Credit: NASA/ESA/CSA, I. Labbe/R. Bezanson/Alyssa Pagan (STScI), Rashi Jain/Yogesh Wadadekar (NCRA-TIFR)
A paper published in Astronomy and Astrophysics, details the discovery of a grand-design spiral galaxy that formed just 1.5 billion years after the Big Bang. Grand design galaxies are similar to the Milky Way in that they display well-formed spiral arms. Prior to this discovery, astronomers thought such objects took billions of years to achieve grand-design status. Apparently not.
Using NASA’s James Webb Space Telescope (JWST), researchers Rashi Jain and Yogesh Wadadekar, working at the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research in India, spotted a galaxy remarkably similar to our own Milky Way, but which apparently formed when the universe was 1.5 billion years old. They named it Alaknanda, after the Himalayan river that is a twin headstream of the Ganga alongside the Mandakini, which is the Hindi name for the Milky Way.
The two astronomers studied JWST images of Alaknanda taken through 21 different filters. This data allowed them to estimate the galaxy’s distance, dust content, how many stars it contains, and how quickly new stars have been forming over time.
They were able to analyze it because Alaknanda lies behind the massive galaxy cluster Abell 2744. The combined gravity of the members of the cluster bent and magnified the young galaxy’s light. This effect, known as gravitational lensing, made Alaknanda appear twice as bright, allowing JWST to capture its spiral structure in stunning detail.
A galactic exception
Until this discovery, astronomers thought that galaxies forming so early should appear irregular and disordered, basically still forming. And for one of them to turn into a grand spiral would take lots of time for gas to flow in from surrounding space, settle into a rotating disk, and be molded into spiral arms by waves inside the galaxy.
But the youthful Alaknanda already has two sweeping arms wrapped around a bright, round central region about 30,000 light-years across. This galaxy is also forming stars at a rate 20 times faster than our Milky Way. In fact, half of Alaknanda’s stars appear to have formed in only 200 million years.
“Alaknanda has the structural maturity we associate with galaxies that are billions of years older,” said Jain, the study’s lead author. “Finding such a well-organized spiral disk at this epoch tells us that the physical processes driving galaxy formation — gas accretion, disk settling, and possibly the development of spiral density waves — can operate far more efficiently than current models predict. It’s forcing us to rethink our theoretical framework.”
How did Alaknanda’s spiral arms form so quickly?
This question is the main one astronomers will be trying to answer. One theory is that the galaxy grew steadily by pulling in streams of cold gas, allowing density waves to naturally carve out spiral patterns. Another is that a gravitational encounter with a smaller companion galaxy triggered the arms. Future observations with JWST could measure how fast the galaxy is rotating and whether its disk is moving in an orderly way or is more turbulent. That data would lend credence to one of the theories.
“Alaknanda reveals that the early universe was capable of far more rapid galaxy assembly than we anticipated,” says Yogesh Wadadekar, the study’s co-author. “Somehow, this galaxy managed to pull together ten billion solar masses of stars and organize them into a beautiful spiral disk in just a few hundred million years. That’s extraordinarily fast by cosmic standards, and it compels astronomers to rethink how galaxies form.”
