JANUARY 22, 1999

Hot afterglow of galaxy formation seen in X-rays

The afterglow from violent explosions that led to the creation of galaxies can still be seen today and the evidence resolves a nagging question among astronomers, according to a study co-authored by University of Victoria astronomer Dr. Julio Navarro.

Navarro and colleagues at the University of Birmingham say the extremely hot gas (created by supernova explosions) they've observed trapped in galaxy groups answers whether structures in the universe start out small and expand, or whether they begin as large objects that then fragment.

In findings published in the Jan. 14 edition of the British science journal Nature, the team concludes that individual galaxies must have preceded the formation of groups and large clusters of galaxies.

In the structure of the universe, stars are grouped into galaxies and most galaxies (including the Milky Way) come together in groups and clusters containing thousands of other galaxies. The space between galaxies is filled with gas as hot as 10 million degrees Celsius that radiates X-rays.

Since X-rays are invisible to optical telescopes, the researchers turned to the orbiting ROSAT satellite and its telescopes, which are specially designed to pick up cosmic X-rays and extreme ultraviolet bands.

The team's calculations lead them to conclude that the amount of energy released when galaxies form could only account for the X-ray observations if it had heated the gas before the group of galaxies was present. In other words, galaxies had to have formed before groups and clusters.

The astronomers also conclude that the presence of the hot gas explains why most galaxy formation stopped billions of years ago. That's because most of the remaining gas is too hot to collapse under gravity and form stars.

The ROSAT observatory was built by a consortium of German, British and American universities. After its launch in 1990, ROSAT took more than 100,000 images of cosmic X-ray sources. It was decommissioned just before Christmas after exceeding its nominal two year lifetime.

Illustrations of some of the images received from ROSAT are available on the Web at <>.

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