A group of astrophysicists has encountered an enormous spiral galaxy, approximately five times the mass of our Milky Way galaxy and extending over an area twice its size, ranking it among the most colossal known galaxies. Nonetheless, the most astonishing characteristic of this galaxy is not its immense scale but rather the fact that it was present in the primitive universe when it was merely 2 billion years old.
“This galaxy is remarkable for being one of the largest spiral galaxies ever detected, which is remarkable for this early epoch of the universe,” remarks Charles (Chuck) Steidel (PhD ’90), the Lee A. DuBridge Professor of Astronomy at Caltech. “Ultimately, this galaxy would have lost its gas and would not have persisted into modern times. It is akin to discovering a living dinosaur before it faced extinction.”
Steidel was a member of an international contingent of astronomers, spearheaded by the University of Milano-Bicocca, that made the discovery and published its conclusions on March 17 in Nature Astronomy. The observations conducted by the team utilized the James Webb Space Telescope (JWST), a collaboration between NASA, the European Space Agency, and the Canadian Space Agency.
The scientists fortuitously spotted the large atypical galaxy in JWST images captured of a nearby quasar—a potent, active supermassive black hole. The team then conducted follow-up observations with JWST to investigate the object’s dimensions, exact distance, rotational velocity, and mass. As the speed of light is not infinite, observations made of distant cosmic objects capture light from an earlier time. The JWST data indicated that the gigantic specimen is not only unexpectedly vast, but also rotates at high velocities, leading the team to affectionately dub the galaxy “Big Wheel.”
Before this discovery, it was believed that disk-shaped galaxies in the nascent universe were significantly smaller. (Disk galaxies encompass spiral galaxies as well as other flat, circular galaxies devoid of spiral arms). Big Wheel is roughly three times the size of any previously identified galaxies with comparable masses existing at similar cosmic intervals, and it is also at least three times larger than what current cosmological simulations predict. The galaxy’s radius spans across 100,000 light-years.
The discovery raises the question: How did the galaxy achieve such considerable size in such a short timeframe? The team remains uncertain but hypothesizes that the answer may lie in its location within a densely populated region of space brimming with many young galaxies that are destined to eventually amalgamate into a massive cluster of gravitationally bound galaxies.
“Extremely dense surroundings like those housing the Big Wheel are still a relatively uncharted realm,” concludes co-author Sebastiano Cantalupo from the University of Milano-Bicocca. “Additional focused observations are essential to establish a statistical sample of giant disks in the primordial universe, thereby opening a new avenue into the early phases of galaxy formation.”
The Nature Astronomy article titled “A giant disk galaxy two billion years after the Big Bang,” received funding from the European Research Council, the Fondazione Cariplo foundation, NASA, and the Australian Research Council.