Hubble Data Upend the Theory That Supernova Winds Cleared the Early Universe's Fog
Nearby starburst galaxies with the fastest winds leaked the least ionizing light, suggesting young stars did the work before the first supernovae went off.
A team using the Hubble Space Telescope has found evidence against a leading explanation for how the early universe became transparent. The results, published in Nature Astronomy, suggest that winds driven by exploding stars did not clear the way for ultraviolet light to escape early galaxies. They may have blocked it.
Within the first billion years after the Big Bang, the universe was filled with a fog of neutral hydrogen that ultraviolet light could not penetrate. Starting about 13 billion years ago, energetic radiation from early galaxies broke that hydrogen apart into charged protons and electrons, an epoch called reionization. "We call reionization the last major phase transition of the universe," said Cody Carr, a postdoctoral researcher at the University of Michigan who led the study while at Zhejiang University in China.
Astronomers know reionization happened, but not exactly which objects drove it or how their light got out. "We don't know exactly what objects caused reionization or how it evolved," Carr said. A leading theory held that winds from supernovae, the explosions of massive stars, swept gas out of early galaxies and cut channels through which ionizing radiation could escape.
To test it, the team used Hubble to observe nearby starburst galaxies, which form stars at a furious pace and resemble the galaxies thought to have helped reionize the early universe. They analyzed the spectra of the galaxies, looking at which wavelengths of light escape. Gas in the winds absorbs light at specific wavelengths, leaving dips whose positions and widths reveal how fast the gas is moving.
The pattern ran against the theory. Galaxies with the fastest winds let the smallest fraction of ionizing radiation out. "It appears that energy input from supernovae has stirred up the surroundings and increased, rather than decreased, the coverage of neutral gas and dust that suppresses ionizing radiation escape," said Stephan McCandliss, a senior author and director of the Center for Astrophysical Sciences at Johns Hopkins University. "Basically, how these galaxies were leaking ionizing radiation isn't what we thought," Carr said.
The findings strengthen an alternative theory in which activity from young stars helps radiation escape before supernova detonations create winds powerful enough to reshape their galaxies. "Our observations show that these photons are escaping before galactic winds form," Carr said. Smaller-scale winds near young stars may still matter, and the team is seeking more Hubble time to observe additional galaxies and test the idea.
The result has consequences for computer models. "Supernovae play a central role in cosmological hydrodynamic simulations that explain how ionizing radiation escapes from galaxies," said Renyue Cen, a senior author and director of the Center for Cosmology and Computational Astrophysics at Zhejiang University. "Our findings highlight the need to examine how radiation and winds from young stars shape the surrounding gas before the first supernovae explode."
The study included nearly 30 collaborators from more than a dozen institutions, including Sally Oey, a University of Michigan professor of astronomy. The paper is titled "Lyman continuum escape during the onset of galactic winds in compact starbursts."
