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Physics graduate student Taotao Fang's thesis project in the late 1990s was to search for a hot, diffuse gas located between galaxies. The gas forms a diffuse cosmic web connecting clusters of galaxies.
Called WHIM (warm-hot intergalactic medium), this million-plus-degree gas may enable astronomers to map the distribution of dark matter in the universe and perhaps understand its origin.
"Although I worked on this project using analytic methods and cosmological simulations, I didn't detect anything with the Chandra X-ray Observatory at that time," Fang recalled. After he finished his thesis in late 2000, he continued his search as a postdoctoral associate at the Center for Space Research with his advisor, Claude Canizares, associate provost and the Bruno Rossi Professor of Experimental Physics.
Luckily, in the two years since Fang finished his thesis, better cosmological simulations have helped researchers understand WHIM's properties and adjust their searching strategy. Fang and Canizares led the MIT team that recently discovered WHIM in a local galaxy.
Four independent teams of scientists, including one at MIT, used Chandra to detect this extremely hot gas that they suspect is part of a gigantic system of hot gas and dark matter that defines the cosmic landscape.
BARYONS AND A WHIM
After the Big Bang, a whole lot of elementary particles called baryons went missing.
According to the standard cosmological model, the total number should have remained constant. Yet there are only a third to half as many baryons in the local universe as the far-off early universe.
Where are the missing baryons, also known as WHIM gas? Apparently with the missing dark matter that makes up 99 percent of the universe.
Far away in the early universe, most of the baryons can be detected easily via hydrogen and/or helium absorption in the optical band.
In the local universe, it's a different story. Cosmological simulations reveal that when galaxies form, they send huge shock waves through intergalactic space that heat the baryons there to between 300,000 and 5 million degrees Celsius. At those temperatures, you have to rely on X-rays to see anything.
The MIT researchers' observations, along with teams from the Harvard-Smithsonian Center for Astrophysics, Ohio State University and the University of Michigan in Ann Arbor, are the first X-ray detection of the existence of such a hot gas.
"Our paper is the only one that detects this hot gas in a very modestly over-dense region in the universe," said Canizares. "So it really ties the gas to where we think the dark matter is slightly concentrated, as the models predict."
HIDE AND SEEK
Fang described how they found WHIM out.
"The basic idea is this: We select an X-ray bright background source such as a distant galaxy that can be detected by Chandra. When photons pass through places where WHIM gas is located, they can be absorbed by the WHIM gas and an X-ray absorption feature will be produced in the spectrum of the background source.
"What we actually found is an absorption feature produced by hydrogen-like oxygen (oxygen with only one electron circulating around the nucleus) along the sight line towards a distant galaxy called PKS 2155-304.
"The absorbing gas that contains this hydrogen-like oxygen is around 800 million light-years away from the Earth, in a small group of galaxies. I didn't expect to find something so close. Typically, we were expecting to detect this WHIM gas at least several billion light-years away. But it didn't surprise me in the sense that all the derived properties are consistent with previous predictions from the standard cosmological model and simulations," Fang said.
"The Chandra observations are showing us, for the first time, that this hot component really exists and that it is indeed associated with over-dense regions in the universe," Canizares said. "The Chandra observations provide strong support for cosmological simulations of structure formation in the universe, for the gravitational effects of dark matter on normal matter and for our current understanding of the amount of normal matter contained in the universe." Lurking below the surface
"What we found is the tip of an iceberg," Fang said. "What we still don't know is whether there is really something huge underneath - but it seems all the observations are pointing in this direction."
The researchers are looking for more evidence of WHIM in the hope it will fully reveal its secrets. "This would address many important questions, such as how did the cosmic structures form and evolve? How did the heavy elements produced in the supernova explosion propagate into the intergalactic space? What is dark matter?" Fang said.
"The final detection of all the WHIM gas will provide a consistent picture of cosmic structure formation and evolution based on the standard model," he said. "On the other hand, failure to detect it will be a serious challenge to the standard cosmological model on which modern astronomy is based."
NASA's Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program, and TRW Inc. in Redondo Beach, Calif., is the prime contractor. The Smithsonian's Chandra X-ray Center controls science and flight operations from Cambridge.
A version of this article appeared in MIT Tech Talk on August 14, 2002.