Galaxies become monsters by repeated mergers

Space 6 December 2005

New Scientist Default Image

Observations of different pairs of galaxies show various stages of the merger process, from the galaxies’ approach (upper left) to their union (lower right)

(Image: P van Dokkum/Yale/NOAO/AURA/NSF)

Half of the largest galaxies near the Milky Way have undergone relatively recent mergers, suggests a new study.

The research offers the first solid evidence that the most massive galaxies in the universe grew gradually – rather than in a brief growth spurt soon after the big bang.

“The best-developed computer models have predicted for a long time that galaxies build up their mass slowly,” says Pieter van Dokkum, an astronomer at Yale University in New Haven, Connecticut, US. In the models, galaxies grow larger by repeatedly merging with others over the 13-billion year history of the universe.


But previous studies have failed to turn up evidence of this steady growth, showing that only about 2% of the largest galaxies have merged within the past 2 billion years. “People argued that the galaxies built up mass early on and that nothing much has happened since,” van Dokkum told New Scientist.

Asymmetrical galaxies

Now, van Dokkum and colleagues have studied the most comprehensive sample of merging galaxies ever made using data from 4-metre telescopes at Kitt Peak National Observatory in Arizona, US, and the Cerro Tololo Inter-American Observatory in Chile.

They found that 53% of the 126 largest galaxies within 1 billion light years of the Milky Way appear asymmetrical, suggesting they have undergone recent mergers.

“If you look at computer simulations of how mergers progress, gravitational forces create large plumes of stars in tails that are stripped from interacting galaxies,” he says. “By looking at things that are not regular, it means they have been disturbed.”

You can view a computer simulation of the formation of a single, large galaxy from the merger of two smaller ones in MPG, AVI or Quicktime formats (Credit: M. Steinmetz). The simulation represents the full, 13.7-billion-year age of the universe.

The new research is published in the December issue of the Astronomical Journal.

Journal reference: Astronomical Journal (vol 130, p 2647)

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