Galaxy Is Most Distant Object Ever Seen

20 October 2010 1:27 pm

Astronomers have measured the distance to the farthest cosmic object known to humankind: a galaxy that lies 13.1 billion light-years away. Imaged last year by Hubble's new Wide Field Camera 3, the galaxy takes researchers back to a mere 600 million years after the big bang. Not only does it smash the previous record for most distant object—a gamma-ray burst that went off 13 billion years ago—but it is also the first object to be detected from an era when the universe was just emerging from an opaque veil of hydrogen gas.

A few hundred thousand years after the big bang, protons and electrons started joining together to form hydrogen. About 150 million years later, the first stars and galaxies began to form, but they remained enveloped in a fog of hydrogen gas which absorbed the light emitted by the earliest stars. Over the next several hundred million years—until the universe was 800 hundred million years old—the radiation from these early stars and galaxies split the surrounding hydrogen into protons and electrons, thus clearing away the fog and making the universe transparent. Researchers call this period the epoch of reionization.

The sighting of the remote galaxy—named UDFy-38135539—supports this theoretical timeline, showing that reionization was well under way 600 million years after the big bang. That's because if the hydrogen "fog" in front of the galaxy had not lifted by then, light from the galaxy would not have reached Earth. The galaxy itself contributed to this reionizing process that shredded the veil obscuring it, although its radiation alone would not have been enough to do the job. "There must be other galaxies, probably fainter and less massive nearby companions of UDFy-38135539, which also helped make the space around the galaxy transparent," says one of the paper's authors, Mark Swinbank of the University of Durham in the United Kingdom. "Without this additional help, the light from the galaxy, no matter how brilliant, would have been trapped in the surrounding hydrogen fog."

The galaxy was one of several faint objects identified as potentially remote in pictures taken last year by Hubble. To measure the distance to the galaxy, lead author Matthew Lehnert of the Paris Observatory, Nicole Nesvadba of the University of Paris, and their colleagues took a spectrum of the object using a spectrograph mounted on the European Southern Observatory's Very Large Telescope in Chile. By analyzing the spectrum, the researchers determined that the galaxy had a red shift of 8.55, corresponding to a distance of 13.1 billion light-years, they report online today in Nature.

Garth Illingworth, an astronomer at the University of California, Santa Cruz, and the leader of a team that identified this and other remote objects in the Hubble images last year, calls the finding a "very interesting result." However, he says he is a "little skeptical" about whether the spectral analysis is accurate because the object in the Hubble image "is really faint."

Avi Loeb, an astrophysicist at Harvard University, has a similar concern, although he too is excited by the find. "The validity of this discovery hangs on whether the authors have correctly identified the signal line and were not misguided by a contaminating line instead," he says, adding that observing such galaxies will become easier when the next-generation James Webb Space Telescope is launched in 2014.

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