Spiral Galaxies May Be a Dime a Dozen in the Early Universe

Today, spiral galaxies like our own Milky Way populate the cosmos. Characterized by puffy spiral arms of gas and dust and usually rich in star formation, these galaxies emerged onto the cosmic scene only about 6-7 billion years ago — or so astronomers thought. Surprising new data from the James Webb Space Telescope has revealed more ancient origins.

A study published in The Astrophysical Journal Letters by researchers at the University of Missouri examined images of galaxies near and far, calculating how the fraction of spiral to non-spiral galaxies changed over time. The team studied light that had traveled as far as 12 billion years (out to a redshift of 4).

“The big question we want to answer is, when do spiral galaxies first start to appear?” says graduate student and team member Vicki Kuhn. As it turns out, the answer is a lot earlier than previously thought.

“JWST has found four times more spirals than Hubble [Space Telescope] did, at a time when the universe was only about 3 billion years old,” says Kuhn, who presented the team’s findings at a press conference during the 244th meeting of the American Astronomical Society. In a surprising twist, the team found that spiral galaxies actually made up around 35% of all galaxies in a universe only 1.5 billion years old. The team suggests that spirals might have first formed only slightly earlier, perhaps 1 billion years after the Big Bang.

Hunting for Spirals

But how do astronomers observe galaxies from the distant past? Because the universe is large and because light takes time to travel, the light from faraway galaxies can take millions to billions of years to reach Earth — and when it arrives, it provides a sort of cosmic time capsule. The distances to these galaxies can be quantified by their redshift, or the shift in light’s color as it travels through an expanding Universe. The team on this study collected 873 galaxies at redshifts between 0.5 and 4 — which means their light has traveled 5 to 12 billion years to Earth.

The team then inspected each galaxy by eye to discern spiral features. Initial measurements indicate that the fraction of observed spiral galaxies decreases from 48% to 8% as the team probes further back in time. Even the small fraction at the earliest times is higher than previously seen with the Hubble Space Telescope.

Although JWST is more sensitive to early galaxies than its predecessor, some of the fainter spiral arms may have escaped detection by both telescopes. So, in addition to their observed spiral fraction, the team also determined the intrinsic spiral fraction, based on the observational limitations that impact distant targets.

They took a sample of their nearer galaxies, artificially shifted them to higher redshifts, and noted how spiral features would disappear. Based on how many spirals escaped detection at higher redshifts, the team was able to deduce that around 35% of the galaxies in the young universe had spiral arms.

A Cosmic Puzzle

How galaxies formed in the early universe remains an open question, and this discovery only complicates our picture of galactic evolution. Numerous theories have been put forth on the formation mechanism of spiral arms and their link to star formation, from rippling density waves to tidal pulls from other galaxies. Knowing when spiral galaxies emerged onto the cosmic frontier provides a crucial piece to this puzzle.

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See the March 2023 issue of Sky & Telescope to learn more about how galaxies form spiral arms.

In addition to spiral galaxies, astronomers also looked at the fraction of disk galaxies over time. Not all disk galaxies host a spiral pattern, but all spiral galaxies contain a disk, and astronomers think spiral galaxies evolved from these gas-rich, turbulent disks. The team found that about 50% of early galaxies were disk-like out to around 9 billion years ago, and other studies have found disk galaxies as far back as almost 13 billion years ago. These hot, gaseous disks would have eventually cooled and settled, thus paving the way for spiral galaxies to proliferate.

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However, some spiral galaxies appear to coexist with disk galaxies even at early times, suggesting a more complicated path to spiralhood. “This is much farther back in time than what was just thought a few years ago,” Kuhn said, “and will change what we know about when spiral galaxies form, how they form at these early times, and what role this early formation has on their kinematic and physical properties.”

As telescopes probe deeper into cosmic time, astronomers can hope to learn more about how galaxies form, evolve, and sustain stellar ecosystems. The discovery of early spiral galaxies provides important clues for unraveling the secrets of a budding universe.