Study: Cosmic ‘Leaf Blower’ May Explain Bright Galaxies

An artist's illustration of gas being cleared from the surroundings of a star-forming region in the early universe. (Credit: Matt Twombly for the Simons Foundation)

New research by scientists at The University of Texas at Dallas and the Center for Computational Astrophysics (CCA) at the Simons Foundation's Flatiron Institute suggests powerful ultraviolet light may have acted like a cosmic leaf blower in some of the universe's earliest galaxies. The study indicates that this radiation could sweep away gas and dust while leaving enough behind to allow stars to form at high rates.

The research, published online Nov. 4 in The Astrophysical Journal, could help explain puzzling observations from the James Webb Space Telescope (JWST), said Dr. Aaron Smith, assistant professor of physics in the School of Natural Sciences and Mathematics and co-author of the study, which was led by CCA research fellow Dr. Shyam Menon.

"Each photon bounces back and forth like a ping-pong ball, transferring momentum with every collision," Smith said. "Those repeated pushes add up, so each photon packs a much stronger punch than it would in a single interaction."

After repeated scattering, the surrounding gas experiences a significant radiation pressure, Smith said, but it was unclear whether that pressure would be enough to overcome gravity, which pulls gas together to form stars.

The researchers analyzed snapshots from an existing simulation of an early-universe, star-forming region to calculate the force that Lyman-alpha radiation pressure would have exerted on the region's gas and dust.

They found that in the densest structures, the pressure was not sufficient to counteract gravity. Enough material was preserved to continue to fuel star formation.

In less-dense regions, however, Lyman-alpha radiation pushed out the gas, clearing away the material that would have otherwise cloaked the young galaxy from view by the Webb telescope.

"Our results show Lyman-alpha radiation can push low-density gas and dust away really, really fast," Menon said. "It can sort of clear the 'fog,' allowing us to see stars in the early universe much more clearly than we would otherwise."

Smith said their calculations show that this radiation can have a huge impact on star-forming environments.

"If you think about a leaf blower, it's not moving trees," Smith said. "But it is very effective at clearing away loose leaves. Once the JWST began observing the early universe, we started seeing bright galaxies in places where our models did not expect to see them. This kind of feedback provides another piece of the puzzle."

The strength of the effect depends on the amount and properties of dust, which can absorb Lyman-alpha photons before they scatter again.

Smith said the study demonstrates how advances in computational astrophysics are allowing researchers to tackle problems once considered too difficult to model.

"Ten years ago, many researchers viewed this as a problem that was simply too computationally expensive to tackle," Smith said. "Now we have the algorithms and computing resources to start addressing it in a meaningful way."

The next step is to run simulations where the radiation and gas evolve together, including how light changes the gas and how the changing gas alters the way light escapes. UT Dallas physics doctoral students Kevin Lorinc BS'26 and Joshua Kasiri BS'26 have conducted related research and are collaborating with Stockholm University doctoral student Olof Nebrin, who has demonstrated this approach in two dimensions.

Smith said future work will focus on incorporating Lyman-alpha radiation directly into fully dynamic 3D simulations and comparing those predictions with Webb telescope observations.

Funding for the research was provided in part by the Space Telescope Science Institute program JWST-AR-08709, which is operated by the Association of Universities for Research in Astronomy under NASA contract NAS 5-03127. The Simons Foundation provided computing resources, and additional support came from the National Science Foundation through the Kavli Institute for Theoretical Physics.