This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: An international team of researchers, including members from the University of Michigan, has revealed surprising results that could rewrite our understanding of a key transition in the early universe. Within the first billion years after the Big Bang, the universe filled with a fog of neutral hydrogen that was impenetrable to ultraviolet light.
Today, however, that fog has cleared thanks to an epoch that began about 13 billion years ago known as reionization. During reionization, energetic radiation from galaxies in the early universe collided with neutral hydrogen that made up the fog and broke it apart into charged, or ionized, protons and electrons. "Eventually, when it ionized the majority of hydrogen filling intergalactic space, that was the end of reionization and we see the universe today," said Cody Carr, a U-M postdoctoral researcher in the Department of Astronomy.
"We call reionization the last major phase transition of the universe." But identifying the sources of this ionizing radiation and understanding how it escaped early galaxies is a central challenge in astronomy. "We don't know exactly what objects caused reionization or how it evolved," said Carr, who led and carried out the study as a postdoctoral scholar at Zhejiang University in China before joining U-M. "We know that it took place, but we don't have complete answers to those questions." A leading theory for what enabled reionization was that winds created by supernovae, or large exploding stars, in early galaxies cleared the way for ionizing radiation to escape.
Now, however, using the Hubble Space Telescope—a NASA mission launched with contributions from the European Space Agency—to observe nearby galaxies, the team has shown that those winds actually prevent radiation from leaving galaxies. The Hubble Space Telescope also supported the work through a grant administered by the Space Telescope Science Institute. "Basically, how these galaxies were leaking ionizing radiation isn't what we thought," Carr said.
The team's findings, published in the journal Nature Astronomy, strengthen an alternate theory behind reionization. This theory puts forth the case that activity from young stars helped radiation escape, before supernova detonations created winds powerful enough to reshape galaxies. "Our observations show that these photons are escaping before galactic winds form," Carr said.
Winds at a smaller scale in the neighborhood of young stars, however, could play an important role. Carr and his colleagues are working to secure more time to use Hubble to observe more galaxies to further test this idea. And, in the meantime, this observation can inform simulations that help astronomers better understand reionization.
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