The early universe was a rough-and-tumble place. Galaxies smashed together with much more regularity than they do today, and the insides of galaxies were chaotic, clumpy pods of stars. It was no place for an orderly, delicate swirl of a galaxy like the Milky Way or Andromeda.
By scanning hundreds of galaxies that existed just a few billion years after the big bang, however, a group of astronomers has turned up a diamond in the cosmic rough. The researchers found a rare early galaxy with pronounced spiral arms, which they reported in the July 19 Nature. (Scientific American is part of Nature Publishing Group.) And that galaxy's unique circumstances may help explain why spirals are so infrequent at that epoch. The newfound galaxy, known as BX 442, was identified in Hubble images as a spiral that existed three billion years after the big bang. It appears to fit the bill for a variety called a grand-design spiral, in which pronounced spiral arms lend a well-defined shape to the galaxy's disk of stars.
Spirals are common in the modern universe, but as astronomers gaze across the cosmos at objects farther and farther away—and hence further and further back in time—spiral structure starts to peter out. Instead of orderly swirls, astronomers see lumpy, blobby galaxies going through the cosmic equivalent of an awkward phase. But somehow a regular spiral structure was imprinted on BX 442, perhaps by a recent grazing encounter with a much smaller galaxy. “What seems to set it apart, as best as we can tell, is that it has this little companion galaxy off to the side,” says lead study author David Law, an astrophysicist at the University of Toronto. If the companion galaxy were the trigger, the spiral arms would “probably fade away within about 100 million years or so,” Law says. The transitory nature of a spiral structure at that epoch could explain why spirals are so rare.
BX 442 could have also generated its own spiral structure without a nudge from its neighbor. Clumps of stars and gas within a galaxy can cause spirals to form, and BX 442 appears to contain at least one large clump along one of its spiral arms.
It may be that numerous different mechanisms can shape a spiral galaxy. Many more examples from different cosmic epochs should be accessible for study once next-generation observatories, such as NASA's James Webb Space Telescope, come online.
COMMENT AT ScientificAmerican.com/oct2012
This article was originally published with the title Primordial Pinwheel.