Most massive disk galaxies like our Milky Way formed gradually, reaching their large mass relatively late in the 13.8 billion-year history of the universe. But the discovery by an international team of astronomers of a massive rotating disk galaxy, seen when the universe was only ten percent of its current age, challenges the traditional models of galaxy formation.
The discovery, reported May 20 in Nature, was made with the Atacama Large Millimeter/submillimeter Array (ALMA). Galaxy DLA0817g, nicknamed the Wolfe Disk after the late astronomer Arthur M. Wolfe, is the most distant rotating disk galaxy ever observed. The unparalleled power of ALMA made it possible to see this galaxy spinning at 170 miles (272 kilometers) per second, similar to our Milky Way.
“Its properties are astonishingly similar to our own galaxy, despite being only 1.5 billion years old,” said coauthor J. Xavier Prochaska, professor of astronomy and astrophysics at UC Santa Cruz.
“While previous studies hinted at the existence of these early rotating gas-rich disk galaxies, thanks to ALMA we now have unambiguous evidence that they occur as early as 1.5 billion years after the Big Bang,” said lead author Marcel Neeleman of the Max Planck Institute for Astronomy in Heidelberg, Germany
How did the Wolfe Disk form?
The discovery of the Wolfe Disk provides a challenge for many galaxy formation simulations, which predict that massive galaxies at this point in the evolution of the cosmos grew through many mergers of smaller galaxies and hot clumps of gas.
“Most galaxies that we find early in the universe look like train wrecks because they underwent consistent and often violent merging,” explained Neeleman. “These hot mergers make it difficult to form well-ordered, cold, rotating disks like we observe in our present universe.”
In most galaxy formation scenarios, galaxies only start to show a well-formed disk around 6 billion years after the Big Bang. The fact that the astronomers found such a disk galaxy when the universe was only ten percent of its current age indicates that other growth processes must have dominated.
“We think the Wolfe Disk has grown primarily through the steady accretion of cold gas,” Prochaska said. “Still, one of the questions that remains is how to assemble such a large gas mass while maintaining a relatively stable, rotating disk.”
Star formation
The team also used the National Science Foundation’s Karl G. Jansky Very Large Array (VLA) and the NASA/ESA Hubble Space Telescope to learn more about star formation in the Wolfe Disk. In radio wavelengths, ALMA looked at the galaxy’s movements and mass of atomic gas and dust, while the VLA measured the amount of molecular mass—the fuel for star formation. Hubble observed massive stars in ultraviolet light.
“The star formation rate in the Wolfe Disk is at least ten times higher than in our own galaxy,” Prochaska said. “It must be one of the most productive disk galaxies in the early universe.”
A ‘normal’ galaxy
The Wolfe Disk was first discovered by ALMA in 2017. Neeleman and his team found the galaxy when they examined the light from a more distant quasar. The light from the quasar was absorbed as it passed through a massive reservoir of hydrogen gas surrounding the galaxy, which is how it revealed itself. Rather than looking for direct light from extremely bright, but more rare galaxies, astronomers used this absorption method to find fainter and more ‘normal’ galaxies 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,” said Neeleman. “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.”
In addition to Neeleman and Prochaska, the coauthors of the paper include Nissim Kanekar at the National Center for Radio Astrophysics in Pune, India, and Mark Rafelski at the Space Telescope Science Institute.
The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organization for Astronomical Research in the Southern Hemisphere (ESO), the U.S. National Science Foundation (NSF), and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia.
