From: yee@atlas.arc.nasa.gov (Peter Yee)
Subject: Mysterious concentration of dark matter discovered [Release 93-1] (Forwarded)
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Originator: yee@atlas.arc.nasa.gov
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Organization: NASA Ames Research Center, Moffett Field, CA
Date: Tue, 5 Jan 1993 07:25:31 GMT

Paula Cleggett-Haleim
Headquarters, Washington, D.C.             January 4, 1993
(Phone:  202/358-0883)                     11:30 A.M. EST

Jim Elliott  
Goddard Space Flight Center, Greenbelt, Md. 
(Phone:  301/286-6256)

RELEASE:  93-1

MYSTERIOUS CONCENTRATION OF DARK MATTER DISCOVERED

	Astronomers have discovered a huge concentration of 
mysterious "dark matter" using the international ROSAT X-ray 
observatory.

	The discovery appears to confirm previous suggestions of 
where most of the dark matter in the universe may be concentrated, 
namely in and around small groups of galaxies, according to 
astronomers.

	"The new findings add much weight to the theory that most of 
the mass of the universe consists of dark matter, the precise 
nature of which remains unknown to scientists," said John S. 
Mulchaey of the Space Telescope Science Institute, Baltimore, Md., 
and the University of Maryland, College Park.

	Dark matter is believed to exist although it has never been 
seen because it emits no radiation.  Its existence has been 
inferred because fluctuations observed in the Big Bang -- the 
explosion presumed to have created the universe -- did not have 
sufficient gravitational pull to cause ordinary matter to coalesce 
immediately.  It is presumed, therefore, that unknown, or dark, 
matter that would be attracted to the weak gravity of the 
fluctuation got the process started.

	Scientists also presume the existence of the invisible 
material because the speeds with which certain galaxies are moving, 
their rotational patterns and their shapes cannot be accounted for 
by the possible gravitational pull of observable matter close 
enough to influence them. 

	Dark matter may, in fact, constitute up to 95 percent of the 
mass of the universe.  Confirming its existence and volume would 
mean that there might be enough mass in space to "close the 
universe."  This means that eventually the expansion of the 
universe, which is being slowed by the pull of gravity, would come 
to a halt or nearly so. 

	The discovery announced today was made with x-ray pictures of 
three galaxies known as the "NGC 2300 group," located about 150 
million light-years from Earth in the direction of the northern 
constellation Cepheus.  (One light-year is the distance light 
travels in 1 year, approximately 5.8 trillion miles.)

	The images were taken with the Position Sensitive 
Proportional Counter instruments on ROSAT during April 25-27, 1992, 
according to Dr. Richard F. Mushotzky, NASA Goddard Space Flight 
Center, Greenbelt, Md.  They show that the small group of galaxies 
is immersed in a huge cloud of hot gas, about 1.3 million light-
years in diameter, he explained.

500 Billion Times The Sun

	Astronomers estimate that the cloud has a mass equal to 500 
billion times that of the sun and is at a temperature of 
approximately 18 million degrees Fahrenheit (10 million degrees 
Kelvin).

	"A cloud like this would have dissipated into space long ago, 
leaving nothing for us to detect, unless it was held together by 
the gravity of an immense mass," Mushotzky said.  "The mass 
required to restrain the cloud is about 25 times greater than the 
mass of the three galaxies that are present."

	This is the first time that a multimillion degree gas has 
been found to pervade a small group of galaxies, the Goddard 
astronomer explained, although such gas has been detected in larger 
clusters of galaxies by earlier satellite telescopes such as NASA's 
High Energy Astronomy Observatory-2.

	Results from the Hubble Space Telescope and other satellites 
already have shown that if the leading version of the Big Bang 
theory is correct, then 90 to 95 percent of the mass in the 
universe must be in the unknown "dark" form, astronomers explain.  
This means, they say, that there must be 10 to 20 times as much 
dark matter by mass as ordinary matter, which scientists call 
"baryonic matter."

	However, in locations observed previous to this work, the 
ratio of dark to ordinary matter has been, at most, a factor of two 
to four.  Much of that work concentrated on studies of the most 
prominent groups of galaxies in space, the "rich clusters," 
according to astronomers.  "Rich clusters" are huge aggregates of 
hundreds to thousands of galaxies.

	"Although they stand out the best and therefore, have been 
the subject of most of the research on dark matter, they are not 
representative of the universe, because most galaxies are in small 
groups like the NGC 2300 group," Mulchaey explained.

	"The universe is like the pre-industrial United States, in 
which the most conspicuous population concentrations were in a few 
big cities, but in which most people actually lived in small towns 
and rural America," he said.

	If small groups of galaxies all have comparable ratios of 
dark to ordinary matter, meaning a factor of 12 to 25 as found in 
the NGC 2300 group rather than a factor of four or less as found in 
rich clusters of galaxies, Mulchaey explained, then the mystery of 
where most of the dark matter in the universe is located has been 
solved.

Closed Universe

	That would mean that there might be enough mass in space to 
"close the universe," indicating that eventually the expansion of 
the universe, which is being slowed by the pull of gravity, would 
come to a halt or nearly so, he continued.

	Although some scientists have suggested that the dark matter 
might be preferentially concentrated in small groups of galaxies, 
direct evidence was lacking until ROSAT observations were made, 
according to the astronomers.  Further work is needed to confirm a 
discovery of this apparent magnitude, they admit.

	This work represents the first case in which the amount of 
dark matter in a small group of galaxies has been determined 
accurately, the science team reported. 

	"For confirmation, we need repeated x-ray observations from 
space of the NGC 2300 group and other representative small groups 
of galaxies," Mushotzky explained.  "Meanwhile, if we are right, 
the theorists need to start thinking about why there is much dark 
matter where there is little ordinary matter (i.e. in small groups 
of galaxies), and there is much less dark matter where there is a 
lot of ordinary matter (in rich clusters of galaxies)," he said.

	The discovery is to be announced today at a meeting of the 
American Astronomical Society in Phoenix, Ariz., by Mulchaey; David 
S. Davis, NASA Goddard Space Flight Center and the University of 
Maryland; Dr. Richard F. Mushotzky; and Dr. David Burstein, Arizona 
State University, Tempe.

	Mulchaey and Davis are University of Maryland graduate 
students who work with Dr. Mushotzky's research group at the 
Goddard Space Flight Center.  Mulchaey currently is employed at the 
Space Telescope Science Institute.  Dr. Burstein is a Professor of 
Astronomy at Arizona State.  

	ROSAT, an acronym for Roentgen Satellite, is a joint project 
of Germany, the United States and the United Kingdom.  It was 
launched on a Delta II rocket from the Cape Canaveral Air Force 
Station, Fla., on June 1, 1990.

                         - end -

EDITORS NOTE:  A video and photograph to illustrate this story is 
available by calling NASA's Broadcast and Imaging Branch on 
202/358-1741.  

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