A cosmic phenomenon involving pockets of hot gas in space which
appear not to cool down has been puzzling astronomers for three
decades. Now new research by Dr Christian Kaiser at the University
of Southampton and Professor Marcus Br¸ggen of the International
University Bremen, Germany, shows that the energy of the hot gas
is actually replenished by jets emitted by black holes.

Their research, which is published in the current issue of the
science journal Nature (Volume 418, 18 July 2002) and an image
from which is featured on the journal’s front cover, involved
extensive computer simulations of the turbulent break-up of jets
in the hot gas of galaxy clusters.

Galaxy clusters are created when a large cloud of gas collapses
under its own gravity, and each can contain around 1,000 galaxies,
such as our own Milky Way, and a large amount of very hot,
leftover gas. This gas radiates X-rays, which is how we can detect
it, and these X-ray emissions should lead to the hot gas cooling
down within a few billion years to form stars in even more

However this is not the case. In their study Christian Kaiser and
Marcus Br¸ggen found that the key to this lies in jets of gas
emitted from massive black holes that lie at the centre of
so-called ‘active’ galaxies within many galaxy clusters. The
black holes swallow up any gas coming close to them and liberate
enormous amounts of energy in the process. This energy drives
very narrow outflows of gas at velocities close to the speed of
light, the ‘jets’.

These jets can carry an amount of energy equivalent to 10
billion supernovae, the violent explosions at the end of the
life of a massive star. This is more than enough to re-heat the
hot gas in galaxy clusters.

"Our results indicate that the black holes in active galaxies
behave like cosmic thermostats," says Christian Kaiser. "The hot
gas in a galaxy cluster cools down to low temperatures and flows
to the cluster centre. There the black hole is waiting. It
swallows some of the cold gas and the energy from this process
drives jets into the cluster gas further out. This heats the
remaining gas and drives it away from the cluster centre. Thus
the black hole runs out of fuel and shuts down. After the gas
has cooled down once more, the whole cycle starts again."

The researchers employed the most advanced software currently
available for the simulation of fluid flows on the 128-processor
parallel supercomputer of the UK Astrophysical Fluids Facility
at the University of Leicester.

They are currently continuing their investigations by extending
their computer simulations and hope to pin down the exact details
of this ‘cosmic thermostat’ and thereby solve one long-standing
mystery in astronomy.

Notes for editors:

The paper ‘Hot bubbles from active galactic nuclei as a heat
source in cooling-flow clusters’ is published in the science
journal Nature this week (Volume 418, 18 July 2002).

Images from the computer simulations are available from Christian
Kaiser or Sarah Watts, contact details below.

For further information:

Dr Christian Kaiser

Department of Physics and Astronomy

University of Southampton

023 8059 2073; email crk@astro.soton.ac.uk

Professor Marcus Brueggen

Bremen, Germany

tel. +49 421 200-3251; email m.brueggen@iu-bremen.de

Sarah Watts

External Relatio

University of Southampton

023 8059 3807; email S.A.Watts@soton.ac.uk

The University of Southampton is a leading UK teaching and research
institution with a global reputation for leading-edge research and
scholarship. The University, which celebrates its Golden Jubilee
in 2002, has 20,000 students and over 4,500 staff and plays an
important role in the City of Southampton. Its annual turnover is
in the region of £215 million.

[http://www.externalrelations.soton.ac.uk/media/coolflow.jpg (58KB)]
The figure shows the distribution of the gas density in one of
the computer simulations. Blue indicates low densities, while red
stands for high densities. The initial jet can be seen at the
bottom. Further out, turbulent eddies have completely destroyed
the smooth structure of the jet. The energy carried by the jet is
distributed over a large volume in the process.