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Early Bloomer: Faraway Galaxy Pushes Cosmic View Closer to the Dawn of the Universe

A galaxy that existed just 600 million years after the big bang is the most distant object ever seen



NASA, ESA, G. Illingworth (UCO/Lick Observatory and University of California, Santa Cruz) and the HUDF09 Team

Astronomers claim to have identified a galaxy in the distant universe that is farther away than any known object in space. The galaxy is so far away that, inasmuch as researchers can see such a faint object, they see it as it looked about 13 billion years ago, just 600 million years or so after the big bang; the light it emitted at that time is only now reaching Earth.

Cosmologists gauge cosmic distances by measuring redshift, a gauge of how much an object's emitted light has been stretched in wavelength during its journey across an expanding universe. The new object, which goes by the unmemorable name of UDFy-38135539, has a redshift of 8.55, topping the previous record holder, a short-lived gamma-ray burst detected in 2009 that had a redshift of about 8.2. The difference in redshift between the two objects corresponds to peering a few tens of millions of years deeper into cosmic history, inching observational astronomy ever closer to the universe's chaotic birth and infancy. Moreover, studying galaxies at high redshifts can bring out details about the epoch of reionization, when light from the first stars and galaxies broke apart neutral hydrogen in the intergalactic medium.

Matthew Lehnert of the Paris Observatory and his colleagues selected UDFy-38135539 from an ultra-deep survey of a small patch of sky made by the Hubble Space Telescope in 2009, shortly after the orbiting observatory had been refurbished with a new camera called Wide Field Camera 3, or WFC3.

The WFC3 survey identified several promising candidates for extremely distant objects, but their true nature remained unclear without follow-up observations. In the October 21 issue of Nature Lehnert and his colleagues report confirmation of the object and a precision estimate of its redshift, thanks to spectroscopic measurements taken at the European Southern Observatory's Very Large Telescope (VLT) in Chile. "Before we took this spectrum, there were only two data points," Lehnert says. "That's not a lot of information." (Scientific American is part of Nature Publishing Group.)

With a more detailed spectrum, astronomers can look for the specific location of Lyman-alpha emission, associated with a transition between energy levels in hydrogen, the most common substance in the universe, as a marker of how much the object's light has been redshifted. For an object as distant as UDFy-38135539 the Lyman-alpha photons have been shifted all the way from the ultraviolet, past visible light, and into the infrared.

James Dunlop, an astrophysicist at the University of Edinburgh's Royal Observatory, notes that UDFy-38135539 is the most distant spectroscopically confirmed object of any kind, despite being a rather mundane galaxy and not a freakishly bright object such as a quasar or a one-off cataclysm such as a gamma-ray burst. At the same time, the detection of a faint Lyman-alpha emission in the spectrum is not unshakable, says Dunlop, who co-authored a paper earlier this year identifying candidate high-redshift galaxies in the Hubble images. "I'm pleased to see this coming because it's so close to what we predicted," he says. "On the other hand, as the authors admit, it's been a real struggle to convince themselves that this line is real."

Earth's atmosphere absorbs many wavelengths of infrared light and contributes its own contaminating molecular emission, known as airglow, making infrared astronomy a tricky business from the ground, even at the VLT's arid, high-altitude perch atop Chile's Cerro Paranal. "In the night sky, you have emission features that cause strong, narrow lines in your spectrum," Lehnert says. "If your astrophysical emission line falls on one of those, then you're doomed."

At the redshift inferred for UDFy-38135539, the hydrogen line happens to fall in a quiet part of the infrared spectrum, but the signal-to-noise ratio leaves some open questions. "It's right on the edge of what you'd believe, to be frank," Dunlop says, noting that confirmation would be much easier with a space-borne infrared spectrograph such as that planned for NASA's James Webb Space Telescope (JWST). But that technology will not be available until JWST launches in 2014, at the earliest. "So you're faced with trying to follow these things up from the largest telescopes on the ground," Dunlop says.

Uncertainties aside, Dunlop credits the researchers for getting solid data on the galaxy with the telescopes available today. "This is two nights integrating on one object with one of the world's best instruments for this stuff," Dunlop says. "It's hard to imagine anyone doing much better with this particular object."

And Lehnert notes that he and his colleagues put their data through a series of statistical tests to try to rule out false positives, a process that he hopes will keep the newly characterized galaxy out of the scrap heap of disputed or retracted claims. "The people that have made these claims that have later been falsified haven't run these tests," he says. "At least if something goes wrong and it's falsified—and I don't think it will be—we can say, 'Hey, we ran every test we could.'"

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