Reionization was the last major phase transition in the universe, according to Cody Carr, a postdoctoral researcher in the University of Michigan Department of Astronomy who led the study while at Zhejiang University. During the universe’s first billion years, neutral hydrogen formed an opaque fog that blocked ultraviolet light. Beginning roughly 13 billion years ago, radiation from early galaxies ionized that hydrogen, separating it into protons and electrons and eventually clearing the intergalactic medium. Astronomers know that this transition occurred, but the objects that produced the necessary radiation and the mechanisms by which it escaped remain incompletely understood.

A leading explanation had proposed that supernovae cleared paths for ionizing radiation to leave early galaxies. The new observations point in the opposite direction. The team used spectra from the Hubble Space Telescope to measure both the wavelengths of light escaping from compact starburst galaxies and the speeds of their winds. Starburst galaxies form stars at unusually rapid rates and may resemble some of the galaxies that contributed to reionization. Neutral hydrogen and dust in their outflowing gas absorb ultraviolet light at characteristic wavelengths, producing spectral dips whose positions and widths reveal the gas’s motion.

Across the observed galaxies, faster winds were associated with lower escape fractions of ionizing radiation. The findings suggest that energy deposited by supernovae can stir the surrounding gas and increase the coverage of neutral hydrogen and dust, suppressing the escape of ultraviolet photons. “It appears that energy input from supernovae has stirred up the surroundings and increased, rather than decreased, the coverage of neutral gas and dust that suppresses ionizing radiation escape,” said Stephan McCandliss, a senior author of the study and director of the Center for Astrophysical Sciences at Johns Hopkins University. Because the research examined nearby galaxies rather than galaxies from reionization itself, it tests the proposed physical mechanism through an analogue rather than directly observing the ancient epoch.

The study, published in Nature Astronomy on 15 October 2026, supports a scenario in which young stars help photons escape before the first supernovae in a galaxy explode. Smaller-scale winds associated with stellar birth regions could still influence that early phase, although the authors said more observations are needed to test the idea. The results also call for cosmological hydrodynamic simulations to examine how radiation and winds from young stars shape surrounding gas before supernova activity begins. “Supernovae play a central role in cosmological hydrodynamic simulations that explain how ionizing radiation escapes from galaxies,” said Renyue Cen, a senior author and professor and director of Zhejiang University’s Center for Cosmology and Computational Astrophysics. “Our findings highlight the need to examine how radiation and winds from young stars shape the surrounding gas before the first supernovae explode.”

Hubble was developed by NASA with contributions from the European Space Agency, and the Space Telescope Science Institute administered a grant supporting the work. The international team included nearly 30 collaborators from more than a dozen institutions, with Sally Oey of the University of Michigan also contributing. The researchers are seeking additional Hubble observing time to study more galaxies and further test whether radiation escapes preferentially before powerful supernova-driven winds develop. The study therefore narrows an important uncertainty about reionization, but it does not identify the precise population of early galaxies responsible for the transition or establish a complete timeline for the escape of their radiation.

Subject of Research: Hubble spectroscopy of nearby starburst galaxies tests how ionizing radiation escaped during cosmic reionization.

Article Title: Shining new light on the 'last major phase transition in the universe'

Article References: Shining new light on the 'last major phase transition in the universe'. Original publication (Bibliographic details unavailable.)

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Keywords: Hubble Space Telescope, cosmic reionization, ionizing radiation, supernova winds, starburst galaxies, young stars, galaxy evolution, Lyman continuum, early universe, Shining, light, last