A Truthful Glow From Galaxy’s Center
Though it is a mere 25,000 light-years from Earth, the center of our own galaxy has long been one of the universe’s most mysterious places. It’s so full of dust and gas that visual telescopes, even those that can gather light from the most distant reaches of space, haven’t been able to see inside.
“It’s invisible to most astronomers because only one photon in a trillion can break through,” said Mark Morris, a professor of astronomy at UCLA who has spent much of the last two decades probing the Milky Way’s center with telescopes that do not detect visible light but pick up electromagnetic radiation emitted in radio, infrared and X-ray wavelengths.
Those images have offered tantalizing but vague hints. When X-ray telescopes first peered at the center of the Milky Way about 20 years ago, they found the area gave off a massive X-ray glow. Recent evidence has pointed to a super-massive black hole in the galaxy’s center. X-rays, a highly energetic form of light, are emitted from some of nature’s most violent creations: supernova blasts and black holes.
But astronomers didn’t know exactly what fearsome objects, if any, emitted the X-rays. The first pictures in the 1980s, from an X-ray telescope called Einstein, were low resolution and too fuzzy. They showed only a diffuse glow of X-rays over a large area. Because X-rays are also created at extremely high temperatures, many astronomers suggested the center of the galaxy was filled with a huge, searing ball of gas.
Now, a modern X-ray telescope 100 times more sensitive than its predecessor has given the first clear glimpse of the Milky Way’s center.
“We thought the image would be fantastic, but it’s better than we thought,” said Q. Daniel Wang, the assistant professor of astronomy at the University of Massachusetts at Amherst who led the team mapping the galactic center.
The image shows that the X-rays don’t come from hot gas but from about 1,000 points of light, a vast collection of what appear to be glittering star remnants, plus black holes. (Black holes give off X-rays when they swallow big chunks of matter or other stars.)
“Our galaxy is like a big city,” said Wang. “The center of the galaxy is downtown, where the action is.”
The finding from NASA’s $1.5-billion Chandra X-ray telescope provides more evidence of how advances in technology in recent years are resolving decades-old mysteries about the skies.
The results “finally settle important aspects of the 20-year-old debate surrounding the origin of X-ray emission” from the galaxy’s center, said Andreas Eckart, an astronomer at the University of Cologne who studies the galactic center and helped confirm the existence of the massive black hole located there.
The new image was unveiled Wednesday at a national meeting of the American Astronomical Society in Washington. It is also being published in today’s issue of the journal Nature.
Calling the images “really fabulous,” Morris said the findings resolved problems he and other astronomers have long had in explaining how enough super-hot gas to generate all the X-rays could be continually created in the Milky Way.
“This relieves the pressure on us to evoke outrageous events and circumstances,” including nonstop supernova explosions or gas traps created by magnetic fields, he said.
The X-rays appear to come from a bright but more mundane collection of objects: hundreds of small, collapsed stellar remnants like white dwarf and neutron stars and also the black holes created when bigger stars collapse.
The new map suggests that the center of the galaxy, unlike the quiet suburb where Earth resides, is a violent place where stars are forming, dying and exploding at furious rates and being buffeted by supernova shock waves.
“It’s a nice place to visit with a telescope, but I wouldn’t want to live there,” said Cordelia Lang, a University of Massachusetts astronomer and coauthor of the new study.
The climate, though, is more benign than previously thought. The hypothesis of a ball of hot gas would have required temperatures of hundreds of millions of degrees. Instead, said Eric Gotthelf, a coauthor of the study from Columbia University, “it’s only a relatively mild 10 million degrees.”
Studying our own galaxy is likely to help astronomers understand the workings of all galaxies, how they formed and why super-massive black holes seem to reside at the centers of many of them.
Our own largest black hole gives off a relatively weak X-ray signal, although a team including UCLA’s Morris did capture a “burp” last year, a dramatic X-ray flare after the hole swallowed some matter.
The study was not able to explain the diffuse X-ray glow that remains or some small dust grains of iron that were detected.
Eckart and Morris said the diffuse X-rays and the iron may have been emitted by the black hole in a tremendous flash of energy as recently as a few hundred years ago.
