This image from the Hubble Space Telescope CANDELS survey highlights the most distant galaxy in the universe with a definitively measured distance, dubbed z8_GND_5296. The galaxy's red color alerted astronomers that it was likely extremely far away, and thus seen at an early time after the Big Bang.
To judge from the headlines, astronomy is nothing more than an ongoing quest to set a Guinness World Record. It’s always seems to be about the tiniest galaxy, or the most Earthlike planet, or the biggest star or the hottest or coldest or weirdest cosmic whatever. And at first glance, the discovery described in the current Nature just continues the gee-whiz trend. Observers using both the Hubble and the Keck telescopes have been conducting a deep field survey of 43 galaxies believed to be extremely distant and have identified a true gem among them: an object whose light has been en route to Earth for more than 13 billion years—and that makes it the most distant galaxy ever found.
But distance alone isn’t what makes this galaxy, known as z8_GND_5296, so important. It’s the fact that astronomers can see it at all, that they’re peering deep into the history of the universe, back to a time when the cosmos was a mere 700 million years old. That’s just five percent of its current age, and not long after the very first stars began to shine. Finding and studying young galaxies like z8_GND_5296 is like finding early human fossils: in both cases, by understanding where we came from, we can figure out how we got here. “We’re really studying the evolution of galaxies,” says lead author Steven Finkelstein, of the University of Texas at Austin.
This particular fossil galaxy has two important features, says Finkelstein. First, it seems to be forming stars at a prodigious rate—about 300 Suns’ worth a year. (That, at least, is what it was doing 13 billion years ago, when the image we see today set out on its journey toward us. By now, z8_GND_5296 may have merged with other galaxies.) “That’s thirty times as much as we expected, and 150 times as fast as the Milky Way,” he says. The explanation could simply be that this galaxy formed in an especially dense region of the early universe, giving it more star-forming material purely by chance. Or it may mean theorists don’t understand galaxy formation as well as they thought.
The second noteworthy feature is that z8_GND_5296’s ultraviolet-light emissions are largely, though not entirely, masked by a shroud of hydrogen gas that doesn’t exist in the modern universe. That’s not a big surprise: astronomers know that hydrogen atoms formed about 400,000 years after the Big Bang, and that intense radiation, probably from young stars, eventually ionized those atoms—that is, knocked off their electrons, leaving just the nuclei behind. The ionized hydrogen that still floats in intergalactic space is transparent to ultraviolet light; the neutral hydrogen in the early universe was mostly opaque to UV. So the older the galaxy, the less UV you see.
“We looked at 43 galaxies with the Keck 1 telescope,” says Finkelstein, “and saw [the signature of UV emissions] in only one.” That reinforces the idea, already suggested by earlier observations, that the ionizing process for hydrogen gas (known, oddly, as “re-ionization”) was far from complete at this point.
These observations are so difficult to make, however—the fact that they required not one but two of the world’s most powerful telescopes is a giveaway—that the conclusions Finkelstein and his colleagues reached about z8_GND_5296 are being received warily by some other astronomers. “I find the high star formation rate to be a dubious result,” says Caltech’s Richard Ellis, who has discovered some pretty faraway galaxies himself.
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Ellis is also not convinced that all 43 of the galaxies in Finkelstein’s survey are really as far from us as they seem. The Hubble can’t actually measure distance for these very faint objects. Instead, it looks for redshift—the stretching and reddening of wavelengths of light caused by the expansion of the universe. The farther away something is, the redder it should look, something Hubble detects with its multiple on-board light filters.
But dusty galaxies are also unusually red, and Hubble is not capable of producing what’s known as a full spectrum—a true reading of exactly what wavelengths of visible light an object is producing. The best it can do is an approximation. Keck does have that capability and has provided a solid reading for z8_GND_5296. Ellis concedes that he’s “fairly confident” of that result, but there’s no spectrum yet for the other 42, which casts some doubt in his mind. And if some of those galaxies are closer than they seem, any statements about the state of intergalactic hydrogen could be shaky as well.
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Of course, when it comes to doubts about distance measurements, Finkelstein can give as good as he gets. Astronomers including Ellis have claimed the probable detection of galaxies significantly farther out than z8_GND_5296, including at least one that seems to lie at a distance that would place it a mere 400 million years after the Big Bang. “There are realistic reasons to be cautious about these,” says Finkelstein, and he doesn’t buy all of them. “But these people are careful,” he says, speaking of his rivals in the record-distance game, and he thinks it’s likely that at least some will ultimately be confirmed.
That gets harder and harder, the farther out and fainter these newborn galaxies are. By 2018 the next-generation James Webb Space Telescope should improve things, however, and the ALMA radio array, dedicated just last spring in Chile, is already starting push back the cosmic horizon as well. It looks like astronomers won’t have much longer to wait before they can truly understand how and when the first stars turned on—and set the stage for everything that came afterward.
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