Planck Institute for Extraterrestrial Physics in Garching,Germany, and John Kormendy of the University of Texas in Austin, madethe Hubble observations. The team's results will be published in theSept. 20, 2005 issue of the Astrophysical Journal.
Hubble Probes Strange Blue LightAstronomer Ivan King of the University of Washington and colleaguesfirst spotted the strange blue light in 1995 with the Hubble SpaceTelescope. He thought the light might have come from a single, brightblue star or perhaps from a more exotic energetic process. Three yearslater, Lauer and Sandra Faber of the University of California at SantaCruz used Hubble again to study the blue light. Their observationsindicated that the blue light was a cluster of blue stars.
Now, new spectroscopic observations by Hubble's Space TelescopeImaging Spectrograph (STIS) reveal that the blue light consists of morethan 400 stars that formed in a burst of activity about 200 millionyears ago. The stars are tightly packed in a disk that is only alight-year across. The disk is nested inside an elliptical ring ofolder, cooler, redder stars, which was seen in previous Hubbleobservations.
The astronomers also used STIS to measure the velocities ofthose stars. They obtained the stars' speeds by calculating how muchtheir light waves are stretched and compressed as they travel aroundthe black hole. Under the black hole's gravitational grip, the starsare travelling very fast: 3.6 million kilometres an hour (1,000kilometres a second). They are moving so fast that it would take them40 seconds to circle the Earth and six minutes to arrive at the Moon.The fastest stars complete an orbit in 100 years. Andromeda's activecore probably made similar disks of stars in the past and may continueto make them.
"The blue stars in the disk are so short-lived that it isunlikely in the long 12-billion-year history of Andromeda that such ashort-lived disk would appear now," Lauer said. "That's why we thinkthat the mechanism that formed this disk of stars probably formed otherstellar disks in the past and will trigger them again in the future. Westill don't know, however, how such a disk could form in the firstplace. It still remains an enigma."
The astronomers credit Hubble's superb vision for finding thedisk."Only Hubble has the resolution in blue light to observe this disk,"said team member Richard Green of the National Optical AstronomyObservatory in Tucson. "It is so small and so distinct from thesurrounding red stars that we were able to use it to probe into thevery dynamical heart of Andromeda. These observations were taken by themembers of our team that built STIS. We designed its visible channelspecifically to seize such an opportunity - to measure starlight closerto a black hole than in any other galaxy outside our own."
Solid Evidence for a Monster Black HoleIn addition to the discovery of the disk of stars, the astronomers usedthis uniquely close look at Andromeda to prove unambiguously that thegalaxy hosts a central black hole. In 1988, in independent ground-basedstudies, John Kormendy and the team of Alan Dressler and DouglasRichstone discovered a central dark object in Andromeda that theybelieved was a supermassive black hole. This was the first strong casefor what are now 40 detections of black holes, most of them made byHubble. Those observations, however, did not definitively rule outother, very exotic, and far less likely, alternatives.
"There are compelling reasons to believe that these aresupermassive black holes," Kormendy said. "But extreme claims requireextraordinarily strong evidence. We have to be sure that these areblack holes and not dark clusters of dead stars."
The STIS observations of Andromeda are so precise thatastronomers have eliminated all other possibilities for what thecentral, dark object could be. They also calculated that the blackhole's mass is 140 million Suns, which is three times more massive thanonce thought.
So far, dark clusters have definitively been ruled out in onlytwo galaxies, NGC 4258 and our galaxy, the Milky Way. "These twogalaxies give us unambiguous proof that black holes exist," Kormendyadded. "But both are special cases - NGC 4258 contains a disk of watermasers that we observe with radio telescopes, and our galactic centeris so close that we can follow individual stellar orbits. Andromeda isthe first galaxy in which we can exclude all exotic alternatives to ablack hole using Hubble and using the same techniques by which we findalmost all supermassive black holes."
"Studying black holes always was a primary mission of Hubble,"Kormendy said. "Nailing the black hole in Andromeda is without a doubtan important part of its legacy. It makes us much more confidant thatthe other central dark objects detected in galaxies are black holes,too."
"Now that we have proven that the black hole is at the centreof the disk of blue stars, the formation of these stars becomes hard tounderstand," Bender added. "Gas that might form stars must spin aroundthe black hole so quickly - and so much more quickly near the blackhole than farther out - that star formation looks almost impossible.But the stars are there."
A Galaxy's Active CoreThe black hole and the disk of stars are not the only pieces ofarchitecture in Andromeda's core. A team led by Lauer and Faber usedHubble in 1993 to discover that the galaxy appears to have a doublecluster of stars at its centre. This finding was a surprise, becausetwo clusters should merge into one in only a few hundred thousandyears. Scott Tremaine of Princeton University solved this problem bysuggesting that the "double nucleus" was actually a ring of old, redstars. The ring looked like two star clusters because astronomers wereonly seeing the stars on the opposite ends of the ring. The ring isabout five light-years from the black hole and its surrounding disk ofblue stars. The disk and the ring are tilted at the same angle asviewed from Earth, suggesting that they may be related.
Although astronomers are surprised to find a blue disk of starsswirling around a supermassive black hole, they also say the puzzlingarchitecture may not be that unusual.
"The dynamics within the core of this neighbouring galaxy maybe more common than we think," Lauer explained. "Our own Milky Wayapparently has even younger stars close to its own black hole. It seemsunlikely that only the closest two big galaxies should have this oddactivity. So this behaviour may not be the exception but the rule. Andwe have found other galaxies that have a double nucleus."
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