This massive collision between two spiral galaxies, NGC 6050 and IC 1179, was seen by the Hubble Space Telescope. Credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and K. Noll (STScI)

Nearly all massive galaxies have undergone at least one major merger since the Universe was 6 billion years old, according to the largest survey of their shape and structure to date.

Reconstructing how galaxies have merged is a vital part of understanding their evolution. But the survey could also help to settle a decades-long debate over whether intense episodes of star formation, known as starbursts, are triggered by large-scale galactic mergers or by processes within individual galaxies.

The survey used data from the Hubble Space Telescope to study 21,902 massive galaxies as they would have appeared when the Universe – now roughly 13.7 billion years old – was between 5.2 billion and 11.2 billion years old.

Mergers tend to leave galaxies with an asymmetric, clumpy appearance, and, based on a computer analysis of these characteristics, astrophysicist Christopher Conselice at the University of Nottingham, UK, and his colleagues found evidence for at least 2,000 mergers during that epoch.

They also found a close match between the timing of the mergers and the starburst episodes. Conselice adds that a significant decline in mergers, when the Universe was about 7 billion years old, coincides with the time of a previously identified decrease in star formation.

Because each of the galaxies was imaged at just one moment in their evolution, the team extrapolated its findings to conclude that almost all of the galaxies will have undergone a merger by the present day.

Star factories

Galaxies can build stars out of their own internal reservoirs of gas, or by receiving a new injection of gas and kinetic energy when one massive galaxy smashes into another. In today's Universe, such massive mergers are rare. But in the past, says Conselice, these mergers have created starburst conditions that spawned stars at a rate of about 200 solar masses per year — 100 times our own Milky Way Galaxy's current star-formation rate.

"Mergers fundamentally transform these galaxies' structure and how they evolve," says Conselice. "Two gas-rich galaxies smash together and gas clouds collide, forming stars."

The results, to be published in the Monthly Notices of the Royal Astronomical Society, are based on over seven times the number of galaxies included in the most recent comparable study

However, some astronomers say that it's still not clear that mergers are the root cause of starbursts. "Whether distant galaxies' irregular appearance is due to mergers, or irregularly distributed sites of very active star formation, is difficult to determine," says Yale University astronomer Pieter van Dokkum.

Mark Dickinson, an astronomer at the National Optical Astronomy Observatory in Tucson, Arizona, notes that there is now evidence that high star-formation rates take place over hundreds of millions of years rather than in 10-million-year bursts. "If the starbursts were driven by mergers, you would expect the latter," says Dickinson.

Conselice hopes that more data will strengthen his case. He says the next step is a similar analysis using future data from the Hubble Space Telescope's Wide Field Camera 3, which is due to be installed in May 2009. Operating in the infrared spectrum, the camera will collect light from more ancient epochs of the Universe's history.

But settling the matter once and for all may require observations from both a new class of ground-based 30-metre telescopes and the orbiting James Webb Space Telescope, due to launch in 2013.