An ancient spiral galaxy offers another tantalizing clue to how nature might create these grand designs.
Take a look through your telescope at our local universe, and you’ll find lots of beautiful, grand-design spirals. Messier objects M51, M81, and M101 all come to mind — perhaps M101 especially because of the supernova that exploded there last year. But if your telescope reached a bit further than our cosmic backyard, you would find that these elegant spiral disks gradually give way to clumpy, irregularly shaped galaxies. So when astronomers found a grand-design spiral gleaming from the universe when it was only 3 billion years old, they were pleasantly shocked.
Despite its great distance, the spiral design of the galaxy Q2343-BX442 is clear in observations by the Hubble Space Telescope and the Keck II Telescope in Hawaii. Measurements made with Keck’s OSIRIS spectrograph confirm that the arms are indeed rotating around a central bulge, so the apparent spiral shape doesn’t result from a chance alignment of two disk-shaped galaxies, the study’s authors conclude in the July 19th Nature.
By all appearances, BX442 is a pretty normal galaxy. With a diameter of 52,000 light-years, the spiral is roughly half the size of the Milky Way. The arms churn out stars at a rate 30 times higher than our galaxy’s neighbors, but these high rates are comparable to other star-forming clumps in the ancient universe.
The only thing that’s not normal about BX442 is the fact that it exists. The galaxy is the only one with a regular spiral shape out of 306 galaxies the team examined with Hubble, all of which lie a similar distance away. Astronomers think spiral patterns are rare in the early universe because newly formed galaxies tend to be turbulent and therefore unfriendly to organized structure. Many of a galaxy’s stars will circle the nucleus in well-behaved elliptical orbits, but other stars will follow random paths that don’t match the galaxy’s overall rotation. As a result, the galaxy becomes “puffy,” making it difficult to shepherd the stars and gas into a spiral pattern.
But BX442 is puffy, the Keck spectroscopic observations reveal. So how did the fluffy galaxy arrange its gas and stars into such sweeping spiral arms? Based on computer simulations, the authors suggest that the gravitational pull of a passing dwarf galaxy (seen to the upper left of BX442 in the image) may have disrupted the galaxy in just the right way to form the spiral pattern. The catch is that the spirality is therefore short-lived, lasting less than 100 million years in the simulation, which might explain why spirals are so hard to find in the early universe.
A galaxy’s disk must be relatively stable to start with if you want to make a spiral, says James Bullock (University of California, Irvine), an expert in galaxy evolution not involved in the study. Last year, Bullock and his colleagues suggested that the Milky Way’s spiral structure might result from its interaction with the Sagittarius dwarf galaxy. Stable galaxies that interact with a neighbor in just the right way to form spiral arms are exceptional in the early universe, he says. “Law and his colleagues seem to have found one of these rare gems!”