JWST finds spiral galaxy about 5 times more massive than Milky Way — scientists call it 'Big Wheel'

A team of astronomers at the University of Milano-Bicocca has uncovered a colossal spiral galaxy that existed just 2 billion years after the Big Bang, which gave birth to the universe some 13.8 billion years ago. Nicknamed "Big Wheel," it is in fact one of the largest galaxies ever observed from this early cosmic era.

The scientists found Big Wheel near a quasar, which is a powerful and active supermassive black hole, using the James Webb Space Telescope (JWST). The galaxy lies 11.7 billion light-years away from our corner of the cosmos, and its given nickname comes from its remarkably fast rotation and huge size. It's five times more massive than the Milky Way, for context, stretches across 100,000 light-years.

More specifically, the astronomers used new spectroscopic observations with the JWST's Near-Infrared Spectrograph (NIRSpec) to confirm that Big Wheel is a rotating disk. The galaxy's rotation curve, an important characteristic of spiral galaxies, shows a pattern typical of flat rotation curves seen in mature galaxies. The velocity of the galaxy's rotation increases as you move outward from the center, reaching a maximum rotational velocity of several hundred miles per second, which is also similar to much more developed galaxies.

Big Wheel's rotational velocity also aligns with the local Tully-Fisher relationship, a correlation between the size and rotation speed of galaxies observed today. What this all means it that, despite its youth, the galaxy behaves in a manner consistent with some of the largest, most mature spiral galaxies we see in the present universe. Big Wheel exists during a time when most galaxies are expected to be small and in their earliest stages of development. Yet, it's fully formed.

"This galaxy is spectacular for being among the largest spiral galaxies ever found, which is unprecedented for this early era of the universe," Charles Steidel, the study's lead author and an astronomy professor at Caltech, in a statement.

So, how could this happen?

One potential clue lies in Big Wheel's environment. The galaxy resides in a dense region of space where galaxy number densities are more than ten times higher than the cosmic average. This dense environment could provide the perfect conditions for rapid galaxy growth. Sebastiano Cantalupo, co-author of the study, suggests Big Wheel may have benefited from efficient gas accretion, which carried the coherent angular momentum necessary for the formation of large disks. Additionally, the frequent mergers of gas-rich galaxies in this crowded region may have contributed to its massive size and rapid growth.

"We think this may open the door to understanding how some galaxies were able to bypass the usual slow process of star formation and grow to enormous sizes in the early universe," Cantalupo said in the statement.

The discovery suggests that galaxy formation might not be as slow or gradual as previously thought, especially in environments rich in gas and merging galaxies.

Big Wheel challenges current cosmological models. Its size and mass far exceed predictions for galaxies at similar redshifts, making it an outlier in the galaxy population. Down the line, astronomers may need to adjust their models to account for the possibility of rapid galaxy growth under such dense conditions.

The study was published on March 17 in the journal Nature Astronomy.