When astronomers studied images of the early Universe obtained by the James Webb Space Telescope, they noticed an oddity: there were too many bright galaxies. According to existing ideas, in the first hundreds of millions of years after the Big Bang such galaxies should have been encountered far less often.
Now researchers from the Flatiron Institute (USA) and the University of Texas at Dallas have proposed an explanation. According to their modeling, powerful ultraviolet radiation from clusters of young stars works like a kind of "leaf blower": it expels gas and dust from the galaxy that block light, and the galaxy appears brighter. The work was published in The Astrophysical Journal.
The radiation in question is Lyman-alpha. It arises when an electron collides with a hydrogen atom or attaches to a hydrogen ion. In environments with young stars such radiation is abundant, and light pressure can influence the formation of stellar "nurseries" - regions where stars are born.
When Lyman-alpha radiation strikes gas or dust, they absorb the energy and are pushed away, like a baseball glove throwing back a caught ball. This effect carries material outward, leaving less material for star formation. Astronomers have long suspected that the radiation could be important, but assessing its role under real conditions was difficult because of modeling challenges.
The peculiarity of Lyman-alpha radiation is that hydrogen atoms, having absorbed it, almost immediately emit light of the same wavelength. Therefore neighboring atoms can absorb and re-emit it many times until it escapes outward. This makes the radiation a powerful source of pressure, but requires enormous computations.
To get around the difficulty, the authors analyzed an existing simulation of a star-forming region in the early Universe and calculated what force Lyman-alpha radiation pressure would create. It turned out that in dense regions this pressure is insufficient to overcome gravity: gas and dust are pushed away but remain bound. But in the rarefied spaces between stars, where gravity is weaker, the radiation accelerates gas to high speeds and triggers explosive winds that eject material beyond the galaxy.
This, according to the researchers, may explain the brightness of early galaxies: the radiation blows away loose gas and dust, clearing the line of sight between the stars and the observer. The authors emphasize that this is only a first step. To fully understand the effect, self-consistent models are needed that simultaneously account for radiation, gas flows and star formation. So far such calculations have been done in two dimensions, and in the future, with growing computing power, the researchers hope to move to three-dimensional simulations.