The existence of galaxies poses much the same conundrum as life itself: They're here, but how did they arise? The prevailing idea is that large galaxies common in the universe, such as the Milky Way, formed within massive clumps of what scientists call cold dark matter, which attracted enough dust and gas to begin igniting stars by the bucketload. Then, over billions and billions of years, these protogalaxies grew by swallowing their neighbors or merging with them.
It's a logical idea. Yet, astronomers have found troublesome exceptions, such as evidence of Milky Way–sized galaxies fully formed just a few billion years after the big bang. How could these galaxies have grown so large so quickly--particularly when the young universe was wracked by enough heat and radiation to disrupt relatively delicate structures?
A team of astronomers led by Michael Disney of Cardiff University in the U.K. decided to see whether galaxies shared any common characteristics. If they did, the team reasoned, those characteristics could lead to a general rule governing their evolution. So, they surveyed 200 galaxies chosen at random but based on radio emissions from their hydrogen gas clouds, an approach that attempted to remove any visual bias in choosing the candidates, because many galaxies emitting radio waves in the hydrogen band are nearly invisible.
As the team reports in the 23 October issue of Nature, the galaxies differed in almost all of their characteristics--such as luminosity, shape, size, and gas content. But those characteristics all seemed to be regulated by a single, as-yet-undetermined quantity, which the researchers suspect could be mass. They found that if you measure a particular quantity for a galaxy, such as its size, you can infer all of its other main properties, such as luminosity, mass, and gas content. "What totally surprised us is the idea that such a diverse population is nevertheless controlled by a single, so-far-unidentified parameter," says Disney. "If you ask me, this throws the whole troubled theory of galaxy formation back into the melting pot."
Astronomer Stacy McGaugh of the University of Maryland, College Park, says it's remarkable how "galaxies exhibit remarkably regular behavior." And it's far from obvious how such a characteristic could emerge from the prevailing theory of galaxy formation.