18 July 2012
Behold, the cosmic Stone Age. In this brutish era 11 billion years ago, galaxies are forming – primitive objects that appear as incoherent jumbles or featureless blobs. All except one. Among the rough-hewn masses sits a single sophisticated spiral with three shapely arms. It seems as finely sculpted as the most glamorous galaxies of today.
“When we saw it we were astounded,” says David Law at the University of Toronto. “Common wisdom was that these things don’t exist.”
Law and his colleagues spotted the cosmic anachronism, dubbed BX442, in infrared images from the Hubble Space Telescope. In their observations, the team counted 306 galaxies whose light has been so stretched and reddened by the expansion of the universe – a phenomenon known as redshift – that it must have been emitted a mere three billion years after the big bang. Of the lot, only BX442 looks like a spiral. Other surveys of such ancient times have turned up no spiral galaxies at all.
Galaxies of this era tend to be turbulent places, where gas and stars churn violently. Such dynamism should prevent any delicate spiral structures from forming. For example, some modern spirals are thought to be carved as gravity gathers their stars and gas into relatively crowded density waves, which ripple through the galactic disk. But the rapid random motions within primordial galaxies such as BX442 should overwhelm that gentle process.
Could the stunning spiral somehow be a closer, more mature galaxy just pretending to be young and far away? Law and his colleagues checked the galaxy’s redshift to make sure. “We have 16 independent measurements of redshift,” says Law, and they all agree.
Law then wondered whether the spiral shape might be a mere fluke, a chance arrangement of colliding lumps that mimics a more mature galaxy. Using theKeck telescope in Hawaii, the team sampled the spectrum of light at different spots across BX442, which revealed that the whole thing is rotating as a unified disk of gas and stars. It is a true spiral galaxy.
Better still, the galactic beauty is a “grand design” spiral. That means the object has clearly defined arms stretching all the way from core to outskirts, unlike most spiral galaxies, including the Milky Way, which are broken-up “flocculent” spirals.
A smudge in the Hubble images may be a vital clue to the galaxy’s comely shape. The group’s observations show a small galaxy in the process of merging with BX442. Today’s grand-design spirals often have merging companions too, says Law, and astronomers have suggested that the right kinds of mergers could stir up spiral patterns.
When two great arrays of stars collide, you might expect the result to be a big mess. But the group simulated this process for BX442 and found that if the merging companion isn’t too heavy and it approaches along the right track, its gravity could create spiral ripples in the larger object’s disk.
If so, the galaxy’s rare glamour won’t last. The merger will eventually be complete, and the turbulent motions within BX442 should wipe out its spiral pattern within 100 million years, a twinkle in cosmic time. The galaxy will then resume the same plain appearance as its contemporaries. Until, perhaps, billions of years later, when a more mature BX442 could stretch out new spiral arms.
Journal reference: Nature, DOI: 10.1038/nature11256