When astronomers looked at photos of the early universe taken by the James Webb Space Telescope, they found something odd: an overabundance of bright galaxies . Based on what they knew about the early universe, such galaxies should have been far less common in the few hundred million years following the Big Bang.
Now, using cutting-edge simulations, researchers at the Simons Foundation's Flatiron Institute in New York City and the University of Texas at Dallas have an answer to this mystery: Powerful ultraviolet radiation produced by clusters of young stars acted like a leaf blower, flinging light-blocking gas and dust out of the galaxy and thereby making the galaxies appear brighter.
The researchers report their findings on this ultraviolet light , called Lyman-alpha radiation, November 4 in The Astrophysical Journal.
"Our results show Lyman-alpha radiation can push low-density gas and dust away really, really fast," says Shyam Menon , a research fellow at the Flatiron Institute's Center for Computational Astrophysics and the study's lead author. "It can sort of clear the 'fog,' allowing us to see stars in the early universe much more clearly than we would otherwise."
Lyman-alpha radiation is created when an electron and a hydrogen atom collide or when an electron joins up with a hydrogen ion. In powerful stellar environments where Lyman-alpha radiation is abundant, the pressure from the light is thought to play a role in sculpting stellar nurseries. When the Lyman-alpha radiation hits gas or dust, the material absorbs the energy and recoils from the impact like a baseball catcher's mitt catching a fastball. This effect launches the material outward, leaving less material available to form a star. Some astronomers wondered whether, in extreme cases, the effect could be strong enough to halt star formation.
"We've known for a very long time that Lyman-alpha radiation could be important because it carries a lot of force," Menon says. "But we've never actually been able to estimate how important it is in a realistic environment because of how challenging it is to model."
Lyman-alpha radiation differs from many other types of radiation in how it interacts with matter. When an atom or dust grain absorbs most kinds of light, it typically re-emits that energy at a different wavelength, making the light much less likely to be absorbed again. But when hydrogen atoms absorb Lyman-alpha radiation, they almost immediately re-emit it at the same wavelength. This means nearby hydrogen atoms can absorb the light over and over before it finally escapes. While this repeated absorption and re-emission makes Lyman-alpha a particularly powerful source of radiation pressure, it also makes it computationally challenging to simulate because each interaction must be modeled.
Instead of directly simulating this radiation recycling, Menon and his collaborator, Aaron Smith at the University of Texas at Dallas, developed a workaround. They 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.
The results showed that in dense environments, the outward pressure from Lyman-alpha radiation wasn't strong enough to counteract gravity. While gas and dust hit by the radiation recoiled, they remained bound to the stellar nursery by gravity. This means that the radiation likely wouldn't have strongly affected star formation in dense regions in the early universe. However, in the spaces between stars where gas was less dense and gravity was lower, the Lyman-alpha radiation was able to win against gravity.
"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.
This effect likely contributes to the brightness of early galaxies. If Lyman-alpha radiation pressure is pushing away loose gas and dust from between stars, it clears the line of sight between those stars and us.
While the results are an important step toward understanding the impact of Lyman-alpha radiation, the findings are just the tip of the iceberg. More research is needed to better grasp the nature of dust in the early universe, Menon says. To fully understand the effects of Lyman-alpha radiation pressure, researchers will need simulations of the early universe that self-consistently incorporate Lyman-alpha radiation, gas flows and star formation.
Recent work by Olof Nebrin of Stockholm University and study co-author Aaron Smith of the University of Texas at Dallas has demonstrated that these effects can be modeled in two dimensions, Menon says. "But the hope is that in the coming years, with improvements to computing power and the development of more effective algorithms, we can eventually do these simulations in three dimensions."
