ROSAT Observes Hot Gas in a Small Galaxy Group       STSCI-93-05

A visible light picture of a three galaxies known as NGC 2300 group,
combined with a false-color image (magenta), taken in X-ray light with
the ROSAT (an acronym for Roentgen Satellite, a joint project of
Germany, the U.S.A., and the U.K.) satellite.

The x-ray image, taken on April 25-27, 1992 with ROSAT's Position
Sensitive Proportional Counter instrument, shows that the small group
of galaxies is immersed in a huge cloud of hot gas, about 1.3 million
light-years in diameter. Astronomers estimate that the cloud has a mass
equal to 500 billion times that of the Sun, and is at a temperature of
about 10 million kelvins  (about 18 million degrees Fahrenheit).

Although such gaseous material has previously been seen in  rich
clusters of galaxies, this is the first time such a hot medium has been
found in a small group that is typical of most clusters in the
universe.

The existence of this gaseous envelope allows astronomers to measure
the total mass of the system. The surprising result is that the total
amount of material in the group is much larger than the sum of the mass
of the  hot gas and the stars in the galaxies.  This means that a
substantial amount of invisible "dark matter" envelops the system,
keeping the hot gas contained.

The NGC 2300 group is located about 150 million light-years from Earth
in the direction of the northern circumpolar constellation Cepheus.

Credit:  John Mulchaey (STScI)
         David S. Davis (NASA GSFC and Univ. of MD)
         Dr. Richard F. Mushotzky (NASA GSFC) 
         Dr. David Burstein (ASU)