UH team makes
Two astronomers identify X-rayBy Helen Altonn
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Two Hawaii astronomers are part of a team that has unraveled one of the greatest mysteries of the universe: the source of most of the X-ray background.
The team found that a substantial number of galaxies have cores that shine brightly in X-rays, but not in visible light. "They are just hidden galaxies," said Lennox Cowie of the University of Hawaii Institute for Astronomy.
Alan Bunner, director of NASA's "Structure and Evolution of the Universe" science theme, said: "Since it was first observed 37 years ago, understanding the source of the X-ray background has been a Holy Grail of X-ray astronomy. Now, it is within reach."
Chandra X-ray Observatory data analyzed by Cowie and Amy Barger, also of the Institute for Astronomy, led to discoveries presented yesterday at an American Astronomical Society meeting in Atlanta.
In an interview at UH, Cowie said that when the first X-ray satellites were launched 30 or 40 years ago, one of the first things they saw was a uniform glow of X-rays in the sky.
Astronomers since have found black holes, quasars and other X-ray sources, but the source of the glow was never solved, Cowie said.
"We are all very excited by this finding," said Richard Mushotzky of NASA's Goddard Space Flight Center. "The resolution of most of the hard X-ray background during the first few months of the Chandra mission is a tribute to the power of this observatory and bodes extremely well for its scientific future."
Besides resolving the origin of most of the X-ray glow throughout the universe, NASA said its Chandra observatory also may have revealed the most distant objects ever seen in the universe and discovered two new types of cosmic objects.
"Not bad for being on the job only five months," the space agency said.
Until Chandra, technology was not available to detect the origin of the hard, or high-energy, X-ray background.
Operated from the Marshall Space Flight Center in Huntsville, Ala., Chandra has been described by NASA as a "time machine" that can study quasars as they were 10 billion years ago.
The X-ray observatory can see exploding stars, black holes, colliding galaxies and other high-energy cosmic objects.
Barger, one of six scientists in the United States named as Chandra Fellows, and Mushotzky discussed the findings at a news conference in Atlanta.
The team's work also was described in an article submitted to the journal Nature by Mushotzky, Cowie, Barger and Keith Arnaud of the University of Maryland-College Park.
The team studied a section of the sky about one-fifth the size of a full moon known as the Hawaii Deep Survey Field.
Scientists already have extensive optical data for the region, so observations could be compared, Cowie said.
"What Richard did was take a very large fraction of time guaranteed on Chandra and pointed at the field we have all the information on. So the minute we knew the X-ray sources, we knew what the optical sources were."
The team determined that about 70 million sources -- mostly galaxies -- were sources for about 80 percent of the cosmic glow in the Hawaii field.
Cowie said it was "somewhat of a shock" to him that a substantial number of galaxies have cores that shine bright in X-rays but not in visible light.
The team reported that there may be tens of millions of these "veiled galactic nuclei" in the universe, saying, "Each of these galaxies likely harbors a massive black hole at its core that produces X-rays as gas is pulled toward it at nearly the speed of light."
Material and dust veil the galaxies and kill soft X-rays, but hard X-rays penetrate the shroud so the activity can be seen, Cowie said. "These things look like normal galaxies, but they've got this stuff going on in the middle. They're normal galaxies with a twist, so to speak, that I hadn't expected at all."
Cowie said scientists were beginning to understand that most normal galaxies have fairly substantial black holes in the center, but it's been a question of why. "Why isn't there more feeding and activity in these things in the local universe? Some people thought maybe there isn't enough gas to feed these things well. At least some are going on eating. They're just hiding it."
The astronomers also identified a second new class of objects, comprising about one-third of the sources, that they believe to be "ultra-faint galaxies."
Mushotzky said the sources may emit little or no optical light, either because the dust around the galaxy blocks the light or because it is absorbed during its journey across the universe.
In the latter case, he said, these sources would be more than 14 billion light-years away--the earliest and most distant objects ever identified.
Cowie said resolving most of the X-ray background "brings a level of closure and completeness to this. You know if many of these galaxies have black hole monsters in the center, they're actually in action. It brings a level of understanding to this whole process."
The team is continuing work with data from the Chandra and Hubble Space Telescopes and from telescopes on Mauna Kea.
Barger, a postdoctoral fellow at UH, is one of about 10 people to receive a Hubble Space Telescope fellowship and the first to receive Chandra and Hubble fellowships at the same time.
Chandra Fellows are named by the Chandra X-ray Observatory Center at the Smithsonian Astrophysical Observatory in Cambridge, Mass.