by Robert Sanders
Extreme ultraviolet observations of two massive nearby clusters of galaxies have revealed a vast cloud of unsuspected"cool" gas permeating them, a surprise mirroring the discovery some 30 years ago of a hot, X-ray emitting gas enveloping these same clusters.
Discovered by Berkeley's EUV Explorer satellite, the puzzling extreme ultraviolet emissions could indicate the presence of a large amount of hidden matter in these clusters, says astronomer Stuart Bowyer.
The findings could have a significance equal to or exceeding that of the detection of X-ray emissions by the early orbiting X-ray satellites, which eventually led astronomers to conclude that the hot cluster gas, primarily hydrogen, equaled or substantially exceeded the mass of all the visible galaxies in the cluster.
Bowyer says that the newly discovered EUV emissions are the first evidence of a large cloud of cooler matter in clusters, totalling as much as 10 trillion of our suns. The discovery could help resolve a long-standing problem of clusters, which is that 80-90 percent of their mass has gone undetected.
"The EUV emitting gas and the gas it cooled to, unless it is being constantly reheated, represents a substantial fraction of or is at least equal to the X-ray emitting gas," says Bowyer, a professor in the graduate school, former director of the Center for Extreme Ultraviolet Astrophysics and a researcher at the campus's Space Sciences Laboratory.
At the least these emissions are evidence of unknown physical processes operating in dense clusters, says Richard Lieu, formerly with Berkeley and now an assistant professor of physics at the University of Alabama, Huntsville.
"The very existence of what we have detected is a major puzzle," he says.
Lieu, Bowyer and a team of astronomers from several other institutions reported the discovery in Science in fall 1996.
Turning EUVE's Deep Survey telescope on the Coma cluster 300 million light years distant, in the constellation Coma Berenices, the team detected EUV emissions that indicate a"cool" intracluster gas at temperatures ranging from about 1.4 million to 3.6 million degrees Fahrenheit, respectively. This is in contrast to the much hotter 167.4 million öF X-ray emitting gas.
Last year Bowyer and Lieu reported finding evidence of a similar 900,000 öF gas in the Earth's nearest supercluster, the Virgo cluster some 60 million light years away. Though the finding was initially dismissed, a subsequent search through earlier observations of the cluster turned up corroborating evidence. The discovery of a similar gas in the Coma cluster adds even more evidence for the existence of a"cooler" gas in clusters of galaxies.
"The story has just switched from, 'the data are clearly wrong,' to 'no, it's right but unexplainable'," Bowyer says."It's now up to the theorists to explain where this gas comes from and where it's going."
If there is a large amount of cool matter in clusters, it would help clear up a major problem with these galactic groupings, that the visible mass is insufficient to keep the cluster from flying apart.
The fact that clusters don't fly apart suggests that some invisible mass, termed cold dark matter, comprises 80-90 percent of most clusters. The debate still rages over whether this dark matter is composed of MACHOs -- massive compact halo objects such as dim and dying stars -- or bizarre and fanciful elementary particles called WIMPs -- weakly interacting massive particles.
"We have on one hand a gravitational mass problem, because we need a lot of mass to make the cluster a bound system," Lieu says."On the other hand, we have an EUV-emitting gas constantly cooling. Both suggest lots of cold matter, and our evidence suggests that a fair amount of it may well be in the form of normal baryonic matter. Perhaps all the missing mass is there in the form of ordinary matter, and we just haven't looked hard enough."
In fact, the EUV Explorer satellite saw the emissions in a new window on the universe, the extreme ultraviolet. The next step, Lieu says, is to determine whether most or all clusters of galaxies emit EUV like Coma and Virgo, and whether they typically are enveloped in cooler gas. Among coauthors of the Coma paper is Chorng-yuan Hwang of the Center for EUV Astrophysics.