How do you build a galaxy? That's a question astronomers continue to ask themselves as they formulate theories on how these gargantuan systems, full of dust, gas and stars, come together. In seeking answers, they turn their telescopes to the sky and look for distant galaxies that could help unravel the mystery.
In a new study, published in the journal Nature on Wednesday, an international team of astronomers detected light from an ancient, huge galactic disk lurking in a far corner of the universe. The light took some 12.5 billion years to reach us on Earth, which means the disk formed around 1.5 billion years after the Big Bang -- in the earliest days of the universe.
Using one of the world's most powerful telescopes, the Atacama Large Millimeter/submillimetre Array, the team found the galaxy when it was studying bright light from a distant, mammoth black hole known as a quasar. Some of the light was absorbed by the galaxy on its way to Earth, revealing it hiding in the dark of space. Studying the galaxy with ALMA and using data from Hubble, the team were able to more clearly resolve some of its features.
"Previous studies hinted at the existence of these early rotating gas-rich disk galaxies," said Marcel Neeleman, an astronomer at the Max Planck Institute for Astronomy and lead author on the study. "Thanks to ALMA we now have unambiguous evidence they occur as early as 1.5 billion years after the Big Bang."
Officially, they've dubbed the galaxy DLA0817g, but they've nicknamed their finding the Wolfe Disk in honor of astronomer Arthur M. Wolfe.
Comparing their observations with analytical models, the team put together a case for what was happening in the galaxy. They found their models most accurately lined up with a galaxy made of a dusty, gaseous disk spinning at approximately 169 miles (272 kilometers) per second, with an estimated mass around 50 to 100 times more than the sun. It also appears to be forming stars at an exceptionally fast rate.
"It must be one of the most productive disk galaxies in the early universe," said Xavier Prochaska, astronomer at the University of California, Santa Cruz and a co-author on the study.
The discovery provides some clues about how galaxies are built and why we so often see the structures resembling huge disks, while others don't.
"Determining when in comic history disks start to emerge within the galaxy population can tell us about the mechanisms by which galaxies grew and formed in the early Universe," says Alfred Tiley, an astrophysicist at the University of Western Australia and author of an accompanying article on the discovery in Nature Wednesday.
The current understanding of galaxy formation proposes huge invisible spheres of dark matter in the cosmos provide a kind of skeleton for gas and dust to fall into, eventually forming stars and entire galaxies. Over eons, the in-falling hot gas and dust create huge disks we see in galaxies dotted throughout the cosmos. Other galaxies collide with each other, a common phenomenon in the early universe where all the gas, dust, stars and galaxies were a little more snug.
But those models suggest you wouldn't see galactic disks at such an early time after the Big Bang. The team propose the reason they've detected the immense Wolfe Disk from such an early point in time is because it was built in a different way -- by cold gas.
Previous theories reasoned these types of cold gas disks should only appear around 3 billion years after the Big Bang. The new analysis pushes this timeframe back another 1.5 billion years.
However, Tiley notes there are other possible explanations for how the disk was built, but proving them requires further observations of DLA0817g.
"One possible explanation is that the gas disk they observe is the result of a merging event between one or more galaxies that could have funnelled cold gas to the center of the resultant halo," he says. "But the authors argue the cold accretion scenario is the more likely."