An international team of researchers has uncovered new findings that challenge existing theories regarding the universe's last major phase transition, known as reionization. The study was published in Nature Astronomy.
Within the first billion years following the Big Bang, the universe was filled with a neutral hydrogen fog that blocked ultraviolet light. Approximately 13 billion years ago, an epoch began where energetic radiation from early galaxies broke apart this hydrogen into charged protons and electrons, ultimately clearing the fog to create the transparent universe observed today. However, astronomers have long debated the exact mechanisms that allowed ionizing radiation to escape these early galaxies.
A leading theory suggested that galactic winds generated by supernova explosions cleared pathways for radiation to escape. To test this, researchers used NASA's Hubble Space Telescope to observe nearby starburst galaxies, which share characteristics with those from the early universe. By analyzing the spectra of these galaxies, the team measured the speeds of their galactic winds and evaluated how light escaped.
Contrary to the prevailing model, the observations revealed that galaxies with the fastest winds actually permitted the smallest fraction of ionizing radiation to escape. According to the researchers, energy from supernovae stirs up surrounding gas and dust, increasing the coverage that blocks radiation rather than clearing it away. The findings instead support an alternate theory that activity from young stars helps radiation escape before supernova-driven winds reshape the galaxies.
The research team included members from the University of Michigan, Zhejiang University, Johns Hopkins University, and nearly 30 collaborators across more than a dozen institutions worldwide. The study received support from NASA, the U.S. National Science Foundation, and several international funding bodies.
