Astronomers say they have captured for the first time the spectral signature of a galaxy as it appeared less than 600 million years after the big bang, making it the oldest, and most distant, galaxy ever observed.
The result helps open a previously closed window on a critical period in the evolution of the early universe, its transition from an opaque fog of hydrogen and helium gas to the galaxy-studded sky we are familiar with.
The team reporting the results said they used images from a 2009 Hubble Space Telescope survey to determine which galaxy "candidate" to study. Their large ground-based telescope, located in Chile, can see in the near-infrared, allowing it to find light-emitting objects that were otherwise hidden in the opaque fog.
"This is the first time we know for sure that we are looking at one of the galaxies that cleared out the fog which had filled the very early universe," says Nicole Nesvadba, a researcher at the Institut d'Astrophysique Spatiale, near Paris, and a member of the team reporting the results in tomorrow's issue of the journal Nature. The team was led by Paris Observatory astronomer Matt Lehnert.
The accomplishment highlights the potent combination that space-based and ground-based telescopes provide in documenting the universe's emergence from its so-called dark ages – a time during the first 150 million years when the cosmos is thought to have been without light.
The Hubble team estimated that the images it gathered last year revealed four galaxy candidates as they appeared roughly 780 million years after the big bang, the sudden release of enormous energy that led to the formation of the universe scientists observe today.
The new data, if they hold up, speak of a younger galactic baby picture than the Hubble team initially estimated. They were gathered by the European Southern Observatory's Very Large Telescope (VLT) near Paranal, Chile.
As cosmologists have recreated the scene, the universe was filled with neutral hydrogen and helium gas once it cooled off from its energetic beginning. No stars, no light, just an atomic fog.
But the gas was slightly more dense in some regions than in others. These subtle differences allowed gravity to kick in, drawing more gas into the regions of higher density. One by one, this gravitational collapse formed stars, then galaxies across the small but expanding cosmos.
Ultraviolet light from the early generations of hot massive stars and the galaxies they came to inhabit began to strip electrons from the electrically neutral hydrogen and helium. This ionization process changed the physical properties of the gas, allowing light to penetrate the fog and, over time, making the universe increasingly transparent.
Some 600 million years after the big bang, the expanding cosmos would have been roughly 3 billion light-years across, compared with a span of nearly 28 billion light-years today, says Michele Trenti, an astrophysicist at the University of Colorado at Boulder. And it would have been a thousand times as dense.
Given the distance to UDFy-38135539 – some 13.1 billion light-years – and the difficulty of the measurements the Hubble and VLT teams made, the relatively small gap in age estimates for the galaxy "is in itself amazing," writes Garth Illingworth, an astronomer at the University of California at Santa Cruz, in an email exchange. Dr. Illingworth leads the Hubble survey team and was not part of the group reporting the spectral results.
Still, he says, he remains cautious about the new results.
"I am still a little skeptical because that galaxy is really faint," he says, making it hard to gather enough light for a clear spectral analysis.
But the Lehnert team posits that UDFy-38135539 wasn't alone; other smaller galaxies in its vicinity may have been present to help ionize the surrounding region enough for UDFy's spectral beacon to escape.
The work truly sits at the cutting edge of observational cosmology, Dr. Trenti adds.
"As of today we have identified more than 500 galaxies within the first billion years after the big bang – the vast majority thanks to the Hubble Space Telescope. So the frontier of discovery is really at 600 million to 700 million years after the big bang," he says.