Light Acts as a Cosmic Leaf Blower, Clearing Early-Universe Fog to Explain Bright Galaxies

When the James Webb Space Telescope turned its cameras on the early universe, it returned images that did not match expectations. There were too many bright galaxies. Given what astronomers understood about the first few hundred million years after the Big Bang, galaxies that luminous should have been much rarer.

On Nov. 4, researchers at the Simons Foundation's Flatiron Institute in New York City and the University of Texas at Dallas published a possible answer in The Astrophysical Journal. Their simulations suggest that intense ultraviolet light from clusters of young stars acted something like a leaf blower. It flung light-blocking gas and dust out of the galaxy, and the galaxies looked brighter as a result.

The light in question is called Lyman-alpha radiation. It is produced when an electron collides with a hydrogen atom, or when an electron joins up with a hydrogen ion. In powerful stellar environments where it is abundant, the pressure of the light is thought to help sculpt stellar nurseries. When it strikes gas or dust, the material absorbs the energy and recoils, much as a catcher's mitt jolts when it takes a fastball. That pushes the material outward and leaves less of it to form stars. Some astronomers have wondered whether, in extreme cases, the effect could stop star formation altogether.

"We've known for a very long time that Lyman-alpha radiation could be important because it carries a lot of force," said Shyam Menon, a research fellow at the Flatiron Institute's Center for Computational Astrophysics and the study's lead author. The trouble was estimating how important it is in a realistic environment, because the physics is hard to model.

The difficulty lies in how Lyman-alpha behaves. Most light, once absorbed by an atom or dust grain, is re-emitted at a different wavelength and is much less likely to be absorbed again. Hydrogen atoms that absorb Lyman-alpha, however, re-emit it almost immediately at the same wavelength. Neighboring hydrogen atoms can then absorb it again, repeatedly, before it escapes. That recycling makes Lyman-alpha an especially strong source of radiation pressure, but every interaction has to be accounted for, which makes direct simulation computationally punishing.

Menon and his collaborator Aaron Smith, of the University of Texas at Dallas, built a workaround. Rather than simulate the recycling directly, they took snapshots from an existing simulation of an early-universe star-forming region and calculated the force Lyman-alpha pressure would have exerted on its gas and dust.

The results depended on density. In dense regions, the outward push was not strong enough to beat gravity. Gas and dust recoiled but stayed bound to the stellar nursery, so the radiation probably did not strongly affect star formation there. In the thinner spaces between stars, where gas is less dense and gravity weaker, the radiation won. "When the density is low enough, the Lyman-alpha radiation can push the gas to really high speeds, launching explosive winds that can shoot material clear out of the galaxy," Menon said.

That likely contributes to the brightness Webb observed. If loose gas and dust between stars are blown away, the line of sight from those stars to us is cleared. Menon described the effect as clearing the fog.

The work is a first step, Menon said, and he called it the tip of the iceberg. Researchers still need a better grasp of the nature of dust in the early universe. A full account will require simulations that self-consistently combine Lyman-alpha radiation, gas flows and star formation. Olof Nebrin of Stockholm University and Smith have recently shown such effects can be modeled in two dimensions. Menon hopes that better computing power and more effective algorithms will make three-dimensional versions possible in the coming years.