When was light created?

How the first light escaped

Astronomers have for the first time observed directly how the radiation from the first stars in the young cosmos escaped into intergalactic space and ionized the hydrogen gas there. Until now, it was unclear how the high-energy photons penetrated the dense gas clouds that surrounded the star-forming regions in the still young galaxies. Apparently the radiation escaped through a narrow channel in the gas, the scientists now report in the journal "Science".

Around 400,000 years after the Big Bang, the universe had cooled down so far that electrically neutral atoms could form - above all hydrogen atoms, which each consist of a proton and an electron. A few hundred million years later, the first stars and galaxies were formed. The high-energy ultraviolet radiation from the young stars in the first galaxies re-ionized the hydrogen gas in the early cosmos. "In order for this so-called reionization epoch to be possible, the ultraviolet radiation had to escape from the galaxies," explains Emil Rivera-Thorsen from the University of Oslo in Norway. However, the star-forming regions are too far away to trace the path of the ultraviolet photons through the dense gas clouds: even with large telescopes, no details of this process can be observed in the galaxies several billion light-years away.

The astronomers around Rivera-Thorsen, however, came to the aid of a chance. The galaxy PSZ1-ARC, eleven billion light years away, lies exactly behind a large galaxy cluster when viewed from Earth - and this galaxy cluster diverts the light rays of the galaxy with its gravity. As a result of this gravitational lensing effect, a star formation region in PSZ1-ARC appears twelve times on images from the Hubble Space Telescope. In addition, the images show the area significantly enlarged thanks to the gravitational lens. As a result, details are visible that could not be seen without a gravitational lens. The team's observations reveal that the ultraviolet radiation escapes the star-forming region through a narrow channel in the otherwise dense gas clouds. How this channel was formed is not yet understood.

At a distance of eleven billion light years, astronomers see the galaxy PSZ1-ARC as it looked about two and a half to three billion years after the Big Bang. At that time the reionization epoch was already over. Nevertheless, the observations provide for the first time an insight into the processes that played an important role during reionization, according to the researchers.