ESA Science & Technology - Cosmic collisions - European HST scientists catch merging galaxies in the act

The cluster is 8 billion light years away, one of the most distant known so far and thus a key target for astronomers facing the problem of how galaxies formed when the Universe was young. The cluster's light has taken so long to reach us that astronomers see it now as it was when the Universe was less than half its present age. And what they see is an amazing picture.

"It has been a real surprise", says team leader Pieter van Dokkum, from Groningen and Leiden universities (The Netherlands). "Collisions had never been observed before at this frequency. Many of the collisions involve very massive galaxies, and the end result will be even more massive galaxies!".

It is a tale of a violent past of the Universe. Although during the collision the stars in the galaxies do not collide, their orbits are strongly disturbed by huge tidal forces caused by the gravitational pull. As a result, the 'parent' galaxies lose their shape and smoother galaxies are formed. Clearly defined spiral galaxies, for instance, produce large featureless elliptical galaxies. The whole merging process can take less than a billion years, a relatively short timescale in astronomy.

"The Hubble image shows the paired galaxies very close together, with distorted morphologies", explains Marijn Franx, from the University of Leiden. "We can even see streams of stars being pulled out of the galaxies. They are old stars in a young galaxy !".

The finding will appear in the 1 August issue of Astrophysical Journal Letters*. To the authors it strongly supports a Big Bang model prediction that says that large galaxies were formed from smaller ones in many 'generations' of mergers. It contradicts the idea that there was, in the past, a kind of 'galaxy boom' event in which all big massive galaxies were born at the same time.

As Franx states, "the evidence for the theories of galaxy formation through collisions had been strong, but circumstantial. Here we finally see a large number of galaxies caught in the act. If observed in other distant clusters, it would represent a general confirmation for a crucial aspect of our galaxy formation theories."

Collisions are much rarer today than they were in the past, but not impossible. Our own galaxy, the Milky Way, is a spiral one that is currently 'eating up' several small satellite galaxies. Within 5 to 10 billion years - some computer simulations show - the Milky Way may collide with the Andromeda galaxy, and the result would be an elliptical galaxy: far enough in the future that we won't have to worry about it!

*Article by P.G. van Dokkum, M.Franx, D. Fabricant, D.D. Kelson, G.D. Illingworth in the Astrophysical Journal Letters, 1 August 1999 (vol 520)

For more information please contact:

Dr Piero Benvenuti, HST (ESA) project scientist
Head of ST- ECF (Garching, Germany)
Tel: +49 89 3200 6291
Fax: +49 89 3200 6480
Email: pbenvenueso.org

Dr Marijn Franx, HST co-investigator
University of Leiden (The Netherlands)
Tel: +31 71 527 5870
Fax: +31-71-5275743
Email: franxstrw.leidenuniv.nl

Dr Pieter van Dokkum, HST co-investigator
University of Groningen and Leiden (The Netherlands)
Tel: +31 71 527 5861
Fax: +31-71-5275743
Email: dokkumstrw.leidenuniv.nl

A 'FAQ' (Frequently Asked Questions) section can be found at: http://www.strw.leidenuniv.nl/~dokkum/mergers/info_eng.txt