Astronomers have determined that so-called “leaky” galaxies may have triggered the last major epoch of transformation in our universe, one that ionized the neutral interstellar gas.
Billions of years ago, our universe was much smaller and much hotter than it is today. At very early times it was so small and hot that it was in a plasma state, where electrons were separated from atomic nuclei. But when the universe was about 380,000 years old, it cooled to the point where electrons could recombine on their nuclei and form a soup of neutral atoms.
However, observations of today’s universe reveal that almost all matter in the universe is not neutral at all. Instead, it is ionized, once again in a plasma state. Something had to happen in the intervening billions of years to transform the neutral gas of the cosmos into an ionized plasma. Astronomers call this event the reionization epoch and suspect it occurred during the first hundred million years after the Big Bang. But they are not sure how this transformational event took place.
One of the great debates in modern cosmology is the source of reionization. One hypothesis is that quasars are responsible. Quasars are the ultraluminous cores around supermassive black holes that pump out enormous amounts of high-energy radiation. This radiation can easily flood the universe and transform it from neutral to ionized. But the problem with this hypothesis is that quasars are relatively rare, and therefore have difficulty covering the volume of the universe.
Another hypothesis is that young galaxies rich in star formation are responsible. In this scenario, the process of ionizing the neutral gas is more widespread throughout the universe. Each individual galaxy is only able to ionize the gas in its vicinity, but since there are so many galaxies it is possible to reionize the entire universe. But the only way to do this is if enough high-energy radiation leaks out of galaxies and into the surrounding medium.
A team of astronomers has used the James Webb Space Telescope to investigate this hypothesis. They can’t study the radiation that comes directly out of the galaxies, because that radiation is absorbed by billions of light-years worth of matter between us and those galaxies. So instead they had to look for other clues. Using James Webb’s ability to study distant galaxies, they were able to measure how compact the galaxies were, and how rich in star formation they were. They were then able to compare these galaxies with similar galaxies found in today’s universe to make an estimate of the amount of radiation leaking from them.
They estimate that galaxies in the early universe leaked, on average, about 12% of their available high-energy photons. This is just enough to potentially reionize the entire cosmos in a relatively short time. The findings are published in the journal Astronomy and astrophysics.
However, the results are inconclusive due to the number of assumptions the astronomers had to make. But it points in an exciting direction for solving this long-standing cosmic puzzle.
S. Mascia et al., Approaching the sources of cosmic reionization: first results from the GLASS-JWST program, Astronomy and astrophysics (2023). DOI: 10.1051/0004-6361/202345866
Astronomy and astrophysics