Almost a Third of Early Galaxies Were Already Spirals

In the years before the JWST’s launch, astronomers’ efforts to understand the early Universe were stymied by a stubborn obstacle: the light from the early Universe was red-shifted to an extreme degree. The JWST was built with extreme redshifts in mind, and one of its goals was to study Galaxy Assembly.

Once the JWST activated its segmented, beryllium eye, the Universe’s most ancient, red-shifted light became visible.

The light emitted by the first galaxies is not only faint but has been stretched by billions of years of cosmic expansion. The galaxies that emitted that light are called high-redshift galaxies, where redshift is indicated by the letter z. Since its shifted into the red, only infrared telescopes can see it. Telescopes like the Hubble and the Spitzer can see some redshifted light. But the JWST has far more power than its predecessors, allowing it to effectively see further back in time.

“Using advanced instruments such as JWST allows us to study more distant galaxies with greater detail than ever before.”

Yicheng Guo, Department of Physics and Astronomy, University of Missouri

Observations have shown that galaxies grow large through mergers and collisions and that up to 60% of all galaxies are spirals. But how did the process play out? When did the first spirals emerge? An answer to that question trickles down and affects other outstanding questions about galaxies.

Spiral arms host active star formation, where successive generations of stars create heavier elements. Those elements allow rocky planets to form and are also a requirement for life. So, an understanding of when spiral galaxies formed helps astronomers understand the parameters of star formation, rocky planet formation, and even, potentially, the appearance of life.

“Knowing when spiral galaxies formed in the universe has been a popular question in astronomy because it helps us understand the evolution and history of the cosmos.”

Vicki Kuhn, Department of Physics and Astronomy, University of Missouri

One of the JWST’s observing efforts is CEERS, the Cosmic Evolution Early Release Science Survey. In CEERS, the JWST was the first telescope to capture images of the Universe’s early galaxies. CEERS found the most distant active supermassive black hole and galaxies that existed in the distant past when the Universe was only about 500 to 700 million years old.

Image of CEERS scientists looking at the Epoch 1 NIRCam color mosaic in TACC’s visualization lab at UT Austin. Credit: R. Larson

New research published in The Astrophysical Journal Letters examined galaxies from CEERS to determine how many of these ancient galaxies were spirals. The title is “JWST Reveals a Surprisingly High Fraction of Galaxies Being Spiral-like at 0.5 ≤ z ≤ 4.” The first author is Vicki Kuhn, a graduate student in the University of Missouri’s Department of Physics and Astronomy.

“Scientists formerly believed most spiral galaxies developed around 6 to 7 billion years after the universe formed,” said Yicheng Guo, an associate professor in Mizzou’s (University of Missouri) Department of Physics and Astronomy and co-author of the study. “However, our study shows spiral galaxies were already prevalent as early as 2 billion years afterward. This means galaxy formation happened more rapidly than we previously thought.”

In their research letter, the authors examined 873 galaxies from CEERS with redshift 0.5 ≤ z ≤ 4 and stellar mass ≤ 1010 solar masses. They found that 216 of them had spiral structures. “This fraction is surprisingly high and implies that the formation of spiral arms, as well as disks, was earlier in the Universe,” the authors write in their paper.

This figure from the research shows some of the galaxies in the sample. Redshift increases from left to right, and the rows from top to bottom show the range of galaxies classified as spiral to nonspiral. “Spiral structure is easier to see at the lower redshift ranges and becomes less pronounced at higher redshifts.” the authors write. The top three rows show galaxies identified as spirals with strong confidence, the middle three rows show galaxies identified as spirals with less confidence, and the bottom row shows non-spirals. Image Credit: Kuhn et al. 2024

“Knowing when spiral galaxies formed in the universe has been a popular question in astronomy because it helps us understand the evolution and history of the cosmos,” said lead author Kuhn. “Many theoretical ideas exist about how spiral arms are formed, but the formation mechanisms can vary amongst different types of spiral galaxies. This new information helps us better match the physical properties of galaxies with theories — creating a more comprehensive cosmic timeline.”

Spiral galaxies started as disks of gas. These results, when combined with other studies of high-redshift galaxies, paint a picture of the history of galaxy evolution in the early Universe. Dynamically hot gaseous disks appear around z = 4 to 5. These disks settled down to become dynamically cold gaseous disks around z = 3 to 4. Since stars form when gas cools and clumps together, large numbers of dynamically cold stellar disks appeared at z = 3 to 4, as indicated by their spiral arms.

This research also illuminates the relationships between spiral arms and other galaxy substructures. Gas-rich disks at high redshifts are very turbulent, and gravitational instabilities form giant clumps of star formation. Later, hot stars disperse young galaxies’ velocities, allowing them to settle down and become less turbulent. These bulges of star formation can also merge, helping to further stabilize the disks. The conclusion is that gravitational instabilities primarily lead to spiral arms, with clumps playing a secondary role since they co-exist with spirals at high redshifts.

The authors point out some caveats in their work. Galaxies that are merging can appear as spirals. The long tails prevalent during mergers can look like spiral arms, so their numbers could be off a little. But on the other hand, spirals can also look like mergers, adding to the uncertainty. “This situation is more severe for galaxies at z > 2, as the merger fraction is believed to be higher then,” the authors write.

But these facts likely don’t affect the conclusion much. “The observed spiral fraction decreases with increasing redshift, from ~43% at z = 1 to ~4% at z = 3,” the researchers conclude. So, while spirals are rarer the further we look back in time, they’re still more plentiful earlier than thought.

“Using advanced instruments such as JWST allows us to study more distant galaxies with greater detail than ever before,” Guo said. “A galaxy’s spiral arms are a fundamental feature used by astronomers to categorize galaxies and understand how they form over time. Even though we still have many questions about the universe’s past, analyzing this data helps us uncover additional clues and deepens our understanding of the physics that shaped the nature of our universe.”

Evan Gough

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