One early galaxy has now illuminated a critical shift in the young cosmos: the first signs of the Universe becoming transparent.
The light that does escape comes in less energetic forms, presenting a serious obstacle to astronomers trying to observe this formative period in cosmic history.
Researchers continue to investigate when and how this process began and what sources triggered it. The prevailing theory held that reionization commenced around 500 million years after the Big Bang and concluded another half-billion years later.
A new finding by astronomers at the Cosmic Dawn Center (DAWN), part of the Niels Bohr Institute and DTU Space, now suggests this process may have started significantly earlier. Their study focused on one of the most ancient galaxies yet discovered and offers compelling evidence of an ionized region surrounding it.
"Young galaxies shine brightest at a very specific wavelength of light, originating from hydrogen. To astronomers, this light goes under the name 'Lyman alpha'. Because of its short UV wavelength, it is easily absorbed by the surrounding medium, and therefore no galaxy from when the Universe was less than half a billion years old has showed us this particular kind of light."
How could such light pass through a Universe still saturated with opaque hydrogen?
"We know from our theories and computer simulations, as well as from observations at later epochs, that the most energetic UV light from the galaxies "fries" the surrounding neutral gas, creating bubbles of ionized, transparent gas around them," Witstok elaborates. "These bubbles percolate the Universe, and after around a billion years, they eventually overlap, completing the epoch of reionization. We believe that we have discovered one of the first such bubbles."
This isolated Lyman alpha signal indicates that a transparent, ionized zone surrounds the galaxy, permitting the light to escape and reach our instruments.
"We knew that we would find some of the most distant galaxies when we built Webb," says Peter Jakobsen, affiliated professor at DAWN, project scientist behind James Webb's spectrograph NIRSpec, and second-author of the study. "But we could only dream of one day being able to probe them in such detail that we can now see directly how they affect the whole Universe."
Although extremely bright stars are one potential source of such ionizing radiation, another hypothesis remains open:
"Most galaxies are known to host a central, supermassive black hole. As these monsters engulf surrounding gas, the gas is heated to millions of degrees, making it shine brightly in X-rays and UV before disappearing forever. This is another viable cause of the bubbles, which we will now investigate," Witstok concludes.
Research Report:Witnessing the onset of reionization through Lyman-a emission at redshift 13
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University of Copenhagen - Faculty of Science
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