New cosmic distance record-holder

Galaxy hails from a time when the universe was a mere 600 million years old

A tiny galaxy from the depths of cosmic space and time has become the most distant astronomical object known. At a distance of 13.071 billion light-years, the galaxy is so remote that the light now reaching Earth left the starlit body less than 600 million years after the Big Bang.

DISTANT GLIMMER Astronomers have identified the most distant galaxy known (circled in red), which lies 13.071 billion light-years from Earth. G. Illingworth, HUDF09 Team, NASA, ESA

Matthew Lehnert of the Observatory of Paris in Meudon, France, and his colleagues describe the discovery in the October 21 Nature.

The galaxy, dubbed UDFy-38135539, was initially identified as a distant candidate from visible-light and infrared images recorded by the Hubble Space Telescope’s Wide Field Camera 3. But the brightness of the body at different wavelengths, while consistent with it being remote, did not constitute proof. To determine the galaxy’s distance, Lehnert’s team divided the faint light into its component colors using a spectrograph on the European Southern Observatory’s Very Large Telescope at La Silla Paranal Observatory in Chile.

The team was initially frustrated because “we thought we would need a year to do the spectroscopy,” recalls study coauthor Nicole Nesvadba of the Institut d’Astrophysique Spatiale in Orsay, France.  However, Lehnert calculated that the light emitted by hydrogen atoms in the galaxy was bright enough to do a shorter study.

In fact, 16 hours of observations with the spectrograph allowed the scientists to measure the galaxy’s redshift — the extent to which light emitted by a body is shifted to longer, or redder, wavelengths by the expansion of the universe. The more distant a body, the greater its redshift. UDFy-38135539 has a redshift of 8.56, beating the previous distance holder, a powerful cosmic explosion known as a gamma-ray burst, by about 35 million light-years (SN Online: 4/28/09).

Studying the galaxy may better illuminate the ancient era known as reionization, in which the universe’s hydrogen atoms were broken apart into their constituent electrons and protons, presumably by ultraviolet light emitted by the first stars in the cosmos. The time at which reionization — a gradual process — began marks the time that the first stars switched on, clearing the fog of neutral hydrogen atoms that had enveloped the cosmos. For UDFy-38135539 to be seen at all, reionization must have already begun in the galaxy’s immediate vicinity. Otherwise, the neutral hydrogen fog would have scattered the galaxy’s light, preventing it from reaching Earth.

But Lehnert and his colleagues calculate that UDFy-38135539 is not bright enough on its own to break apart most of the hydrogen atoms in the galaxy’s neighborhood. The body must have had several helpers — small companion galaxies at the same distance from Earth, but too faint to be seen — to reionize this neck of the cosmic woods.

Observing the light from UDFy-38135539 and much fainter galaxies will become easier with the 2014 launch of the infrared James Webb Space Telescope, comments Avi Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. Astronomers using Hubble have already identified a slew of other galaxies that may lie farther away than UDFy-38135539  (SN: 1/30/10, p. 5), but will need the new infrared telescope — located above the contaminating infrared radiation emitted by Earth’s atmosphere — to measure the distances.

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