Peering way back in time, astronomers have discovered the faintest galaxy ever detected in the early universe.
No more than a few pixels in Hubble images, the galaxy appears as it did 13.1 billion years ago — just 700 million years after the big bang.
The discovery, reported in the journal Nature Astronomy, sheds light on a critical period in the evolution of the universe.
Up to a billion years after the big bang, light from the earliest stars and galaxies ionised hydrogen atoms.
This process produced charged plasma instead of opaque, neutral hydrogen gas, bringing an end to the cosmic “dark ages” and creating the transparent universe we know today.
Study co-author Michele Trenti of the University of Melbourne said that faint galaxies, which are the most common in the universe, were thought to be responsible for this crucial “cosmic reionisation”.
But up until now, astronomers had only found a handful of bright galaxies at this distance.
Austin Hoag, who led the study, said the newly discovered galaxy, known as MACS1423-z7p64, was about 10 times fainter than other galaxies that have been found at the same distance.
“That might not sound like much, but when you are looking for galaxies at such a vast distance, even the very brightest ones are hard to confirm,” said Mr Hoag, a PhD candidate at the University of California, Davis.
Dr Trenti said the galaxy was estimated to have about 500 million stars.
“To put that in context, our own galaxy, the Milky Way, has about 100 billion stars. Thus we are talking about a galaxy that is about 200 times less massive than the Milky Way.”
He said the discovery pushed the limits of what was possible using current telescopes.
“The smallest galaxies might have just one million stars, so there is still room for future discoveries of even less luminous distant galaxies as our telescope technology improves,” Dr Trenti said.
How to find a distant galaxy
The international team of astronomers made the discovery using the Hubble Space Telescope and the Keck Observatory in Hawaii.
First, they used a technique known as gravitational lensing to locate and identify distant galaxies.
“It happens when the galaxy you are looking for is behind a very massive structure, in our case a cluster of galaxies at a much closer distance than the galaxy itself,” explained Mr Hoag.
“The mass from the structure bends the light of the more distant galaxy, and in some cases causes the galaxy to be magnified.”
The galaxy was sitting in a sweet spot behind a giant cluster that magnified its brightness by a factor of 10, so it could be detected by the Hubble telescope.
Then, to get a precise measurement of the distance, a team led by Mr Hoag used the Keck Observatory to analyse the light spectrum coming from the galaxy.
“Because the light has to travel all the way from the distant sources to us in an expanding universe it gets shifted towards the red wavelength,” Dr Trenti explained.
The galaxy had a redshift of 7.6, meaning its light came from when the universe was only about 700 million years old.
A team led by Dr Trenti repeated the observations again a year later in 2016 to confirm the distance was correct.
More powerful telescopes will reveal more galaxies
Dr Trenti said it was likely more faint galaxies at this distance would be revealed by more powerful telescopes, such as the James Webb Telescope, which is due to be launched in 2018.
“The James Webb will get much higher quality spectrum in one or two hours of observation. It will be the next really giant leap forward in the field.”