A research team led by Professor Renyue Cen of the School of Physics at Zhejiang University, in collaboration with the University of Michigan, Johns Hopkins University, and other institutions in the United States, has used observations from the Hubble Space Telescope to uncover a surprising pattern: in compact starburst galaxies, ionizing radiation may escape primarily during the early stages of intense star formation. As supernova-driven galactic winds develop, the neutral gas and dust they carry may instead hinder the escape of ionizing radiation. The findings provide new observational evidence for understanding how ionizing radiation escaped from galaxies during the reionization of the early universe. The study was published online in Nature Astronomy on October 15, 2026.
After the Big Bang, the vast space between galaxies was once filled with neutral hydrogen, forming a kind of “cosmic fog” that blocked the propagation of high-energy ultraviolet radiation. As early galaxies produced increasing numbers of high-energy ionizing photons, neutral hydrogen in intergalactic space was gradually ionized into protons and electrons. The universe thus underwent a major transformation, from a predominantly neutral state to a highly ionized one—a process known as cosmic reionization. Yet two fundamental questions remain in astrophysics: Which celestial sources supplied enough ionizing radiation, and how did those photons pass through the gas and dust within galaxies to reach intergalactic space?
Because the abundant neutral hydrogen in the early universe absorbs Lyman continuum radiation, astronomers have difficulty measuring directly the escape fraction of ionizing photons from galaxies during the epoch of reionization. Studying nearby galaxies with similar physical properties has therefore become an important way to investigate this process.
One long-standing explanation holds that supernova explosions at the end of massive stars’ lives drive powerful galactic winds, clearing away some of the gas within galaxies and opening channels through which ionizing photons can escape. Recent observations, however, have found that in some galaxies with high escape fractions, the ionizing radiation is still produced mainly by very young stars. This suggests that stellar radiation and stellar winds, even before the first supernovae explode, may play a crucial role in enabling ionizing photons to escape. Could the galactic winds once thought to clear the way for these photons also become an obstacle to their escape?
To investigate this question, the international collaboration selected six compact starburst galaxies in the nearby universe with similar stellar masses, metallicities, and star formation rates. The team obtained high-quality ultraviolet spectra using the Cosmic Origins Spectrograph aboard the Hubble Space Telescope. In some physical respects, these galaxies resemble those that may have contributed to reionization in the early universe, making them valuable analogues for investigating a process that is difficult to observe directly. Intriguingly, despite their broadly similar basic properties, the six galaxies exhibit markedly different escape fractions of ionizing radiation, ranging from approximately 20% to nearly zero. What accounts for such a striking difference?
Figure 1. Illustration of the interaction between stellar radiation and galactic winds in a compact starburst galaxy. Supernova-driven galactic winds carry gas outward and may impede the escape of ionizing radiation from the galaxy. Illustration by Siyi Wang (CC BY-ND 4.0).
By examining absorption lines of neutral hydrogen Lyman-beta (Lyβ) and other ions, the researchers analyzed the motions of gas at different temperatures. They then compared these measurements with stellar population ages and Lyman continuum (LyC) escape fractions. Within the sample studied, they found that higher galactic wind velocities and larger amounts of entrained neutral gas tended to be associated with lower escape fractions of ionizing photons. By contrast, during the early stages of a starburst (within approximately three million years), intense radiation and stellar winds from massive stars could already facilitate photon escape, while fast, large-scale galactic winds had yet to fully develop.
The findings highlight a possible dual role for stellar feedback. Early in a starburst, intense radiation and stellar winds from young, massive stars may create favorable conditions for ionizing photons to escape. As supernovae begin to explode, however, the strengthening galactic winds may make it increasingly difficult for those photons to leave the galaxy.
The team therefore proposes the following evolutionary picture: over roughly the first ten million years of a starburst, successive supernova explosions accelerate galactic winds and cause them to entrain more neutral gas and dust. This material may obscure the low-density channels that previously allowed ionizing photons to propagate, reducing the escape fraction. Importantly, this picture is based primarily on comparisons among nearby galaxies at different evolutionary stages and will need to be tested with observations of larger samples.
“In many cosmological hydrodynamical simulations that seek to explain how ionizing radiation escapes from galaxies, supernovae play a central role. Our results indicate that we need to examine more closely how radiation and stellar winds from young stars alter their surrounding gaseous environments before the first supernovae explode,” said Prof. Renyue Cen, a co-corresponding author of the paper. The findings also provide new constraints for improving numerical simulations of feedback processes in early galaxies and cosmic reionization.
Paper and Author Information
The paper’s first author is Dr. Cody Carr, who conducted postdoctoral research at Zhejiang University during the study and is now a postdoctoral researcher at the University of Michigan. Dr. Carr, Professor Renyue Cen, director of the Center for Cosmology and Computational Astrophysics at Zhejiang University, and Stephan McCandliss, director of the Center for Astrophysical Sciences at Johns Hopkins University, are the paper’s co-corresponding authors. The collaboration brought together nearly 30 researchers from more than ten research institutions worldwide, including Sally Oey, a professor of astronomy at the University of Michigan.
Zhejiang University is the first-listed affiliation on the paper. In the article’s affiliation list, the Center for Cosmology and Computational Astrophysics at Zhejiang University’s Institute for Advanced Study in Physics and the Institute of Astronomy at its School of Physics appear as the first and second affiliations, respectively. The research was supported by the National Natural Science Foundation of China (Research Fund for International Young Scientists, grant No. W2433001), Zhejiang University research start-up funding, the Zhejiang Provincial Top Talent Program, and research funding from international partner institutions. Part of the data analysis was performed using Zhejiang University’s “SilkRiver” supercomputing platform.
Paper title: Lyman Continuum Escape During the Onset of Galactic Winds in Compact Starbursts
Paper link：https://www.nature.com/articles/s41550-026-02999-z
