A giant reservoir of intergalactic gas was detected lying along a wall-shaped structure of galaxies about 400 million light-years away from Earth, providing the strongest evidence yet that the "missing matter" in the nearby universe is located in an enormous web of hot, diffuse gas.
Observations from NASA's Chandra X-ray Observatory and ESA's XMM-Newton observatory have allowed astronomers to examine the missing matter, which is not the same as dark matter. The missing matter is composed of baryons, which are particles such as protons and electrons that make up most of the mass of the visible matter in the universe.
A variety of measurements of distant gas clouds and galaxies have provided a good estimate of the amount of "normal matter" that was present when the universe was only a few billion years old. But, an inventory of the much older, nearby universe discovered only about half as much normal matter ? a surprising deficit.
So, where does this missing matter reside in the nearby universe? A recent study predicts that the missing matter is mostly found in a web of hot, diffuse gas, known as the Warm-Hot Intergalactic Medium (WHIM). Scientists believe that the WHIM is composed of material left over following the formation of galaxies, enriched by elements that were blown out of the galaxies. ??
"Evidence for the WHIM is really difficult to find because this stuff is so diffuse and easy to see right through," said lead author Taotao Fang of the University of California at Irvine. "This differs from many areas of astronomy where we struggle to see through obscuring material."
To locate the WHIM, researchers examined X-ray observations of a rapidly growing supermassive black hole known as an active galactic nucleus, or AGN. The AGN is located about two billion light-years away, and generates immense amounts of X-ray light as it sucks matter inwards.
Along the line of sight to this AGN, at a distance of approximately 400 light-years away, is the so-called Sculptor Wall. The "wall" is a large diffuse structure that stretches across tens of millions of light-years and contains thousands of galaxies.
If the scientists' theoretical simulations are correct, the Sculptor Wall could potentially contain a significant reservoir of the WHIM as well. The WHIM in the wall should absorb some of the X-rays from the AGN as they make their journey across intergalactic space to Earth.
Absorption of X-rays by oxygen atoms in the WHIM was clearly detected by Fang and his colleagues. The characteristics of the absorption are consistent with the distance of the Sculptor Wall, as well as the predicted temperature and density of the WHIM.
The results of this study give scientists confidence that the WHIM can also be located in other large-scale structures.
Previous claimed detections of the hot component of the WHIM have been controversial because they were made using only one X-ray telescope, so the statistical significance of majority of the results have been questioned.
"Having good detections of the WHIM with two different telescopes is really a big deal," said co-author David Buote, also from the University of California at Irvine. "This gives us a lot of confidence that we have truly found this missing matter."
The new study also removes another uncertainty from previous claims: since the distance of the Sculptor Wall is already known, the statistical significance of the absorption detection is greatly enhanced over previous "blind" searches.
These previous searches attempted to find the WHIM by observing bright AGN at random directions on the sky, in the hope that their line of sight would intersect a previously undiscovered large-scale structure.
In the past, confirmed detections of the WHIM were difficult because of the structure's extremely low density. Using observations and simulations, scientists calculated that the WHIM has a density that is equivalent to only 6 protons per cubic meter. As a comparison, the interstellar medium, which is the diffuse gas that is in between stars in our own galaxy, typically has about a million hydrogen atoms per cubic meter.
"Evidence for the WHIM has even been much harder to find than evidence for dark matter, which is invisible but can be detected because of its gravitational effects on stars and galaxies," Fang said.
The results of the study were published in the May 10th issue of The Astrophysical Journal.
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