NASA's Hubble Space Telescope has revealed that supernovae may actually hinder rather than help ultraviolet radiation escape from early galaxies, according to research published in Nature Astronomy by an international team including astronomers from the University of Michigan.
The findings challenge the widely accepted theory that winds from exploding stars cleared pathways for radiation to escape. Instead, the researchers found that galaxies with the fastest winds allowed the smallest fraction of ionizing radiation to escape. "Basically, how these galaxies were leaking ionizing radiation isn't what we thought," said Cody Carr, a postdoctoral researcher at the University of Michigan who led the work.
The team examined nearby starburst galaxies using Hubble to study their light spectra, which reveal gas movement and wind speeds. When comparing wind speeds with the amount of ionizing radiation escaping from each galaxy, stronger galactic winds correlated with less escaping radiation. "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 at Johns Hopkins University.
This finding relates to reionization, a period roughly 13 billion years ago when intense radiation from early galaxies transformed neutral hydrogen filling intergalactic space by stripping electrons from atoms. This process made the universe transparent to ultraviolet light, a transformation astronomers call the last major phase transition of the universe.
The new observations suggest that radiation from young, massive stars may escape galaxies before those stars explode as supernovae. Renyue Cen, a senior author and professor at Zhejiang University, noted that the findings highlight the need to examine how radiation and winds from young stars shape surrounding gas before the first supernovae explode. Carr and colleagues are seeking additional Hubble observation time to examine more galaxies and determine how widespread these patterns are.