Berkeley -- 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 UC Berkeley's EUV Explorer satellite, the puzzling extreme ultraviolet (EUV) emissions could indicate the presence of a large amount of hidden matter in these clusters, says astronomer Stuart Bowyer at the University of California at Berkeley.
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, that 80-90 percent of all 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 at UC Berkeley, 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 UC 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 report the discovery in the Nov. 22 issue of Science.
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 800,000 Kelvin to about 2 million Kelvin (1.4 million and 3.6 million degrees Fahrenheit, respectively). This is in contrast to the much hotter 93 million Kelvin (167.4 million sF) X-ray emitting gas.
The gas cloud extends well beyond the central region of the cluster, to a diameter of at least 30 arcminutes -- about 2.6 million light years across.
Last year Bowyer and Lieu reported finding evidence of a similar 500,000 Kelvin (900,000sF) gas in the Earth's nearest supercluster, the Virgo cluster some 60 million light years away. Though initially dismissed, a subsequent search through earlier ROSAT (Roentgen Satellite) 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."
Bowyer, Lieu and UC Berkeley astrophysicist Michael Lampton argue strongly against theories that have already been proposed to explain the Virgo emissions in a second article in this week's Science.
For the Virgo cluster, Bowyer, Lieu and their colleagues have calculated that the amount of EUV-emitting gas is too large to be simply the product of cooled X-ray emitting gas. They point out that at the slow rate at which Virgo's 20 million Kelvin (36 million sF) X-ray emitting gas cools off, it produces far too little gas that could emit EUV, even at the center of the cluster where the gas is so dense that it cools faster and creates a so-called "cooling flow."
"Maintenance of the EUV-emitting gas would require 35 times more cooling than that expected from the X-ray emitting gas," Bowyer says.
In the Coma cluster there is no cooling flow at the center, creating an even greater discrepancy.
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 to astronomers 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 the Coma and Virgo clusters, and thus whether they typically are enveloped in cooler gas.
"Now we need to look at more clusters to gather statistics, to determine what the range of behavior is in this zoo," Lieu says.
The research is supported by NASA.
Coauthors on the Coma cluster paper include Jonathan P. D. Mittaz of the Mullard Space Science Laboratory in England; astronomers Jeffrey O. Breen and Edward M. Murphy of the University of Virginia, Charlottesville; Felix J. Lockman of the National Radio Astronomy Observatory in Green Bank, West Virginia; and Chorng-yuan Hwang of UC Berkeley's CEA.