15 July 1999

Cosmic Collisions - Watching Merging Galaxies in the Act

by Kate Melville

Astronomers using the NASA Hubble Space Telescope have obtained new images of more than a dozen distant galaxies banging into each other. These colliding objects are part of a large concentration of galaxies, or galaxy clusters. Though collisions have been observed in other clusters, this particular cluster displays the greatest number ever seen. And to astronomers the finding indicates that, at least in some cases, big massive galaxies form from collisions between smaller ones, in one big and never ending crash and bash cycle.

The Hubble Space Telescope studied 81 galaxies in the galaxy cluster MS1054-03, 13 of which are remnants of recent collisions or pairs of colliding galaxies. The 10-meter W.M. Keck Telescope was used to select these 81 cluster galaxies.

The cluster is 8 billion light-years away, one of the most distant known, making it a key piece in the how-were-galaxies-formed puzzle. 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.

"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."

Although during the collision the stars in the galaxies do not run into each other, 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 time scale 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 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 spiral galaxy, the Milky Way, is currently "eating up" several small satellite galaxies. Some believe that within 5 to 10 billion years the Milky Way may collide with the Andromeda galaxy, and the result would be an elliptical galaxy.