A surprisingly large disk galaxy in the early universe – about two billion years after the Big Bang – with dimensions more typical of the giant galactic disks of the current universe. This is the discovery of the “Cosmic Web” research group, part of the Astrophysics Unit of the Physics Department at the University of Milano-Bicocca, reported in an article published today in “Nature Astronomy” (“A giant disk galaxy two billion years after the Big Bang”, DOI: 10.1038/s41550-025-02500-2), signed by Weichen Wang and Sebastiano Cantalupo, respectively research fellow (post-doc) and full professor at the university, in addition to the other members of the “Cosmic Web” group and international collaborators. The discovery is based on the data obtained by the researchers of Milano-Bicocca from the James Webb Space Telescope (JWST), the largest and most powerful space observatory ever built to date, the result of a partnership between NASA, ESA and the Canadian Space Agency.
"When and how galactic disks form is still a mystery in modern astronomy," says Sebastiano Cantalupo. "The first years of observations with the James Webb Space Telescope revealed several galactic disks in the early universe, which corresponds to a cosmic epoch of eleven billion years ago, or two billion years after the Big Bang. Before our observation, however, only galactic disks much smaller than those we see in the local universe had been discovered by JWST. For this reason, it was thought until now that the formation of the largest disks required as much time as most of the age of the universe. To shed new light on the issue, we turned our attention to the early universe and, in particular, to a special cosmic environment."
The Cosmic Web Group researchers conducted their study using new observations from JWST, integrated with data from other facilities such as the Hubble Space Telescope, the Very Large Telescope (VLT) and the Atacama Large Millimeter/submillimeter Array (ALMA). These observations were aimed at a specific region of the sky, which is 11-12 billion light years away from us and which is embedded in a large-scale structure that will probably evolve into a cluster of galaxies. This is an almost unique region in the universe, exceptionally dense, with a high concentration of galaxies, gas and black holes. "A laboratory in which we can study the mechanisms of galaxy formation. In fact, thanks to the finite speed of light, observations and images of the telescope are literally a picture of that region of the sky when the universe was ‘only’ 2 billion years old."
"Using data from two instruments - continues Weichen Wang - the Near-Infrared Camera and the Near-Infrared Spectrograph, on board of JWST, we identified the galaxies within this hyperdense region and analyzed their redshifts, their morphology and their kinematics, which are all necessary for the identification of galactic disks. The observations led us to the discovery of a surprisingly large disk in the large-scale structure. This galaxy, which we called “Big Wheel” due to its enormous size (Figure 1), has an effective radius (i.e. the radius that contains half of the total light) of about 10 kiloparsecs. “Big Wheel” is about three times larger than any previously discovered galaxies with similar stellar masses at similar cosmic times, and is also at least three times larger than predicted by current cosmological simulations. It is instead comparable to the size of most massive disks seen in the current universe». Further spectroscopic analyses confirmed that “Big Wheel” is a disk that rotates like a spiral galaxy, that is, like the Milky Way, our galaxy».
The early and rapid growth of this disk could be related to its highly overdense environment, which could offer surprisingly favorable physical condition to support this early giant disk formation. This was unexpected given current galaxy formation models. "Exceptionally dense environments such as the one hosting the Big Wheel are still a relatively unexplored territory," concludes Sebastiano Cantalupo. "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."
BIG WHEEL AND THE COSMIC WEB GROUP
