UT Dallas simulations suggest ultraviolet light swept the haze from some of the universe's first galaxies

A physicist at the University of Texas at Dallas helped build a new explanation for one of the puzzles raised by the James Webb Space Telescope: why the early universe appears to hold more bright galaxies than astronomers' models predicted.

The study, published online Wednesday in The Astrophysical Journal, was led by Dr. Shyam Menon, a research fellow at the Flatiron Institute's Center for Computational Astrophysics, with Dr. Aaron Smith, an assistant professor of physics in the UT Dallas School of Natural Sciences and Mathematics, as co-author, the Richardson university said.

Light that pushes

The pair looked at Lyman-alpha radiation, ultraviolet light emitted by the hydrogen that surrounds newborn stars. A Lyman-alpha photon can bounce between hydrogen atoms many times before it escapes or is absorbed by dust, and each bounce transfers a bit of momentum to the gas.

"Each photon bounces back and forth like a ping-pong ball, transferring momentum with every collision," Smith said in the university's announcement.

Working from frames of a previously run simulation of a region where stars were forming soon after the Big Bang, the researchers calculated how hard that radiation would push on the surrounding gas and dust. In the densest pockets, the push was not enough to beat gravity, so material stayed put and kept feeding star formation.

In thinner regions, the light drove gas away quickly, clearing material that would otherwise hide a young galaxy from Webb's view.

Smith compared the effect to a leaf blower. "It's not moving trees," he said. "But it is very effective at clearing away loose leaves."

Menon put it in terms of visibility: the radiation "can sort of clear the 'fog,' allowing us to see stars in the early universe much more clearly than we would otherwise," he said in the release.

Limits and next steps

How strong the effect is depends on how much dust is present and what it is like, since dust can absorb the photons before they scatter again. The researchers' next step is simulations in which the radiation and the gas change together over time, and eventually full three-dimensional models whose predictions can be checked against Webb observations, the university said.

UT Dallas physics doctoral students are working on related research with a Stockholm University doctoral student who has already shown the technique works in a two-dimensional model.

Smith said problems like this one were long considered too expensive to compute. "Now we have the algorithms and computing resources to start addressing it in a meaningful way," he said.

The work was funded in part by a Space Telescope Science Institute program, with computing resources from the Simons Foundation and further support from the National Science Foundation by way of the Kavli Institute for Theoretical Physics.

Sources

news.utdallas.edu