Seems like yesterday (though it was really late last year) that I described a remarkable image from the Hubble Space Telescope that had cosmologists salivating.
To be sure, the 2009 version of the Hubble Ultra-Deep Field (HUDF) was a challenging feat, involving 48 hours' exposure with HST's new Wide Field Camera 3. The idea behind this effort was to record faint blips from galaxies that formed soon after the Big Bang and thus gauge the state of the universe at the youngest possible age.
A first pass through the HUDF yielded an assortment of candidate galaxies with redshifts, or z, of at least 7. (Redshift is what astronomers use to gauge both age and distance in the cosmos. The farther away the source, the longer its light has taken to reach us, the faster it's moving away from us, and the more its light has been stretched by that expansion to longer wavelengths.)
Hubble's handlers recorded the HUDF in infrared light, because they were searching for the highly redshifted ultraviolet light from bursts of star formation in those primordial galaxies. By comparing the appearance of very dim galaxies at three different infrared wavelengths, the HUDF team could identify candidates with high — but approximate — values of z. Then it was up to ground-based astronomers to try to pin down the exact ages.
Now that's been done, at least for one of the HUDF's dim blips, and it's a record-buster. In today's issue of Nature, a team led by Matt Lehnert confirms that the galaxy designated UDFy-38135539 has a redshift of 8.6. This means its light started crossing the giant void less than 600 million years after the Big Bang. It edges out the previous redshift record of 8.3, attributed to a powerful burst of gamma rays, and smashes the previous high of 7.0 for a galaxy.
Two things are remarkable about UDFy-38135539. The first is that it could be spotted from the ground at all. Lehnert's team collected nearly 15 hours of exposures using one of the giant 8.2-m reflectors of the European Southern Observatory's Very Large Telescope in Chile. Feeding its light to the high-performance SINFONI spectrograph, the observers just barely recorded the galaxy's near-infrared signature. In fact, it took months to convince themselves that the spectrum is real — and there's still a tiny chance they're being fooled by a galaxy that's actually much closer.
Second, unlike the one-and-done gamma-ray burst, this galaxy's steady light is a critical probe of a primordial period known as the reionization era. During the infant universe's first 400 million years, hydrogen created in the Big Bang was too hot to form stars — so it was completely dark everywhere. Intense, high-energy light from those first-generation stars ionized the intergalactic medium and made it largely transparent, at least in spots. (The animation here shows how this all works.)
The light leaving UDFy-38135539 initially was at the ultraviolet wavelength of 121.6 nm, the strong Lyman-alpha emission of neutral hydrogen atoms. But after traveling across more than 13 billion years of expanding space those weary photons had become redshifted in wavelength nearly tenfold, into the infrared at 1.16 microns, by the time they reached the VLT and SINFONI.
Lehnert and his team speculate that intense radiation from billions of young stars created a local ionized "clearing" in the primordial fog roughly 10 million light-years across. This would have given the ultraviolet photons enough running room to redshift away from the Lyman-alpha wavelength and thus keep them from being reabsorbed by any hydrogen atoms along the line of sight to Earth. Yet UDFy-38135539 likely contained only a billion stars, far too few to create a clearing big enough, so it must have had help from unseen galaxies in its neighborhood.
This result represents "a fundamental leap in observational cosmology," according to Michele Trent, an astrophysicist at the University of Colorado, because we finally have a direct probe of what the universe was like when it was just 1⁄25 of its present age. Moreover, the HUDF field, only 2 arcseconds square, contains several other high-redshift candidates that observers are chasing down. Chances are we haven't heard the last about this tiny patch of space.