Galactic Winds May Prevent Light from Reionizing the Universe

Using Hubble Space Telescope observations, astronomers, including researchers from the University of Minnesota, found that newly launched galactic winds inhibit the escape of ionizing radiation from compact galaxies undergoing recent bursts of star formation. These nearby systems resemble some of the galaxies thought to have helped reionize the early Universe. 

In a paper published in Nature Astronomy, researchers analyzed the spectra of nearby compact galaxies undergoing intense star formation to measure the speeds of their galactic winds. Gas in these winds absorbs light at specific wavelengths, leaving dips in the spectra whose positions and widths reveal how the gas is moving. The team found that galaxies with the fastest winds allowed the smallest fraction of ionizing radiation to escape. 

Early in its history, the Universe underwent a major phase transformation. Energetic light stripped electrons from the neutral hydrogen filling intergalactic space, a process known as reionization. Identifying the sources of this ionizing radiation—and understanding how it escaped their galaxies—is a central challenge in astronomy. Because intervening neutral hydrogen absorbs this light, astronomers cannot directly measure its escape from galaxies during reionization. Nearby galaxies with similar properties offer an accessible laboratory for investigating the process. 

How ionizing radiation escapes remains debated. In many theoretical models, galactic winds driven by stellar feedback—including supernovae and radiation pressure—open pathways through a galaxy’s surrounding gas, allowing radiation to escape after a delay, as a burst of star formation begins to fade. Yet observations of nearby galaxies show that strong escape can occur while their light is still dominated by very young stars. The new study examines this early stage, finding that newly launched winds can initially hinder, rather than facilitate, the escape of ionizing radiation. 

The results suggest that galactic winds increasingly suppress the escape of ionizing radiation as they accelerate and grow in column density. These massive outflows are driven primarily by supernovae, indicating that early radiative feedback preceding supernova explosions may instead be the primary facilitator of ionizing photon escape in compact galaxies. 

Lead author Dr. Cody Carr, who conducted the study at Zhejiang University in China and is now at the University of Michigan, said the key was measuring absorption directly in neutral hydrogen lines—tracing the gas that blocks ionizing radiation. Previous studies that relied on indirect tracers, such as absorption lines from metal ions, had yielded conflicting results. Carr was a graduate student at the University of Minnesota, where he worked with Professor Claudia Scarlata of the Minnesota Institute for Astrophysics, a co-author of the study. 

Co-author Professor Renyue Cen of Zhejiang University emphasized the implications for the astrophysics of feedback processed in simulations of galaxy formation.

“Supernovae play a central role in many simulations that explain how ionizing radiation escapes from galaxies. Our findings highlight the need to examine how radiation and winds from young stars shape the surrounding gas before the first supernovae explode.” 

Other collaborating institutions included Johns Hopkins University, Stockholm University, University of Minnesota, Williams College, University of Edinburgh, Instituto de Astrofísica de Andalucía, Arizona State University, University of Arizona, Univ Lyon, Université de Genève, Aix Marseille Univ, NASA Goddard Space Flight Center, Catholic University of America, Minnesota State University-Mankato, University of Virginia, Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, The Pennsylvania State University, and Northwestern University. 

The paper, “Lyman Continuum Escape During the Onset of Galactic Winds in Compact Starbursts”, published in the journal Nature Astronomy, can be found here.