Early in the universe's history, a dense haze of neutral hydrogen blocked ultraviolet light, until radiation from the first galaxies split those atoms apart and cleared the way for the cosmos we see now. Astronomers have long agreed that this reionization happened, but not who did the work or how the light got out of its galaxies in the first place. A new study of nearby star-forming galaxies, drawing on Hubble observations, suggests that the light escaped before the violent winds of exploding stars had a chance to shape those galaxies, a finding that gently shifts the story toward the quieter
Hubble Data Suggest Early Galaxies Leaked Ionizing Light Before Supernova Winds Formed
The photons are escaping before galactic winds form.
Mark
So what's the headline here? Supernovae aren't what let light out?
Mimi
In the galaxies they studied, yes. The fastest winds went with the least ionizing radiation escaping. That's the opposite of what the leading theory predicted.
Mark
How do they even measure wind speed?
Mimi
Through spectra. Gas in the winds absorbs specific wavelengths, and the position and width of those dips show how the gas is moving.
Luke
Worth flagging though: these are nearby starburst galaxies, not the early universe itself. They resemble some galaxies thought to have helped reionize it, but that's an analog, not a direct observation.
Mark
Fair. So does the result prove the alternate theory?
Mimi
The reporting says it strengthens it, not proves it. Carr says the photons escape before winds form.
Luke
And McCandliss's explanation, that supernova energy stirred up gas and raised neutral gas and dust coverage, is his reading of the pattern. The source doesn't say it's been confirmed as the mechanism.
Mark
What changes practically?
Mimi
Cen says supernovae are central in the simulations, so models may need to treat young-star radiation and winds before the first explosions.
Luke
Whether that actually changes the models is still open. The team is seeking more Hubble time, so more evidence isn't in hand yet.
The Pulse
- A long-favored explanation, that supernova winds carved open exits for ionizing light, now faces spectra showing the fastest winds let the least radiation through.
- Models of the early universe that lean heavily on supernovae are suddenly under pressure to account for a different escape route.
- Researchers are reaching for more Hubble observing time to test how young stars' radiation sculpts the surrounding gas before the first explosions.
- The result is landing as a reshaping of simulations, with the alternate theory gaining ground while the mechanism awaits confirmation.
Early in the universe's history, a dense haze of neutral hydrogen blocked ultraviolet light, until radiation from the first galaxies split those atoms apart and cleared the way for the cosmos we see now. Astronomers have long agreed that this reionization happened, but not who did the work or how the light got out of its galaxies in the first place. A new study of nearby star-forming galaxies, drawing on Hubble observations, suggests that the light escaped before the violent winds of exploding stars had a chance to shape those galaxies, a finding that gently shifts the story toward the quieter influence of young stars.
Roughly thirteen billion years ago, the young universe was wrapped in a fog of neutral hydrogen that ultraviolet light could not pass. Reionization burned that fog away, and astronomers still struggle to say which early objects were responsible and how their radiation found its way out. Cody Carr of the University of Michigan calls it the last great phase transition of the cosmos, and he admits the field still lacks complete answers.
For a time, the leading idea held that supernova winds swept through early galaxies and opened channels for escaping light. A team using the Hubble Space Telescope tested that picture on nearby starburst galaxies, which resemble the hungry young galaxies of the early universe. Reading the dips that gas leaves in their spectra, they found the opposite of what was expected: the galaxies with the fastest winds let the smallest share of ionizing radiation escape.
Stephan McCandliss of Johns Hopkins suggested that supernova energy may stir up surrounding neutral gas and dust, thickening the barrier instead of thinning it. The findings, published in Nature Astronomy, lend support to an alternate account in which young stars cleared the way before supernovae detonated. Renyue Cen of Zhejiang University noted that current simulations lean heavily on supernovae, and that the role of early stellar radiation deserves closer study.
The work drew on nearly thirty collaborators across many institutions. The team is now seeking more Hubble time to gather further evidence, and in the meantime the result offers a new lens for rebuilding the story of how the universe became transparent.
Within the first billion years after the Big Bang, the universe was filled with a fog of neutral hydrogen that ultraviolet light could not get through. That fog is gone now. It was cleared by an epoch called reionization, which began roughly 13 billion years ago, when energetic radiation from early galaxies struck the neutral hydrogen and split it into charged protons and electrons.
Cody Carr, a postdoctoral researcher in the University of Michigan's Department of Astronomy, describes it in large terms. Once the majority of hydrogen filling intergalactic space had been ionized, reionization ended and the universe took the form we see today. He calls it the last major phase transition of the universe.
What remains unsettled is who did the work and how. Astronomers do not know exactly which objects drove reionization, or how it unfolded over time. Carr, who led the study while a postdoctoral scholar at Zhejiang University in China before moving to Michigan, put it plainly: the field knows reionization happened, but lacks complete answers to those questions. A central puzzle is how ionizing radiation got out of early galaxies at all.
One leading theory held that winds from supernovae, the explosions of massive stars, swept through early galaxies and opened paths for the radiation to escape. A team using the Hubble Space Telescope has now found evidence pointing the other way. Looking at nearby galaxies, they found that those winds are missing in the galaxies where ionizing radiation is escaping. Hubble is a NASA mission launched with contributions from the European Space Agency, and it also supported the work through a grant administered by the Space Telescope Science Institute. As Carr summarized it, how these galaxies leak ionizing radiation is not what researchers had thought.
The targets were nearby starburst galaxies, where stars form at an astronomical rate and which resemble some of the galaxies believed to have helped reionize the early universe. The team analyzed their spectra, measuring which wavelengths of light escape and how fast the galactic winds move. Gas in those winds absorbs light at particular wavelengths, leaving dips in the spectra whose positions and widths show how the gas is moving. The result: galaxies with the fastest winds let the smallest fraction of ionizing radiation escape.
Stephan McCandliss, a senior author and director of the Center for Astrophysical Sciences at Johns Hopkins University, offered a reading of that pattern. Energy from supernovae appears to have stirred up the surroundings and increased, rather than decreased, the coverage of neutral gas and dust that suppresses escape of ionizing radiation.
Published in Nature Astronomy, the findings strengthen an alternate theory: that activity from young stars helped radiation escape before supernova detonations produced winds powerful enough to reshape galaxies. Carr said the observations show these photons escaping before galactic winds form.
That matters for the models. Renyue Cen, a senior author and professor and director of the Center for Cosmology and Computational Astrophysics at Zhejiang University, noted that supernovae play a central role in cosmological hydrodynamic simulations of how ionizing radiation leaves galaxies. He said the results highlight the need to examine how radiation and winds from young stars shape surrounding gas before the first supernovae explode.
The study involved nearly 30 collaborators from more than a dozen institutions, including Michigan astronomy professor Sally Oey. Funding came from Zhejiang University, the Zhejiang Provincial Top-Level Research Support Program, the National Natural Science Foundation of China, Spain's Ministry of Science, Innovation and Universities, the European Regional Development Fund, the EU's Horizon 2020 program, NASA and the U.S. National Science Foundation. Carr and colleagues are now seeking more Hubble time to gather further evidence, and in the meantime the result can inform simulations of reionization.
Notable Quotes
Our observations show that these photons are escaping before galactic winds form.— Cody Carr, University of Michigan
Findings highlight the need to examine how radiation and winds from young stars shape surrounding gas before the first supernovae explode.— Renyue Cen, Zhejiang University
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