Groundbreaking JWST observations reveal the key role of low-mass galaxies in the reionization of the early universe and challenge existing theories of cosmic evolution. Working with data from NASA’s James Webb Space Telescope (JWST), scientists have obtained the first full spectrum of some of the oldest star explosions in the universe. The images provide the clearest picture yet of very low-mass newborn galaxies that formed less than a billion years after the Big Bang, and suggest that small galaxies played a central role in the universe’s origin story.

An international research team, including two Pennsylvania astrophysicists, recently published their findings in a journal. Nature. The spectra show some of the first visible light from a period in the universe known as reionization, caused by the formation of the oldest stars and galaxies.

Primitive universe: transition from darkness to light

Joel Leja, professor of astronomy and astrophysics at the University of Pennsylvania and author of the paper, explained that ordinary matter in the universe began as a hot, dense nebula composed almost entirely of hydrogen and helium nuclei. As it expanded and cooled, single protons and electrons began to bond together to form neutral hydrogen for the first time. Then, about 500-900 million years after the Big Bang, the neutral hydrogen that dominated the early universe began to break down again into ionized gas, fueling the formation of stars and galaxies and clearing the primordial haze so that light could pass through unimpeded. eats for the first time.

“Something opened up that started pumping very high energy photons into the intergalactic space,” Leia said. “These sources operated as cosmic beacons that burned off the neutral hydrogen fog. Whatever it was, it was so energetic and so persistent that the entire universe reionized.”

Galactic progenitors: The role of low-mass galaxies

By analyzing the spectra of young, low-mass galaxies, scientists have shown that the small galaxies are strong candidates for “something” that is causing the universe to reionize by heating the dense primordial gas around them and ionizing once-neutral hydrogen.

“If other low-mass galaxies in the universe are as common and energetic as these, we think we finally understand the beacons twinkling in the cosmic fog,” Leia said. “They were incredibly energetic stars in many tiny little galaxies.”

Leia added that most galaxies in the early universe are expected to be relatively small, making it difficult to study their frequencies and properties. Thanks to the technological achievement made possible by the unique combination of JWST’s sensitivity and the gravitational lensing effect of the Abell 2744 cluster (neighbouring galaxies that act as cosmic magnifiers, distorting space and amplifying the light of background galaxies), numerous smaller galaxies and their ionizing properties have now been detected throughout the first billion years of the universe’s existence. can be determined.

“We found that during the reionization era of the universe, small galaxies outnumbered large galaxies by about a hundred times,” said astrophysicist Hakim Atek of Sorbonne University, a researcher at the Paris Institute of Astrophysics and first author of the paper. “These new observations also show that these small galaxies produce significant numbers of ionizing photons, four times the canonical values ​​usually assumed for distant galaxies. This means that the total flux of ionizing photons emitted by these galaxies significantly exceeds the threshold required for reionization.” “

Space Evolution Planning: Future Directions

The Penn State team led simulations of the UNCOVER survey, targeting a large foreground galaxy cluster that obscures smaller, more distant galaxies. Researchers at the University of Pennsylvania analyzed all the tiny points of light during the study to understand the objects’ properties, possible masses and distances. This analysis was later used for more detailed observations of JWST, which led to this discovery, Leia explained.

These discoveries were preceded by a number of hypotheses identifying other sources responsible for cosmic reionization, such as supermassive black holes; large galaxies with masses exceeding one billion solar masses; and small galaxies with masses less than 1 billion solar masses. The researchers say that the hypothesis of low-mass galaxies is particularly difficult to confirm, given their low luminosity; But the new findings provide the clearest evidence that low-mass galaxies play a central role in the reionization of the universe.

The researchers now want to expand the study to a larger scale to confirm that the specific location they analyzed is consistent with the average distribution of galaxies in the universe. In addition to the reionization process, their observations shed light on the process of early star formation, how galaxies formed from primordial gas, and how they evolved into the universe we know today.