***EDITORS NOTE: NASA and the Chandra X-ray Observatory Center at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., also released a story on this today. Contacts are Megan Watzke of the Chandra X-ray Observatory Center, 617-496-7998, and Steve Roy, Marshall Space Flight Center, Huntsville, Ala., 256-544-6535.***
Astronomers have for the first time "X-rayed" a galaxy to directly measure its oxygen content by using a quasar like an X-ray machine. The discovery, which could have been made only by X-ray observation, demonstrates a new way to get chemical information needed to understand how galaxies evolve.
They made a second major discovery, too: They discovered what was believed to be a relatively quiescent type of infant quasar to be producing an enormous X-ray jet, at least a million light years long. The result overturns a branch of quasar lore that says a certain type of dust-enshrouded quasar is too young to be very active.
Jill Bechtold of the University of Arizona and Aneta Siemiginowska of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., collaborate in quasar surveys using NASA's Chandra X-ray Observatory and Hubble Space Telescope.
They used Chandra in May 2000 to observe X-rays from a distant quasar called PKS 1127 - 145 as the radiation passed through an intervening galaxy about 4 billion light years from Earth.
Atoms of oxygen and other elements in the galaxy absorb some of the X-rays, dimming the quasar's X-ray emission. The astronomers observed this X-ray "shadow" from the galaxy, and for the first time determined directly how much oxygen it contained by measuring absorption.
Scientists have measured so-called absorption line systems at radio, optical and ultraviolet wavelengths, but never before in X-ray. Oxygen absorbs X-rays.
"Because this galaxy and our sun formed about the same time, out of the same interstellar gas, the galaxy should have an oxygen abundance similar to our sun," Bechtold said. "So it was a surprise to find the galaxy had only about one-fifth the oxygen that we now measure for typical stars in our Galaxy." Bechtold and her colleagues reported the results in the Nov. 20, 2001 Astrophysical Journal.
Scientists believe that oxygen, silicon and sulfur are spewed throughout a galaxy primarily by explosions of massive stars. As galaxies age, they become richer in oxygen and other elements necessary for life. Using the fact that the light travel time from distant galaxies can range from hundreds of millions to billions of years, astronomers can study the rate of this enrichment.
There are tens of thousands of quasars, and all emit X-rays. But most are too faint to detect with current X-ray telescopes. "There are very few quasars bright enough in X-ray to do this observation, and we couldn't do it before Chandra," Bechtold said in an interview. A next-generation X-ray telescope called Constellation-X would excel at such observations, added Bechtold, who is on a science working group for the planned NASA observatory.
And although hundreds of absorption systems are known, "There are only two or three systems known that have enough gas to study with X-rays from quasars," Bechtold added. "That's three among more than 500 known X-ray-bright quasars."
She and Siemiginowska now are hunting for more galactic systems with enough absorbing gas to study using the quasar "X-ray" technique. Such absorbers are bright at optical and ultraviolet wavelengths. Bechtold uses the 6.5-meter UA/Smithsonian MMT on Mount Hopkins, Ariz., and the 6.5-meter Magellan telescopes at Las Campanas, Chile, in the search for optically bright candidate systems.
"The search for oxygen absorption was the reason we made the observations," Bechtold said. "Finding that PKS 1127-145 was producing an X-ray jet hundreds of thousands of light-years long was a surprise."
The jet reveals explosive activity that occurred 10 billion years ago around the quasar's central supermassive black hole.
"The X-rays from the jet are likely due to the collision of microwave photons left over from the Big Bang with a high-energy beam of particles," Siemiginowska concluded. She is lead author on a paper which will appear in the May 10, 2002 issue of the Astrophysical Journal. "The intensity of these microwaves today is much less than it was 10 billion years ago, due to the expansion of the universe."
Chandra observations of quasar PKS 1127-145 demonstrate that scientists can image these jets even though they are billions of light years away. Studies of these extraordinarily large structures, made possible by the superb resolution and resolving power of the Chandra X-ray Observatory, will allow astronomers to test models for quasars and the supermassive black holes that power them.
The length of the jet and the prominent knots of X-ray emission observed suggest that the activity in the vicinity of the central supermassive black hole is long-lived but intermittent, perhaps due to the mergers of other galaxies with the host galaxy.
Bechtold and Siemiginowska collaborate in the Hubble Space Telescope and Chandra observations to search for X-ray jets and absorption in a sample of 12 systems.
Other members of the research teams were Thomas Aldcroft, Martin Elvis, Dan Harris and Adam Dobrzycki (Harvard-Smithsonian Center for Astrophysics). The Advanced CCD Imaging Spectrometer (ACIS) X-ray camera, which was used in both observations, was developed for NASA by Pennsylvania State University, University Park, Pa., and the Massachusetts Institute of Technology, Cambridge, Mass.. NASA's Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program. TRW, Inc., Redondo Beach, California, is the prime contractor for the spacecraft. The Smithsonian's Chandra X-ray Center controls science and flight operations from Cambridge, Mass.
PHOTO/CAPTION: QUASAR PKS 1127-145
A low-resolution (72dpi) image of quasar PKS 1127-145 is available at http://graucho.opi.arizona.edu/graphix/images/pksjetpink.jpg For the image at high resolution, contact Megan Watzke at the Chandra X-ray Observatory Center, CfA, Cambridge, Mass. phone: 617-496-7998, email@example.com
Caption:The X-ray image of the quasar PKS 1127-145, a highly luminous source of X-rays and visible light about 10 billion light years from Earth, shows an enormous X-ray jet that extends at least a million light years from the quasar. The jet is likely due to the collision of a beam of high-energy electrons with microwave photons.
The high-energy beam is thought to have been produced by explosive activity related to gas swirling around a supermassive black hole. The length of the jet and the observed bright knots of X-ray emission suggest that the explosive activity is long-lived but intermittent.
On their way to Earth, the X-rays from the quasar pass through a galaxy located 4 billion light years away. Atoms of various elements in this galaxy absorb some of the X-rays, and produce a dimming of the quasar's X-rays, or an X-ray shadow. In a similar way, when our body is X-rayed, our bones produce an X-ray shadow. By measuring the amount of absorption astronomers were able to estimate that 4 billion years ago, the gas in the absorbing galaxy contained a much lower concentration of oxygen relative to hydrogen gas than does our galaxy -- about 5 times lower. These observations will give astronomers insight into how the oxygen supply of galaxies is built up over the eons.Credit: NASA/CXC/A. Siemiginowska (CfA) & J. Bechtold (University of Arizona)