Flatiron Institute and University of Texas at Dallas propose Lyman-alpha winds explain early galaxies

Astronomers using images from the James Webb Space Telescope found more bright galaxies in the early universe than earlier models predicted, according to work reported Nov. 4 in The Astrophysical Journal. The same study proposes that winds driven by starlight may explain why those galaxies appear so bright.

The finding ran contrary to earlier expectations that bright galaxies should be far less common so soon after the Big Bang. They appear in the universe’s first few hundred million years, challenging prior ideas about how common bright galaxies should be so early.

An electron produces Lyman-alpha radiation when it strikes a hydrogen atom or binds to a hydrogen ion. When the radiation hits gas or dust, the material absorbs the energy and moves outward, launching material away and leaving less behind for new stars. Hydrogen atoms absorb the radiation and re-emit it at the same wavelength almost immediately, letting nearby atoms absorb it again before it escapes.

Shyam Menon of the Flatiron Institute and Aaron Smith of the University of Texas at Dallas developed a workaround by studying snapshots from an existing simulation of star formation in the early universe. They used those snapshots to calculate the force the radiation would apply to gas and dust, bypassing direct simulation of radiation recycling. Menon said the radiation carries force, but researchers had never estimated its real-world importance because the modeling is difficult.

In dense star-forming regions, gravity overpowered the radiation pressure, so gas and dust struck by the radiation stayed bound to the nursery. Those regions probably would not strongly shape star formation. In the low-density space between stars, where gravity is weaker, the radiation overcame gravity and drove gas to high speeds, launching winds that blast material out of the galaxy. Removing loose gas and dust clears the view between stars and observers, which likely makes early galaxies appear brighter.

Menon said the pressure can push low-density gas and dust away quickly, clearing the view so stars in the early universe are easier to see. He said fully grasping the pressure requires simulations that combine radiation, gas flows and star formation in a self-consistent way. Menon noted that Olof Nebrin of Stockholm University and Aaron Smith of the University of Texas at Dallas have shown these effects work in two-dimensional models. He said he hopes better computing power and improved algorithms will eventually let researchers run these simulations in three dimensions.

A survey led by the University of Utah uses Hubble’s Cosmic Origins Spectrograph to examine massive stars in nearby galaxies that stand in for early-universe stars. Grace Telford is an assistant professor in the University of Utah’s Department of Physics and Astronomy and leads the study. She said the telescope raised questions about how these early galaxies evolved because they look unusual. The Astrophysical Journal Supplement Series published the survey on Sept. 21, 2026.

Stars with more than 10 times the sun’s mass are rare, yet they drive powerful change in galaxies. They release intense radiation, lose mass through stellar winds and explode as supernovae that return energy and matter to nearby gas. The term metallicity refers to how much of a star consists of elements heavier than hydrogen and helium. Early galaxies held far fewer heavy elements than the Milky Way, so their massive stars probably had different physical traits.

The survey looked at 29 massive stars in six nearby dwarf galaxies whose metallicities fall below one-fifth of the sun’s. Each target needs as much as 35 hours on Hubble, since stars like these are faint outside the Milky Way. The project added fresh observations of 12 stars to older data, building a larger, consistent dataset.

TEMPOS results show that wind speed drops as metallicity falls, but below roughly 10% of the sun’s level the decline steepens much more than expected. Metal ions connect a star’s radiation to nearby material, so astronomers expect weaker winds and less mass loss over the lives of lower-metallicity stars. If these stars lose less mass through weak winds, they may keep more of their starting mass, changing how they live, die and influence their galaxies.

Researchers gauged the strength of faint iron absorption lines in ultraviolet spectra to see how much light iron takes out of the stellar ultraviolet continuum. Measuring iron is difficult in metal-poor settings, so researchers often estimate metallicity using oxygen instead. Stars in oxygen-rich galaxies show much stronger iron absorption than stars in oxygen-poor galaxies, suggesting metal-poor stars cover a wide range of iron abundances. Telford said this gives the team the statistical power to spot that trend across many stars in six galaxies with different chemistry.

Researchers are pairing Hubble ultraviolet spectra with visible-light data from the Keck Observatory in Hawaii to improve models and measure chemical abundances and mass-loss rates. Observations came from the NASA/ESA Hubble Space Telescope, with support from NASA grant numbers GO-16767, GO-16920 and GO-17491. Researchers from California State University Fresno, Rutgers University, the Space Telescope Science Institute, Christian-Albrechts-Universitat zu Kiel, the University of Texas at Austin, Rensselaer Polytechnic Institute, Princeton University and the University of Notre Dame collaborated on the project.