Enlarge (credit: NASA, ESA, P. Oesch (Yale University), G. Brammer (STScI), P. van Dokkum (Yale University), and G. Illingworth (University of California, Santa Cruz))
When did the first stars and galaxies form? The earliest thing we can see in the Universe is the Cosmic Microwave Background, created about 400,000 years after the Big Bang. The event that created the CMB filled the Universe with neutral hydrogen, which efficiently absorbs most of the wavelengths of light we would normally use to detect the first galaxies. But their light gradually reionized this neutral hydrogen, bringing an end to the Universe's "dark ages."
By the time reionization was complete, however, there was already substantial population of galaxies (not surprising, since they were caused by the reionization). So, how do we find the earliest galaxies?
In the case of new results announced this week, the authors used a combination of the Hubble and Spitzer space telescopes to identify a candidate galaxy that was exceptionally bright and distant. Then they used the absorption of light by all that neutral hydrogen to determine that it dates from only 400 million years after the Big Bang, when the Universe was only three percent of its current age.
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