The “Epoch of Reionization” marked a cosmic dawn, transforming the universe from a dark, cold void filled with neutral hydrogen gas into a bright, star-studded cosmos. During this era, the first stars and galaxies ignited, emitting powerful radiation that stripped hydrogen atoms of their electrons, creating an ionized plasma and allowing light to permeate space. While the exact sources of this trans formative light have eluded scientists for years, new findings, published in Nature, suggest that ancient dwarf galaxies likely generated the photons that heralded the dawn of re ionization.

In an effort led by astrophysicist Hakim Atek of Sorbonne University, an international team used the James Webb Space Telescope (JWST) to study galaxies formed in the universe’s first billion years. Their analysis focused on extremely faint dwarf galaxies whose radiation may have been strong enough to initiate reionization. This work was carried out under the Ultradeep NIRSpec and NIRCam ObserVations before the Epoch of Reionization (UNCOVER) program, which combines the JWST’s powerful infrared imaging and spectroscopy to capture some of the oldest and faintest galaxies ever observed.

To examine these faint sources, astronomers leveraged gravitational lensing, a technique that utilizes massive objects as cosmic magnifying glasses. In this case, the team observed light from galaxies behind the galaxy cluster Abell 2744, also called Pandora’s Cluster, an immense cluster formed by at least four smaller clusters. The gravitational pull of Abell 2744 magnified the light from eight ancient galaxies, making them visible for analysis. The combination of gravitational lensing with JWST’s NIRSpec instrument allowed scientists to detect galaxies that were over 100 times fainter than the Milky Way.

The power of JWST’s spectroscopy provided the UNCOVER team with the first-ever detailed images of these ancient galaxies. Using NIRSpec’s Multi-Shutter Assembly, scientists performed multi-object spectroscopy, capturing the galaxies’ light spectra to confirm that their radiation could have played a critical role in ending the “dark age” of the universe. These early dwarf galaxies, despite their small size, emitted energetic radiation at a volume significant enough to transform neutral hydrogen into ionized plasma. As Atek explained, these low-mass galaxies existed in such abundance that their combined impact was powerful enough to reionize the universe, effectively setting the stage for the complex structures we observe in space today.

This milestone in astronomy underlines the immense potential of JWST, a $10 billion telescope that is continually unveiling distant secrets of the universe. Iryna Chemerynska, a co-author of the study from the Institut d’Astrophysique de Paris, emphasized the importance of these discoveries, explaining how ultra-faint galaxies play a crucial role in understanding the early universe. By studying these early galaxies, astronomers can piece together how low-mass galaxies influenced cosmic evolution, adding to our knowledge of the forces that have shaped the universe.

The JWST’s exploration of the universe’s distant past is only beginning. Atek, also a principal investigator of another research program, GLIMPSE (Gravitational Lensing & NIRCam Imaging to Probe Early Galaxy Formation and Sources of Reionization), aims to investigate an even earlier period known as the “Cosmic Dawn,” when the universe was only a few million years old. Through these studies, scientists hope to unlock more secrets of the universe’s formative years and gain new insights into the origins of the galaxies, stars, and structures that populate space today.