Did supernovas help make the early universe transparent? A study offers new clues

The radiation emitted by the first galaxies split hydrogen atoms and initiated the process of reionization in the universe. (Illustrative Image Infobae)During the earliest times of the cosmos, the space between galaxies was covered by a fog of neutral hydrogen that blocked ultraviolet radiation. About 13 billion years ago, that barrier began to disappear. The energy emitted by the first galaxies split hydrogen atoms into protons and electrons. That process, by which the universe became transparent, is called reionization. The question remains open: astronomers do not know precisely how the photons capable of causing that change managed to escape from those young galaxies. A frequent explanation held that supernovae, explosions that mark the end of some massive stars, expelled gas and opened channels for radiation to reach the space between galaxies. A study published in the journal Nature Astronomy provides evidence of a different sequence. Ionizing radiation escapes before the fast galactic winds linked to supernovae. The team examined six nearby galaxies that allow the study of processes similar to those of the early universe. Therefore, the result is not a direct observation of the era of reionization. In simple terms, the study seeks to find out why light was able to escape from the first galaxies. The answer matters because it helps explain how the universe went from being covered by a fog of gas to letting light travel through space. The escape of photons in the Lyman continuum peaks during the first three million years of the starburst. (Illustrative Image Infobae)To test that hypothesis, an international team used the Hubble Space Telescope to observe six nearby galaxies where stars are forming at high speed. They are not galaxies from the early universe: they were chosen as present-day references because they share features with the compact and luminous galaxies that would have contributed to reionization. The researchers analyzed the spectrum of each galaxy, that is, the way its light is distributed according to wavelength. Gas absorbs part of that light and leaves signals that allow calculating how it moves. In this way they measured the speed of the galactic winds, currents of gas that flow out of a galaxy, and the fraction of ionizing radiation that manages to pass through it. The comparison challenged the expectation that supernovae would clear the way. The galaxies with faster winds let escape a smaller proportion of ionizing radiation, while the younger systems showed a greater escape, according to the University of Michigan press release. Cody Carr, a postdoctoral researcher in the Department of Astronomy at the University of Michigan and lead author of the study, summarized the contrast with the previous explanation. "The way these galaxies were letting ionizing radiation escape was not what we thought," Carr said, according to the institutional press release. Supernova explosions increase the covering of gas and dust that blocks the passage of ultraviolet light. (Illustrative Image Infobae)Ionizing radiation is made up of photons with enough energy to strip electrons from atoms. The study focused on the Lyman continuum, a part of ultraviolet light whose photons can ionize hydrogen. Put another way, they can modify it so that it no longer blocks radiation as easily. The data indicate that the escape peaks during the first three million years of the starburst. In that stage, intense radiation and the winds of young stars predominate, flows of matter expelled by those massive stars. A fast galactic wind has not yet appeared. In the galaxies with bursts older than three million years, the team detected faster winds and a greater amount of dust. "The observations show that these photons escape before the galactic winds form," said Carr, a postdoctoral researcher at the University of Michigan, according to the press release. The result suggests that radiation and stellar winds act first on the nearby gas. Afterwards, the stage associated with supernovae could hinder the escape of that radiation. Stephan McCandliss, senior author and director of the Center for Astrophysical Sciences at Johns Hopkins University, attributed that change to the effect of supernovae on the environment. "The energy provided by the supernovae seems to have stirred up the environment and increased the covering of neutral gas and dust that reduces the escape of ionizing radiation," he said, according to the University of Michigan press release. The Hubble Space Telescope analyzed the spectrum of six reference galaxies to measure the speed of the gas. (Illustrative Image Infobae)The difference between the winds of stars and galactic winds matters for the models that reconstruct the early universe. As detailed in the study, the escape of ionizing radiation decreases after a supernova-driven wind begins, during the first ten million years of star formation. That wind incorporates gas and dust, so it increases the material capable of absorbing the photons. Renyue Cen, senior author, professor and director of the Center for Computational Astrophysics and Cosmology at Zhejiang University, pointed out that simulations should pay more attention to the stage prior to the first explosions: "Our findings highlight the need to examine how the radiation and winds of young stars shape the surrounding gas before the first supernovae explode." The press release notes that the team is seeking more observation time with Hubble to gather data on the escape of ionizing radiation. Those measurements will allow a more detailed examination of how the processes observed in nearby galaxies can be incorporated into models of reionization.