Early supernovae helped, then hindered, universe reionization

Researchers led by Dr. Cody Carr reported that early supernova explosions initially helped ionizing radiation escape galaxies by carving low-density channels in galactic gas, while later explosions filled those outflows with neutral gas and dust and suppressed that escape.

The work appeared in Nature Astronomy on October 15, 2026, and relied on Hubble Space Telescope observations of six nearby starburst galaxies used as proxies for the first galaxies. An international team of nearly 30 collaborators from more than a dozen institutions traced the launch of multiphase galactic winds through spectral lines and measured the escape fraction of Lyman continuum photons. Carr led the research as a postdoctoral scholar at Zhejiang University before joining the University of Michigan, where he now works as an Extremely Large Telescope Fellow; Renyue Cen, a professor at Zhejiang University, served as senior author, and Stephan McCandliss contributed to the analysis.

The Epoch of Reionization is the last major phase transition in the universe, marking the shift from a neutral hydrogen-dominated cosmos to an ionized one. That change allowed ultraviolet light to travel freely, ending the Dark Ages and making the universe transparent to light.

Astronomers estimate the Epoch of Reionization occurred around redshift z ~ 6–8, roughly 600 million to 1 billion years after the Big Bang, with star-forming galaxies as the primary drivers. Lyman continuum photons, which have wavelengths shorter than 912 ångströms, are responsible for ionizing the intergalactic medium, though neutral gas and dust often block their escape.

The Dark Ages began after Recombination and lasted from around 150,000 to 1 billion years after the Big Bang, until ultraviolet radiation from early stars, galaxies, and quasars ionized neutral hydrogen. That opaque state ended when the intergalactic medium became transparent.

Early supernova explosions carved low-density channels in galactic gas, creating pathways for Lyman continuum photons to leak out and aiding the Epoch of Reionization. Researchers tracked those channels through spectral features associated with galactic winds driven by stellar feedback and supernovae.

As more supernovae occurred, they drove neutral mass and dust into the galactic outflow, reducing the average escape fraction despite the earlier formation of low-density channels. Faster supernova-driven winds correlate with lower escape fractions of ionizing radiation, contrary to the theory that supernovae aid reionization by clearing pathways.

The findings show that young stars, rather than supernovae, were primarily responsible for driving ionizing radiation out of galaxies before supernova explosions became dominant. Radiation and stellar winds from young stars shaped the surrounding gas early in galactic evolution, creating optimal conditions for Lyman continuum escape. Cosmological simulations often assume supernovae play a central role in enabling ionizing radiation to escape galaxies.

Renyue Cen, a senior author of the study and professor at Zhejiang University, emphasized the need to refine simulations by incorporating the role of young stars and their radiation in shaping galactic gas before supernovae become significant.

A separate line of evidence comes from LCEz4-M1, a galaxy at redshift z = 4.444 observed 1.4 billion years after the Big Bang. Identified as one of the most distant “leaky” galaxies known to emit ionizing radiation, it was detected using combined data from the Very Large Telescope, the Hubble Space Telescope, and the James Webb Space Telescope, with its distance confirmed by hydrogen emission lines. The observation was reported on June 22, 2026.

LCEz4-M1’s lower star-formation surface density compared with typical local leakers suggests it may be in a postburst phase, where gas channels opened by earlier stellar winds and supernovae still allow ionizing radiation to escape. The galaxy’s compact nature indicates it may not be in a classical starburst phase but rather in a period when earlier stellar activity created escape routes.

The researchers say they need further observations and simulations to refine understanding of how ionizing radiation escapes galaxies before supernovae become dominant. Cen stressed that addressing this gap in current models requires incorporating the early influence of young stars and their radiation.