A faint glow first spotted by the Hubble Space Telescope belongs to a galaxy that is the most distant object yet found, new observations suggest. The galaxy helps provide a window into the primordial cosmos, when a thick fog gave way to the transparent universe we see today.
The Hubble Space Telescope’s Wide Field Camera 3 picked up the object as a dim infrared blob in observations made in August and September 2009.
It was suspected to be a very distant galaxy because its light is strongly reddened or skewed towards longer wavelengths, as expected for light that has travelled for billions of years to reach Earth. The intervening expansion of the universe stretches light waves, pushing them to longer, redder wavelengths.
But astronomers initially could not rule out the possibility that the object might be intrinsically red and much closer to Earth, such as a brown dwarf star in our own galaxy. The Hubble camera is not equipped to measure the detailed light spectrum needed to distinguish between such possibilities.
Now, follow-up observations made with an 8.2-metre telescope at the European Southern Observatory in Chile suggest the object sits far outside the Milky Way. Its light appears to be more than 13.1 billion years old, making it the most distant object confirmed to date.
A team led by Matt Lehnert of the Observatoire de Paris, France, used the telescope to stare at the object for 16 hours to measure its light spectrum. The peaks and troughs in the spectrum – which correspond to light that is emitted or absorbed – included a spike at a wavelength of 1.16 micrometres. The team says the peak is most likely created by the glow of hot hydrogen gas from a distant galaxy. The light originally had a wavelength of 0.122 micrometres but has been greatly stretched en route to Earth.
The amount of this stretch, measured by a quantity called redshift, is 8.55, suggesting the light has taken more than 13.1 billion years to reach us. We see the galaxy as it appeared less than 600 million years after the big bang occurred, some 13.7 billion years ago.
“We have confirmed that a galaxy spotted earlier using Hubble is the most remote object identified so far in the Universe,” says Lehnert. The record for most distant object was previously held by a self-destructing star with a redshift of 8.2, suggesting the blast occurred when the universe was 630 million years old.
Tentative candidates at even higher redshifts than the newfound galaxy have been reported, but they have not been confirmed with light spectrum measurements. Until now, the farthest galaxy to be spectroscopically confirmed had a redshift of 6.96.
The extreme distance of this new galaxy – dubbed UDFy-38135539 – provides hints about how a primeval fog was lifted to make the universe more transparent to light. Much of the starlight in the first few hundred million years of the universe’s existence was quickly absorbed by hydrogen gas permeating the cosmos.
These hydrogen atoms were eventually split into their constituent protons and electrons – a process called reionisation – by sources of radiation whose identity is still debated, leading to the more transparent universe we see today.
UDFy-38135539 existed at a time when the fog was still in the process of lifting. But the fact that it is visible at all indicates that much of the hydrogen around it has already been reionised.
Lehnert and colleagues calculate that radiation from the galaxy itself is insufficient to do the job, so it must have had help, perhaps from a retinue of smaller galaxies around it that are too faint to see.
“It’s very exciting that we have possible evidence for a galaxy at a record breaking redshift,” says Avi Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. But Loeb, who is not a member of Lehnert’s team, says that contributions from Earth’s atmosphere could add some uncertainty to the galaxy’s distance. “I wouldn’t say it’s a slam dunk,” he says.
The team admits there is a chance that the spike in the spectrum is due to oxygen instead of hydrogen, which would give it a much lower redshift of 2.12. The jittering of Earth’s atmosphere could cause light from oxygen, which produces a pair of spikes, to blur into a single peak that looks like the one produced by hydrogen. But the team calculated this blurring should only occur in about 0.1 per cent of observations, making it unlikely to be the case.