A team of astronomers has stumbled upon a humungous spiral galaxy, about five times more massive than our Milky Way galaxy and covering an area two times as big, making it among the largest known galaxies. The most surprising trait of the galaxy, however, is not its colossal size but the fact it existed in the early cosmos when the universe was only 2 billion years old.
"This galaxy is spectacular for being among the largest spiral galaxies ever found, which is unprecedented for this early era of the universe," says Charles (Chuck) Steidel (PhD '90), the Lee A. DuBridge Professor of Astronomy at Caltech. "Ultimately, this galaxy would have been stripped of gas and would not have survived to the modern day. It is like finding a live dinosaur, before it became extinct."
Steidel was part of an international team of astronomers, led by the University of Milano-Bicocca, that made the discovery and published its findings on March 17 in Nature Astronomy. The team's observations were made using James Webb Space Telescope (JWST), a partnership between NASA, the European Space Agency, and the Canadian Space Agency.
The researchers serendipitously noticed the large anomalous galaxy in JWST images taken of a nearby quasar—a powerful, active supermassive black hole. The team then followed up with JWST to learn more about the object's size, precise distance, rotation speed, and mass. Because the speed of light is finite, observations made of objects in the distant universe capture light from a bygone era. The JWST data revealed that the colossal specimen is not only surprisingly large, but also spins at great speeds. This led the team to nickname the galaxy "Big Wheel."
Prior to the discovery, it was thought that disk-shaped galaxies in the early universe were considerably smaller. (Disk galaxies include spiral galaxies as well as other flat, circular galaxies without spiral arms). Big Wheel is about three times larger than any previously discovered galaxies with similar masses at similar cosmic times, and it is also at least three times larger than what is predicted by current cosmological simulations. The galaxy's radius stretches across 100,00 light-years.
The finding begs the question: How did the galaxy get this big so fast? The team is not sure but suspects the answer has to do with the fact that it lives in a very dense area of space packed with a lot of young galaxies that will eventually coalesce into a giant cluster of gravitationally bound galaxies.
"Exceptionally dense environments such as the one hosting the Big Wheel are still a relatively unexplored territory," concludes co-author Sebastiano Cantalupo of the University of Milano-Bicocca. "Further targeted observations are needed to build a statistical sample of giant disks in the early universe and thus open a new window on the early stages of galaxy formation."
The Nature Astronomy study titled "A giant disk galaxy two billion years after the Big Bang," was funded by the European Research Council, the Fondazione Cariplo foundation, NASA, and the Australian Research Council.