The James Webb Space Telescope (JWST) has detected one of the earliest known galaxies in the process of reionization, pushing the time of cosmic reionization (an important transformation in the early universe) to at least 330 million years after the Big Bang, according to a new astronomy paper published in the internationally renowned academic journal Nature.
The paper describes a period sometimes called the Dark Ages of the Universe after the extremely hot Big Bang, when the universe gradually cooled until free protons and electrons combined into neutral (uncharged) gas, mostly hydrogen and helium. The first galaxies lit up the universe, in particular, photons of a specific ultraviolet wavelength (the so-called Lyman continuum) were absorbed by neutral hydrogen, while shorter wavelength photons reionized the gas, making the universe transparent to Lyman photons and enabling them to penetrate to Earth. This time is called cosmic reionization, and its occurrence is uncertain. Recent observations by JWST have found bright galaxies that produced ultraviolet radiation when the universe was less than 300 million years old, but direct evidence of reionization is lacking.
Joris Witstok, the first author and corresponding author of the paper, and colleagues and collaborators at the Niels Bohr Institute at the University of Copenhagen, Denmark, found that only 330 million years after the Big Bang, JWST observed that a galaxy named JADES-GS-z13-1-LA had a reionization signal. This bright emission was identified as Lyman-α, which is a signal of neutral hydrogen transitioning from an excited state to a ground state. This means that the galaxy produced enough ultraviolet photons to excite neutral hydrogen, and there was almost no neutral hydrogen between it and the Earth to reabsorb the Lyman-α photons released when hydrogen returned to a stable, lowest energy state.
The authors of the paper pointed out that the possible sources of this reionization are either massive hot stars or supermassive black holes. They concluded that these findings provide new insights into the properties of the earliest galaxies and will help narrow the scope of the beginning and timeline of cosmic reionization.
