Light-Driven Winds May Explain Why the Early Universe Held So Many Bright Galaxies

The photographs from the James Webb Space Telescope showed astronomers something they had not expected: too many bright galaxies. In the few hundred million years after the Big Bang, what researchers understood about the early universe said such galaxies should have been far less common. The surplus has been a puzzle since the images came in.

Now a team at the Simons Foundation's Flatiron Institute in New York City and the University of Texas at Dallas has offered an explanation based on simulations. Their proposal is that ultraviolet light from clusters of young stars acted like a leaf blower, flinging light-blocking gas and dust out of the galaxy. With the haze gone, the galaxies appear brighter. The work is reported in The Astrophysical Journal.

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 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 does when it takes a fastball. The material is launched outward, leaving less of it to form stars. Some astronomers wondered whether, in extreme cases, that push could stop star formation altogether.

Shyam Menon, a research fellow at the Flatiron Institute's Center for Computational Astrophysics and the study's lead author, says the force has long been recognized. "We've known for a very long time that Lyman-alpha radiation could be important because it carries a lot of force," he says, but no one had been able to estimate its importance in a realistic environment because it is so 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, by contrast, re-emit Lyman-alpha almost immediately at the same wavelength, so neighboring atoms can absorb it again and again before it escapes. That repeated cycle makes the radiation an especially strong source of pressure, but it also means each interaction has to be modeled, which makes simulation costly.

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

The results split by density. In dense regions, the outward pressure was not strong enough to overcome gravity. Gas and dust recoiled but stayed bound to the stellar nursery, so the radiation likely did not strongly affect star formation there. In the lower-density spaces between stars, where gravity is weaker, the radiation won out. "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 says. Clearing that loose material opens the line of sight between the stars and us, which likely contributes to the brightness of early galaxies.

Menon calls the findings the tip of the iceberg. More work is needed to understand the nature of dust in the early universe, and a full picture will require simulations that handle Lyman-alpha radiation, gas flows and star formation together and self-consistently. Recent work by Olof Nebrin of Stockholm University and Smith has shown this can be done in two dimensions. Menon hopes that better computing power and more effective algorithms will allow three-dimensional versions in the coming years.