GALAXIES CAME BEFORE CLUSTERS
Astronomers at the University of Birmingham, using observations from the orbiting ROSAT X-ray telescope, have discovered evidence that the afterglow of galaxy formation can be seen today in the hot gas trapped in groups of galaxies. The high temperatures seen in this gas can be explained by the energy released from the supernova explosions accompanying galaxy formation, but only if galaxies formed *before* they grouped together into galaxy clusters. This resolves a long-standing debate in cosmology over whether structures in the Universe have formed "bottom-up" (i.e. small objects forming first and then clustering) or "top-down" (large objects form first and then fragment). The results are published in the Jan.14th issue of Nature.
The Universe has a hierarchical structure: stars are grouped into galaxies, and most galaxies (including the Milky Way) congregate in small groups or large clusters which may contain up to thousands of galaxies. The space between the galaxies in these groups and clusters is not empty, but is filled with gas at a temperature of many millions of degrees Celsius, which radiates X-rays (see Figures).
A team led by Dr. Trevor Ponman of Birmingham University has used the X-ray telescope on board the German/UK/US ROSAT satellite to image these X-rays, and to compare the hot gas in small groups to that in large clusters. Most current cosmological theories predict that as the Universe expands, galaxies clump together to form groups, which in turn merge together to form clusters. In this picture the hot gas in small groups should look just like a scaled-down version of that in large clusters. In practice, the ROSAT images show that this is not the case. The gas in galaxy groups is hotter than expected, and is spread out more thinly than that in clusters, so that they shine less brightly in X-rays than would be expected. These differences in the hot gas are almost certainly due to the energy injected into it when the galaxies formed.
The formation of galaxies should be a spectacular process. As large numbers of stars are formed, the most massive of them live only a short time before exploding as supernovae, releasing large bursts of energy. This should heat the gas within the young galaxies to tens of millions of degrees, giving it enough energy to escape altogether, as high speed `galaxy winds'. These stream out of galaxies, heating the gas which surrounds them. This extra heat has little effect in large clusters (where the gas gets very hot anyway, due to the large amount of energy released as gas falls in the strong gravitational field), but in small groups it is very significant, and pushes the gas outward, causing the differences in X-ray brightness which are seen in the ROSAT images.
Calculations by the team, Drs. Trevor Ponman and Damian Cannon from the University of Birmingham, and Dr. Julio Navarro from the University of Victoria in Canada, show that energy has a bigger effect if it is released into the gas before the galaxy groups formed. (It is harder to push gas out of a group than it is to stop it falling in in the first place.) In fact, the amount of energy released when galaxies form could only account for the observations if it had heated the gas before the group was present. In other words, galaxies must have formed before groups and clusters.
This epoch of violent heating also explains why most galaxy formation stopped billions of years ago - there is plenty of gas left, but this does not generally form more galaxies, since most of it is now too hot to collapse under gravity and form stars. Most of the normal matter in the Universe has therefore been left in the form of hot gas filling intergalactic space.
Captions to Illustrations
Note on ROSAT
PublicationPaper: 'The thermal imprint of galaxy formation on X-ray clusters' Appearing in: 'Nature', January 14th 1999 Authors: Dr. Trevor Ponman & Dr. Damian Cannon
School of Physics & Astronomy, University of Birmingham, UK
Dr. Julio Navarro
Physics & Astronomy, University of Victoria, Canada
Illustrations for this release available at: http://www.sr.bham.ac.uk/public (from 10.00 GMT, Thursday 14 January)