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Primordial Pinwheel: Astronomers Spot Oldest Prominent Spiral Galaxy Yet

Three billion years after the big bang most galaxies were clumpy and odd-shaped, but at least one had already assumed a familiar form, and may elucidate how modern galaxies got their shapes
Spiral galaxy at z = 2.18, in the early universe



David Law; Dunlap Insitute for Astronomy & Astrophysics

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.

But by scanning hundreds of galaxies that existed just a few billion years after the big bang, a group of astronomers has turned up a diamond in the cosmic rough. The researchers found a rare early galaxy with pronounced spiral arms, they reported in the July 19 issue of Nature. And that galaxy's unique circumstances may help explain why spirals are so rare at that epoch. (Scientific American is part of Nature Publishing Group.)

The newfound galaxy, known as BX 442, was identified as a spiral in Hubble images targeting 306 galaxies at redshifts of 1.5 to 3.6, corresponding to times roughly 9.3 billion to 11.9 billion years ago. (Redshift is a measure of cosmological distance that indicates how much an object's light has been stretched toward longer wavelengths as it traverses an expanding universe.) BX 442, the only identifiable spiral of the bunch, resides at redshift 2.18, some 10.7 billion years ago, or just 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.

"We see the grand-design spiral pattern in this galaxy, and it's kind of astounding. We hadn't expected to find that," says lead study author David Law, an astrophysicist at the University of Toronto. "The 300-odd other galaxies in our own survey, and many others in different surveys, don't show this pattern."

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, which is why BX 442 makes for such an interesting case study.

"It is unique, they're right about that," says astronomer Bruce Elmegreen of IBM's T. J. Watson Research Center in Yorktown Heights, N.Y. "It's been known for awhile that there are disks at these redshifts. Disks are not surprising, and usually when there are disks locally there are spirals in the disks," he adds. "But at these redshifts spirals have not been seen, and that had been a puzzle."

Instead of orderly swirls, astronomers see lumpy, blobby galaxies going through the cosmic equivalent of an awkward phase. "At these redshifts, the galaxy population if you look with Hubble mostly looks irregular and peculiar," says astrophysicist Christopher Conselice of the University of Nottingham. "Most of those galaxies are probably undergoing mergers, smashing together." And their stars are not usually confined to a flat, thin, evenly rotating disk.

BX 442, too, appears to have a somewhat chaotic stellar population, as if it its contents have been churned up. But somehow a regular spiral structure was imprinted on the galaxy's stars, perhaps by a recent grazing encounter with a much smaller galaxy. "What seems to set it apart as best as we can tell is it has this little companion galaxy off to the side," Law says of the rare spiral.

A study last year showed that the Sagittarius Dwarf Galaxy, a satellite of the Milky Way, could be responsible for some of the spiral structure of our own galaxy. As Sagittarius fell inward and passed through the plane of the larger galaxy, its impact would have been strong enough to disrupt a uniform disk of stars and morph it into a familiar spiral. A similar process could explain the atypical spiral nature of BX 442. Using a computer simulation, the researchers found that an interaction could indeed stir up spiral structure in the distant galaxy. But that transformation would not be permanent if indeed the companion galaxy were the trigger. "If that's the case, then what we're seeing now will 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 Law and his colleagues found just one spiral in their galactic survey. In fact, BX 442 may have evolved into a different shape in the billions of years since it emitted the light now reaching Earth. "It's impossible to say what happened to that spiral galaxy," Conselice says. "It's not like the galaxy types are set in stone."

BX 442 could also have generated its own spiral structure without a nudge from its neighbor, Elmegreen notes. 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. "They perturb everything around them, and each clump sort of makes its own tidal tail," Elmegreen says. "That's a rather easy way to get three spiral arms, and this galaxy has three arms."

It may be that numerous different mechanisms can shape a spiral galaxy. Many more examples should be accessible for study once next-generation observatories, such as NASA's James Webb Space Telescope, come online. Until then, astronomers will have to rely on unique specimens such as BX 442 to help clear up which are the dominant forces creating spiral structure at different times in the universe's history. "We don't really have a good explanation for how the spiral arms exist," Conselice says. "There's a lot of questions that we just haven't answered."

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