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Some 2 billion to 3 billion years after the big bang, the number of new stars in the universe mushroomed. In the past, many scientists suggested that the bursts of new stars were created because smaller galaxies collided – and as they did, their gravity caused gases in each galaxy to clump together and form stars. But now, a team of US and British astronomers says the main process that drove the boom was probably less chaotic.

Astronomers are keenly interested in this critical period because star formation rates directly affect the structure and evolution of galaxies. Researchers have focused on some of the biggest and brightest galaxies formed during this epoch and then looked at how the range of velocities across the width of the galaxies as they rotated. By comparing these changes from one edge of the galaxy to the other, they found that the transition was smooth, not the jumbled mess that collisions or mergers would trigger.

The next question: Was this observation unique to larger galaxies? The team, led by California Institute of Technology astronomer Daniel Stark, found a small galaxy from the same time span. The team found that the smaller galaxy also showed smooth changes in velocity across its expanse, pointing to a collision-free trigger for stars forming in and around it. Combined with the data from the larger galaxies, the results suggest that galaxies like the Milky Way may gain most of their mass by accretion rather than collision, the team says.

The results appear in today’s issue of the journal Nature.

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