Astronomers have spotted a massive disk galaxy, not unlike our own, that formed 12.5 billion years ago when our 13.8 billion-year-old universe was only a tenth of its current age. But according to what scientists know about galaxy formation, this one has no business being in the distant universe.
This discovery is challenging how astronomers think about galaxy formation in the early universe.
It’s known as Galaxy DLA0817g, but astronomers nicknamed it the Wolfe Disk after late astronomer Arthur M. Wolfe, former doctoral advisor to three of the study’s four authors. It represents the most distant rotating disk galaxy they have ever observed, thanks to the Atacama Large Millimeter/submillimeter Array of telescopes in Chile known as ALMA.
According to their observations, the galaxy’s disk has a mass of 70 billion times that of our sun. It’s also rotating at 170 miles per second, which is similar to our Milky Way galaxy. But galaxies with stable, well-formed disks, like the Milky Way, formed gradually and appeared later in the universe’s timeline, with some dated to 6 billion years after the Big Bang.
In the early days after the Big Bang, the universe was largely a blank slate. Eventually, this was followed by galaxy formation that was pretty messy. Small galaxies merged and crashed together along with hot gas clumps.
“Most galaxies that we find early in the universe look like train wrecks because they underwent consistent and often ‘violent’ merging,” said Marcel Neeleman, lead study author and postdoctoral researcher at the Max Planck Institute for Astronomy in Heidelberg, Germany, in a statement. “These hot mergers make it difficult to form well-ordered, cold rotating disks like we observe in our present universe.”
The study published this week in the journal Nature.
So how did a well-formed rotating disk galaxy appear during this turbulent period? This galaxy formed and grew, researchers concluded, in a different way, known as cold-mode accretion.
Much of what astronomers know about galaxy formation is based on hierarchy. In the beginning, halo-like structures of dark matter, a large, unseen component of the universe known by its effect on surrounding matter, drew in gas. Mergers created something larger where star formation was possible, and eventually, a galaxy was born.
The gas drawn in by the dark matter halos was heated by the collisions, and it would form a disk once it cooled — which could take place over billions of years.
Cold brew
But in the cold scenario, much cooler gas is drawn into a new galaxy and allows for quicker formation of a disk.
“We think the Wolfe Disk has grown primarily through the steady accretion of cold gas,” said J. Xavier Prochaska, study coauthor and professor of astronomy and astrophysics of the University of California, Santa Cruz, in a statement. “Still, one of the questions that remains is how to assemble such a large gas mass while maintaining a relatively stable, rotating disk.”
The researchers also used data from the Hubble Space Telescope and the National Science Foundation’s Karl G. Jansky Very Large Array of radio antennae in New Mexico to understand what kind of star formation was occurring in the galaxy.
“The star formation rate in the Wolfe Disk is at least 10 times higher than in our own galaxy,” explained Prochaska. “It must be one of the most productive disk galaxies in the early universe.”
Neeleman and his colleagues first spotted the Wolfe Disk using ALMA in 2017 when light from a quasar passed through hydrogen gas around the galaxy and revealed it. A quasar, which looks a bit like a star through a telescope, is actually a remote object that emits a large amount of energy likely powered by matter falling on a black hole at the center of a galaxy. The light helped them identify this normal galaxy, rather than the direct light emitted by extremely bright galaxies.
Otherwise, distant galaxies are hard to observe because they’re so faint. But this “absorption” of light method using quasars can happen when the telescopes, galaxy and quasar are in alignment, which is rare — unless galaxies like this were more common in the early universe.
“The fact that we found the Wolfe Disk using this method, tells us that it belongs to the normal population of galaxies present at early times,” Neeleman said. “When our newest observations with ALMA surprisingly showed that it is rotating, we realized that early rotating disk galaxies are not as rare as we thought and that there should be a lot more of them out there. Thanks to ALMA, we now have unambiguous evidence that they occur as early as 1.5 billion years after the Big Bang.”
Future research and observation is needed to understand how common this cold method of galaxy formation was in the early universe.