Washington: Astronomers have announced a robust detection of a vast reservoir of intergalactic gas about 400 million light years from earth.
This discovery is the strongest evidence yet that the "missing matter" in the nearby Universe is located in an enormous web of hot, diffuse gas.
This missing matter -- which is different from dark matter -- is composed of baryons, the particles, such as protons and electrons, that are found on the earth, in stars, gas, galaxies, and so on.
These discoveries are based on observations from NASA`s Chandra X-ray Observatory and European Space Agency`s XMM-Newton, or X-ray space observatory.
XMM-Newton is the biggest scientific satellite ever built in Europe, its telescope mirrors were amongst the most powerful ever developed in the world, and with its sensitive cameras it will see much more than any previous X-ray satellite.
A variety of measurements of distant gas clouds and galaxies have provided a good estimate of the amount of this "normal matter" present when the universe was only a few billion years old.
However, an inventory of the much older, nearby universe has turned up only about half as much normal matter, an embarrassingly large shortfall.
The mystery then is where does this missing matter reside in the nearby universe?
This latest work supports predictions that it is mostly found in a web of hot, diffuse gas known as the Warm-Hot Intergalactic Medium (WHIM).
Scientists think the WHIM is material left over after the formation of galaxies, which was later enriched by elements blown out of galaxies.
"Evidence for the WHIM is really difficult to find because this stuff is so diffuse and easy to see right through," said Taotao Fang of the University of California-Irvine (UC-I), who led the study. "This differs from many areas of astronomy where we struggle to see through obscuring material."
To look for the WHIM, the researchers examined X-ray observations of a rapidly growing supermassive black hole known as an active galactic nucleus, or AGN. This AGN, which is about two billion light years away, generates immense amounts of X-ray light as it pulls matter inwards.
Lying along the line of sight to this AGN, at a distance of about 400 million light years, is the so-called Sculptor Wall.
This "wall," which is a large diffuse structure stretching across tens of millions of light years, contains thousands of galaxies and potentially a significant reservoir of the WHIM if the theoretical simulations are correct.
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, says a NASA release.
Using new data from Chandra and previous observations with both Chandra and XMM-Newton, absorption of X-rays by oxygen atoms in the WHIM has clearly been 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.
This result gives scientists confidence that the WHIM will also be found in other large-scale structures.
These results appeared in the May 10 issue of The Astrophysical Journal.