Spiral arms and bars are galactic fuel pumps for star formation

Peak star formation took place during cosmic noon, between 2–3 billion years after the Big Bang. The star formation rate (SFR) back then was up to 100 times greater than it is today. For the SFR to be so high, gas had to move through galaxies efficiently.

Astronomers have struggled with this fact because early galaxies were thought to be messy and chaotic due to mergers and turbulence. But new research reveals something different. It shows that massive disk galaxies with bars and spiral arms moved cold gas around efficiently, driving their high SFRs.

These results are reported in two new papers. The first paper is “Galaxy morphologies at cosmic noon with JWST: A foundation for exploring gas transport with bars and spiral arms,” and it’s published in Astronomy and Astrophysics. The lead author is Dr. Juan Manuel Espejo Salcedo, from the Max Planck Institute for Extraterrestrial Physics.

The second paper is “NOEMA3D: Resolving radial gas flows in disk galaxies at z~1.1-1.6 with high-resolution CO observations,” and it’s available on the arXiv preprint server. The lead author is Jean-Baptiste Jolly, also from the Max Planck Institute for Extraterrestrial Physics.

Stars can form only from cold gas. If gas is heated by something like an active galactic nucleus or is turbulent because of a merger, then star formation suffers. Only cold, dense gas can collapse to form stars. Inside galaxies, this means that cold gas has to flow from the outer disk into the central regions, where stars form.

The two new papers are based on NOEMA3D, a survey of how cold gas moves around in star-forming galaxies during cosmic noon. NOEMA3D examined massive main-sequence galaxies with JWST and NOEMA, the Northern Extended Millimeter Array, to generate a high-resolution study of molecular gas kinematics. The first paper is based on a subset of 10 of NOEMA3D’s galaxies, and the second paper considers a much larger sample.

“A fundamental question in galaxy evolution is how early star-forming galaxies assembled the well-ordered structures seen in the present-day universe,” the authors of the first paper write. While previous observations have shown that cosmic noon galaxies were lumpy and chaotic, more powerful observations with JWST and NOEMA have revealed something else.

“While early morphological studies suggested that high-redshift galaxies were highly irregular and dynamically unstable, kinematic surveys have since revealed that disk-like rotation is widespread at cosmic noon,” the authors explain. In a more chaotic, irregular galaxy, velocity dispersions would be high. But that’s not what astronomers are finding today.

“With the advent of the HST Wide Field Camera 3 (WFC3) and near-infrared imaging, deeper surveys began to reveal a growing number of galaxies with more regular morphologies,” Espejo Salcedo and his colleagues write.

These cosmic noon galaxies are well-ordered spirals, and four of the 10 also have bars. At these redshifts, astronomers thought these features were rare. But, not for the first time, JWST is helping show us how wrong we were about galaxies in the universe’s early periods.

By measuring the gas velocities in the galaxies, the authors of the second paper found that some of the gas moved just like it would in an ordinary rotating galaxy. But in nearly every one, rotation couldn’t explain all of the gas movement. JWST showed that the excess gas movement is spatially correlated with the galaxies’ bars and spirals.

“For the first time, we can directly link spiral arms and bars to the motions of cold gas within galaxies,” Jolly says. “This provides compelling evidence that these structures were already driving gas transport when the universe was at the peak of its star-forming activity.”

This means that arms and bars are channeling gas into the galaxies’ inner regions. They actively redistribute gas. The rate of inflow is comparable to the galaxies’ SFRs. So the gas is feeding star formation and may also be contributing to supermassive black holes.

“The depth of the NOEMA observations allows us to trace the cold-gas reservoirs that fueled galaxy growth during cosmic noon,” said Jianhang Chen, co-author of the first study. “We can now see, in unprecedented detail, how galaxies sustained star formation across their disks over billions of years.”

These results are helping paint an entirely new picture of the first galaxies, their morphologies and how they had such high SFRs. With their arms and bars already well-established, these cosmic noon galaxies were able to efficiently channel cold, star-forming gas from their outer regions into their centers. This contradicts our previous understanding, in which early galaxies were clumpy and messy.

Many of these ancient galaxies were very similar to the present-day Milky Way, with its clear spiral arms and bar. But the speed at which gas moved through them was much higher than in local galaxies.

“These flows would be sufficient to fuel the high SFR of galaxies at cosmic noon, promoting bulge formation and possibly the feeding of central SMBHs,” Jolly and his co-authors conclude.

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Lisa Lock

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Andrew Zinin

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