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An international research collaboration led by researchers at the University of Michigan and Zhejiang University has revealed findings that challenge long-standing assumptions about how ionising light escaped early galaxies to clear the cosmic fog during the epoch of reionisation
Published in Nature Astronomy, the study utilises data from the Hubble Space Telescope to show that galactic winds driven by supernovae actually hinder, rather than facilitate, the escape of high-energy ultraviolet radiation.
The physics of cosmic reionisation and galactic winds
During the first billion years following the Big Bang, the universe was filled with a dense, neutral hydrogen gas that absorbed ultraviolet light. Over the epoch of reionisation, radiation from early star-forming galaxies ionised this neutral hydrogen, transforming the cosmos into the transparent universe observed today.
Astronomers have long debated how high-energy Lyman continuum (LyC) photons escaped the dense gas and dust of early starburst galaxies:
The supernova wind hypothesis:
Previous cosmological hydrodynamic simulations suggested that powerful galactic winds driven by supernova explosions created channels through interstellar gas, allowing ionising radiation to escape into intergalactic space.
Observational contradiction:
Analysing nearby compact starburst galaxies that serve as local analogues to early universe systems, the team measured the Doppler shifts in spectral absorption lines to trace wind velocities.
Radiation-driven escape:
The observations demonstrated that Lyman continuum escape peaks early (within the first 3 million years of a starburst), driven by intense radiation pressure and stellar winds from young, massive stars before major supernovae detonations occur.
Impact of Supernovae on Photon Suppression
The study revealed a strong negative correlation between the speed of supernova-driven galactic winds and the fraction of escaping ionising radiation. As starburst systems age past 3 million years, exploding supernovae drive fast, mass-loaded galactic winds.
Rather than clearing clean pathways, these large-scale outflows stir up surrounding interstellar neutral gas and dust, increasing cloud coverage and suppressing escaping radiation.
Co-author Professor Renyue Cen (Zhejiang University) noted that these observational constraints require cosmologists to update hydrodynamic simulations by incorporating early radiative feedback and local stellar winds prior to the onset of supernova activity.