Dense Galaxies Are Carving Out A Bubble in the Early Universe

When the James Webb Space Telescope (JWST) began delivering its first scientific results, it marked a triumphant moment for scientific vision and persistence. The telescope, which had faced budget overruns and cancellation threats for many years, was finally launched on December 25, 2021. Astronomers, cosmologists, exoplanet scientists, and astrophysicists around the world eagerly awaited its findings.

Initial observations from the JWST revealed surprisingly bright, well-developed galaxies only several hundred million years after the Big Bang, challenging existing scientific understanding. One of the JWST's first and most significant surveys, the JWST Advanced Deep Extragalactic Survey (JADES), has contributed to many of the telescope's surprising early Universe findings.

In a new study published in The Astrophysical Journal, researchers identified a galaxy overdensity candidate in JADES that could shed light on the early Universe's reionization process. Reionization refers to the era when the first stars and galaxies formed, illuminating their surroundings and reionizing hydrogen atoms in the primordial Universe.

Prior to this, the Universe was opaque and dark. The overdensity, detected at a redshift of approximately 10.5, contains 18 galaxies in a very dense arrangement, with a galaxy number density four times higher than the field expectation. This region is about four times as dense as the average galaxy density in the area. The galaxies within the overdensity have higher stellar masses and star formation rates than their field counterparts, but their interactions are more frequent than field galaxies.

Astonishingly, the researchers detected Lyman-alpha radiation in this region, which is a specific UV spectral line emitted by hydrogen when an electron loses energy. Normally, Lyman-alpha radiation would be blocked or highly attenuated by neutral hydrogen in the early Universe, making its detection before reionization nearly impossible.

However, the detected Lyman-alpha radiation shows spatial variation, with elevated transmission near the center of the overdensity and reduced transmission toward the outskirts. This behavior suggests the presence of an ionizing bubble with a radius of about 6 comoving megaparsecs. If confirmed, this discovery would provide evidence of the first stages of cosmic reionization, with galaxy overdensities like this one potentially responsible for carving bubbles in the neutral hydrogen that dominated the early Universe.

The spatial variation in Lyman-alpha transmission represents a rare laboratory to study the structure of ionizing bubbles, offering a direct test of their role in cosmic reionization within the first 500 million years of the Universe's existence.