Gas reveals secrets of universe evolution

25 February 1999

New observations using the ROSAT X-ray telescope endorse a “bottom-up” model of the evolution of the universe.

Most ordinary matter in the universe is in the hot gas that makes up the intergalactic medium. ROSAT observed X-ray emission from this gas,
both in small groups of galaxies (where it has a temperature of around
10 million degrees) and in large clusters (around 100 million degrees). The gas in the small groups contained more energy than expected. This extra energy comes from galaxy formation,
and modelling the evolution of the system showed that it had to be injected before the group formed. This supports a “bottom-up” picture,
with small objects being formed,
then clustering together.

This is the
first time that such energy comparisons have been made.
Observations are difficult because the
intergalactic gas is so thin.
The result has important implications for cosmology and
may help to explain why we see no galaxies forming today.
The original phase of galaxy formation heated up the
intergalactic gas so much that it is too energetic to collapse and
form stars.

The ROSAT satellite is a collaboration between Germany,
the UK and the US. After eight years in orbit and recording 20 times more X-ray sources than had been seen before,
ROSAT ceased to function in December 1998.

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