Astronomers have discovered an unexpected cache of spiral galaxies that appear to have formed recently, long after the period early in the history of the universe that most galaxies were thought to have been created.

These younger galaxies are big and bright, like our own Milky Way. The reigning hypothesis of galaxy formation holds that such well-established spirals would have formed about 13 billion years ago, shortly after the Big Bang.

But the new discovery of a group of 15 spirals that look to be much younger may upset that thinking.

Though they are just as luminous and large as normal spirals, these galaxies appear to have all the hallmarks of youth. They don't have nearly as many heavy elements, called metals, as would be expected for older galaxies. All elements heavier than helium are created through nuclear reactions inside stars, and elements heavier than iron are made when stars die in supernovae. Thus, the longer a galaxy has been alive and forming stars, the longer its stars have been churning out heavy metals, so the greater abundance you would expect.

But the chemical abundances of the newly discovered galaxies would suggest they are only about 3 billion or 4 billion years old.

"We're not saying there's a complete breakdown in the theory of galaxy evolution, but that these objects do run counter to the standard model," said Indiana University astronomer John Salzer, the lead author of a paper detailing the study in the April 10 issue of the Astrophysical Journal Letters. "These potentially could have formed much more recently. The significance is that they give us the opportunity to study galaxy formation and evolutionary processes which would otherwise be veiled at the vast distances involved in looking at things at these early stages of the universe."

If this age estimate proves to be correct, these galaxies could present an unprecedented opportunity to study massive spirals at a younger point in their evolution, and at closer range, than is generally possible.

"These objects may represent a unique window on the process of galaxy formation, allowing us to study relatively nearby systems that are undergoing a phase in their evolution that is analogous to the types of events that, for most galaxies, typically occurred much earlier in the history of the universe," Salzer said. Another possible explanation for the unusual galaxies is that they are the product of collisions between two smaller galaxies. This could explain why they have low levels of metals, since dwarf galaxies tend to have few heavy elements to begin with. Also, the chaos of a collision can stimulate a burst of star formation, which might explain why the galaxies appear so bright. However, this scenario would require the galaxies' luminosity to multiply by about 30 times after merging ? a boost much greater than is usually seen.

"It's just hard for me to fathom, and hard for models to account for, an increase by a factor of 30," Salzer told SPACE.com.

Salzer discovered the galaxies through the Kitt Peak National Observatory International Spectroscopic Survey (KISS), a multi-year project to observe more than 2,400 star-forming galaxies. He and his team have requested time on the Hubble Space Telescope to try to distinguish between the two possible explanations by hunting for evidence that the galaxies have undergone collisions.

The KISS project was funded by the National Science Foundation.