Object allows astronomers a glimpse of Universe's era of 'reionization'.
Zeeya Merali
Galaxy UDFy-38135539Light from a distant galaxy has provided a snapshot of the early universe.ESO/L. Calçada
Observations of the most distant object yet discovered go a long way in supporting astronomers' models of the early Universe. But the far-flung galaxy, details of which are published in Nature today1, also raises questions about the source of the first light in the cosmos.
Light from the galaxy, named UDFy-38135539, left the object just 600 million years after the Big Bang, giving a snapshot of the cosmos in its infancy. This value smashes the previous record held by a galaxy by 150 million years2. The image shows the galaxy as it was when it was around 100 million years old and is just 1-10% of the mass of the Milky Way.
The galaxy is particularly fascinating because, 600 million years after the Big Bang, the Universe was thought to be going through a phase called reionization. However, there has been little direct observational evidence for this, says astronomer Matt Lehnert at the Paris Observatory in France, who led the team involved in the study. According to astronomers' best models, the early Universe burst out of the Big Bang around 13 billion years ago as an ionized fireball. This ball of gas gradually cooled, becoming neutral as protons and electrons combined to form hydrogen. "Then stars and galaxies began to form, lighting up the Universe, heating up the gas and reionizing it," says Lehnert. "This galaxy allows us to peek at the reionization era."
At the limit
The first hint of the galaxy's existence came when astronomers scrutinized a near-infrared image taken by the Hubble Space Telescope's Wide Field Camera and saw "a faint blob", says Lehnert3. To confirm its distance, Lehnert and his colleagues searched for a characteristic signature, called the Lyman-α line, that is seen in the spectrum of light emitted by galaxies. The Lyman-α line is produced as electrons move between two energy levels in a hydrogen atom. The wavelength of the light is shifted towards the red end of the spectrum by an amount that is related to the motion and distance of the source from which it is emitted. As this 'redshift' is greater the older the object being observed, it allows astronomers to calculate an object's age. Using the ground-based Very Large Telescope in Paranal, Chile, the team detected the line, and calculated that it had a redshift of 8.55, indicating that the light had set out 600 million years after the Big Bang.
“They are really pushing the instrumentation to its limit.”
James Dunlop, an astronomer at the University of Edinburgh, UK, who was part of the team that found the galaxy candidate in the Hubble data, says the result is "exciting, if proved correct". But he adds that it is also "slightly controversial" because it is based on the discovery of just one spectral line, making it tough to establish that this is not just an artefact of the measuring process. "They are really pushing the instrumentation to its limit," says Dunlop.
Lehnert emphasizes that the team took pains to rule out the possibility that the line was caused by background contamination from molecules in Earth's atmosphere. "It took months for us to convince ourselves that this is real," he says.
Strong signal
The galaxy seems to confirm astronomers' models of the early Universe, which predict that young galaxies were responsible for reionization around 600 million years after the Big Bang4, says Martin Haehnelt, a cosmologist at the University of Cambridge, UK. But he notes that typical galaxies do not produce Lyman-α lines that are as strong as the one seen by Lehnert's group, making the finding puzzling.
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Lehnert's team argues that the line may be unusually strong because there are additional, as yet undetected, galaxies surrounding the newly discovered one, giving it a helping hand in reionization. "This could help explain it, but even then, the strength is still surprisingly large," says Haehnelt. "This is a very nice result, but it is important to be cautious about it."
It will be difficult to investigate the galaxy further using ground-based telescopes, as the data will be contaminated by 'noise' from Earth's atmosphere. However, the James Webb Space Telescope, due to launch in 2015, will train its spectrographic instruments on this region, in an effort to delve even further back into the Universe's infancy. It will also help astronomers to unpick the puzzle of the strength of the Lyman-α line, by revealing exactly what kind of galaxies are responsible for reionization, says Dunlop. "This is the sort of galactic archaeology that the next generation of telescopes will be able to do," he adds.
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