EMBARGOED for A vast fog of hot gas infusing the space between galaxies has been firmly detected, apparently filling in an important blank in the cosmic ledger involving up to 80 percent of normal matter. The work builds on and confirms preliminary results, by four separate groups, that were
announced last summer.The gas surrounds our own galaxy and appears to weave through about three dozen others that make up what's called the Local Group of galaxies. The hot, intergalactic fog is thought to be a relic of the early universe, material that did not fall into galaxies back when the
first great structures formed more than 13 billion years ago. In all, the gas makes up a giant cloud, estimated to weigh a trillion times more than our Sun, all surrounding the Local Group.
The discovery most likely squares away an important piece of astronomical accounting by revealing the missing normal matter. Perhaps more significantly, it hints at the locations of far greater quantities of more elusive stuff, called dark matter, which remains unaccounted for.
The gas is widely dispersed and 150 times hotter than the Sun's surface, making it practically invisible. It could only be detected by the most advanced space-based observatories, and three of them were needed to do the job.
"This gas is so hot that it radiates at energies too high to be seen at visible wavelengths, so we had to look at it in the ultraviolet," said Ohio State University astronomer Smita Mathur. X-ray observations were required, too.
The study was led by Fabrizio Nicastro of the Harvard Smithsonian Center for Astrophysics and will be detailed in the Feb. 13 issue of the journal Nature.
What's out there
Astronomers have long known that direct observations were well out of balance with the amount of matter that is perceived to exist. Unseen matter makes its presence known by gravitational effect, holding galaxies and even clusters of galaxies together.
Strong but indirect observations had shown that the universe is composed of about 4 percent normal matter -- protons, electrons and neutrons that are collectively called baryons, a figure confirmed this week in a
detailed study of the universe's first radiation. This normal, so-called baryonic matter makes stars, planets, flowers, you, and everything else that is visible. Co-existing with normal matter is an unseen sort called dark matter. Astronomers don't know what it is, but they know it makes up about 23 percent of the universe. It is thought to have played a
crucial role in the development of the first galaxies.The rest of the cosmic mass-energy budget, some 73 percent, involves an even more exotic thing called
dark energy, which appears to work across large distances and in the opposite manner to gravity. This anti-gravity force, as it is sometimes referred to, seems responsible for the accelerating pace of the universe's ongoing expansion.Shadow of dark matter
Today's announcement directly concerns only normal matter. But astronomers have realized in recent years that this baryonic variety acts as a sort of shadow for the totally undetectable dark matter.
"We believe baryons are drawn to the gravity of the dark matter, so they trace the location of dark matter through space," Mathur said. "One provides a map to the other."
The new work does not address the composition of dark matter, however. One idea is that it is made of tiny but oddly massive particles. No one knows whether these might somehow be detected one day.
Mathur said the cloud that was observed, though vast, is probably part of even larger rivers of gas that wind between all the galaxies of the universe.
She explained why the baryons in the gas were so hard to find: At some point after the Big Bang, the baryons likely collided and ignited in a "heat shock" that created so much energy as to render the particles invisible. The task is made more daunting because the baryons are widely dispersed. Space is not empty, but it's not exactly crowded, either.
Relic of formation
The observations were made with NASA's Chandra X-ray Observatory and the Far Ultraviolet Spectroscopic Explorer (FUSE), as well as the European Space Agency's XMM-Newton satellite. The gas was detected indirectly, by noting how radiation from more distant galaxies was affected as it passed through the apparent cloud.
The reservoir of baryons around the Milky Way may be a relic of the original matter that formed our galaxy and its nearest neighbors, explains Amiel Sternberg of Tel Aviv University, Israel.
During the first few billion years of the universe, theory holds, about 20 percent of the matter came together under the influence of gravity to form groups and clusters of galaxies. Scientists think most of the remaining normal matter and dark matter formed a giant
web-like structure that connects the galaxy clusters together. However, Sternberg, who was not involved in the work but wrote an analysis of it for Nature, said more observations are needed to determine exactly how the newly detected gas might be related to primordial gas and the initial structure of the early universe.
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