Some of the galaxies that might have turned the universe from a dark place to a bright one have been identified

A scheme of the passage from the universe full of neutral hydrogen and dark to the bright one following the epoch of reionization
An article published in the journal “Astronomy & Astrophysics” reports the identification of a group of primordial galaxies that could be among the ones that contributed to the reionization of the universe, making it go from dark to bright. A team of researchers coordinated by the Italian National Institute of Astrophysics used observations conducted with the James Webb space telescope within the GLASS-JWST program to study 29 very distant and therefore ancient galaxies. The examination of those galaxies’ physical characteristics led the researchers to conclude that 80% of them contributed significantly to reionization.

The epoch of reionization was a pivotal period in the history of the universe in which hydrogen, which was neutral in the early life of the universe, was separated into protons and electrons. Neutral hydrogen blocked light, so reionization turned the universe from a dark place to one illuminated by the light of primordial stars. Astronomers are trying to pinpoint the causes of that process.

The image (ESA – C. Carreau) shows a scheme of the passage from the universe full of neutral hydrogen and dark to the bright one following the epoch of reionization. On the left side, a portion of the cosmic microwave background (CMB) radiation map is shown.

Quasars have been suspected of being the cause of reionization due to the extremely powerful electromagnetic emissions they generate, believed to be able to ionize hydrogen over enormous distances. However, the amount of quasars in the early universe seems too small. Another hypothesis sees young galaxies with considerable star formation as responsible for reionization but this is only possible if enough energy leaked from most of those galaxies into the intergalactic medium.

Obtaining enough detail of the most distant galaxies to estimate their possible contribution to the reionization of the universe was nearly impossible until recently. The James Webb Space Telescope showed right from the start of its scientific mission that it can offer details in quantity and quality impossible for other instruments. One of Webb’s main goals is precisely the study of the early universe and the GLASS-JWST program is focused on the epoch of reionization.

The observed galaxies are so far away that astronomers had to use the Abell 2744 galaxy cluster, also known as the Pandora Cluster, as a gravitational lens to amplify the very faint light coming from the galaxies behind the cluster. This study shows the first results of the GLASS-JWST program.

29 very distant galaxies we see as they were when the universe was between 650 million and 1.3 billion years old were observed with the James Webb Space Telescope’s NIRSpec and NIRCam instruments. That enabled to obtain estimates of their compactness and rate of star formation within them. Even for Webb, it’s impossible to directly detect the ionization capabilities of those galaxies due to the impossibility of directly detecting the high-energy photons that caused the reionization of the intergalactic medium. For this reason, the researchers estimated their ability to emit high-energy radiation by comparing them with similar but much younger and closer galaxies.

According to the researchers, 80% of the distant galaxies examined contributed to the reionization of the universe with an average emission of 12% of their high-energy photons, the ones useful for ionizing neutral hydrogen. This may have reionized the entire universe in an astronomically relatively short time.

These conclusions were reached using information deduced from observations and not relying on directly collected data. For this reason, it will be more necessary than ever to obtain more confirmations to be gained through other observations and analyses. The samples of galaxies examined are scheduled to be expanded with more observations that will be conducted again with the James Webb Space Telescope which will include galaxies with higher masses or are more distant. The search is long and complex but slowly astronomers are exploring a crucial period in the history of the universe.

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