Astronomers using the NASA/ESA Hubble Space Telescope have identified the source of a mysterious blue light surrounding a supermassive black hole in our neighbouring Andromeda Galaxy (M31). Though the light has puzzled astronomers for more than a decade, the new discovery makes the story even more mysterious.
The images and illustration reveal that the Andromeda Galaxys (M31s) core is composed of a ring of old, red stars and a newly discovered disk of young, blue stars. The disk is trapped within a supermassive black holes gravitational field. The mass of Andromedas monster black hole is 140 million times greater than that of our Sun.
The illustration at bottom, right shows the structure of Andromedas unusual core and is based on Hubble images of the region. The disk of blue stars is nested inside the larger ring of red stars. The tiny black dot within the blue disk is the monster black hole.
Astronomers deduced the structure of Andromedas core from Hubble images taken over the past decade. The image at upper, right, taken with the Wide Field and Planetary Camera 2, shows that the galaxy appears to have two cores. Normal galaxies only have one core. Astronomers now believe that Andromeda has one core. The two bright blobs are actually the ring of red stars and the disk of blue stars. In fact, the bright blob on the right has a bluish cast.
The image at left is a view of the entire Andromeda Galaxy. The active core is in the center of the galaxy. Andromeda is 2.5 million light-years from Earth.
Credit: Photo Credit for image at left: ©2002, R. Gendler, Photo by Robert Gendler. Credits for image at upper, right: NASA, ESA and T. Lauer (NOAO/AURA/NSF). Credits for illustration at lower, right: NASA, ESA and A. Feild (STScI).
The blue light is coming from a disk of hot, young stars. These stars are whipping around the black hole in much the same way as planets in our solar system are revolving around the Sun. Astronomers are perplexed about how the pancake-shaped disk of stars could form so close to a giant black hole. In such a hostile environment, the black hole’s tidal forces should tear matter apart, making it difficult for gas and dust to collapse and form stars. The observations, astronomers say, may provide clues to the activities in the cores of more distant galaxies.
By finding the disk of stars, astronomers also have collected what they say is ironclad evidence for the existence of the monster black hole. The evidence has helped astronomers rule out all alternative theories for the dark mass in the Andromeda Galaxy’s core, which scientists have long suspected was a black hole.
Solid Evidence for a Monster Black Hole
In addition to the discovery of the disk of stars, the astronomers used this uniquely close look at Andromeda to prove unambiguously that the galaxy hosts a central black hole. In 1988, in independent ground-based studies, John Kormendy and the team of Alan Dressler and Douglas Richstone discovered a central dark object in Andromeda that they believed was a supermassive black hole. This was the first strong case for what are now 40 detections of black holes, most of them made by Hubble. Those observations, however, did not definitively rule out other, very exotic, and far less likely, alternatives.
"There are compelling reasons to believe that these are supermassive black holes," Kormendy said. "But extreme claims require extraordinarily strong evidence. We have to be sure that these are black holes and not dark clusters of dead stars."
The STIS observations of Andromeda are so precise that astronomers have eliminated all other possibilities for what the central, dark object could be. They also calculated that the black holes mass is 140 million Suns, which is three times more massive than once thought.
So far, dark clusters have definitively been ruled out in only two galaxies, NGC 4258 and our galaxy, the Milky Way. "These two galaxies give us unambiguous proof that black holes exist," Kormendy added. "But both are special cases - NGC 4258 contains a disk of water masers that we observe with radio telescopes, and our galactic center is so close that we can follow individual stellar orbits. Andromeda is the first galaxy in which we can exclude all exotic alternatives to a black hole using Hubble and using the same techniques by which we find almost 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 doubt an important part of its legacy. It makes us much more confidant that the other central dark objects detected in galaxies are black holes, too."
"Now that we have proven that the black hole is at the centre of the disk of blue stars, the formation of these stars becomes hard to understand," Bender added. "Gas that might form stars must spin around the black hole so quickly - and so much more quickly near the black hole than farther out - that star formation looks almost impossible. But the stars are there."
A Galaxys Active Core
The black hole and the disk of stars are not the only pieces of architecture in Andromedas core. A team led by Lauer and Faber used Hubble in 1993 to discover that the galaxy appears to have a double cluster of stars at its centre. This finding was a surprise, because two clusters should merge into one in only a few hundred thousand years. Scott Tremaine of Princeton University solved this problem by suggesting that the "double nucleus" was actually a ring of old, red stars. The ring looked like two star clusters because astronomers were only seeing the stars on the opposite ends of the ring. The ring is about five light-years from the black hole and its surrounding disk of blue stars. The disk and the ring are tilted at the same angle as viewed from Earth, suggesting that they may be related.
Although astronomers are surprised to find a blue disk of stars swirling around a supermassive black hole, they also say the puzzling architecture may not be that unusual.
"The dynamics within the core of this neighbouring galaxy may be more common than we think," Lauer explained. "Our own Milky Way apparently has even younger stars close to its own black hole. It seems unlikely that only the closest two big galaxies should have this odd activity. So this behaviour may not be the exception but the rule. And we have found other galaxies that have a double nucleus."
Lars Christensen | alfa
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