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 of 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 supernovae 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.
"Supernovae play a central role in cosmological hydrodynamic simulations that explain how ionizing radiation escapes from galaxies," said Renyue Cen , a senior author of the study and professor 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.”
With the Hubble Space Telescope, the team observed nearby starburst galaxies—galaxies where stars are forming at an astronomical rate—which resemble some of the galaxies thought to have helped reionize the early universe. The researchers analyzed the spectra of these galaxies, measuring which wavelengths of light escape the galaxies, to measure the speeds of their galactic winds.
Gas in these winds absorbs light at specific wavelengths, leaving dips in the spectra whose positions and widths reveal how the gas is moving. The team found that galaxies with the fastest winds allowed the smallest fraction of ionizing radiation to escape.
"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 , senior author of the new study and director of the Center for Astrophysical Sciences at Johns Hopkins University,
The study included nearly 30 collaborators from more than a dozen institutions worldwide. Sally Oey, U-M professor of astronomy was also a contributor. Additional funding for the project was provided by Zhejiang University; Zhejiang Provincial Top-Level Research Support Program; National Natural Science Foundation of China; Ministry of Science, Innovation and Universities in Spain; European Regional Development Fund; and European Union's Horizon 2020 Research and Innovation Program.
The study was supported in part by federal funding from NASA and the U.S. National Science Foundation.
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