Astrophysics & Particle Physics-APPC 2026

Reionization era galaxies were crucial in shaping the young universe during the reionization epoch (EoR), an era that took place approximately between 400 million and 1 billion years post-Big Bang. This period brought an end to the cosmic dark ages, when the first light sources—galaxies, stars, and quasars—began to form and reionized the intergalactic medium's neutral hydrogen.According to Peter A. Shaver, after the recombination era, the universe was filled with neutral hydrogen, which rendered it opaque to ultraviolet (UV) radiation. The formation of the first galaxies altered this setting tremendously. Violent UV radiation from early, massive stars ionized nearby hydrogen atoms so light could flow freely across space. This cosmic reionization reshaped the universe's structure, making it transparent as we see it today.Examination of reionization epoch galaxies offers clues about early star formation, galaxy evolution, and the development of cosmic structures. High-redshift galaxy observations show that they were smaller, irregular, and less chemically developed than today's galaxies, but they were extremely efficient producers of ionizing photons.

They are thought to be the main contributors to reionization, with contributions from quasars and potentially Population III stars—the earliest stars to have formed from untouched hydrogen and helium.New telescopes like the James Webb Space Telescope (JWST) have made dramatic leaps in the exploration of reionization-era galaxies. JWST's detection capability of distant, weak galaxies at redshifts above 10 has opened a new window on the cosmic dawn, with evidence for star formation and structure just a few hundred million years after the Big Bang. Lyman-alpha emission lines are also important observational traces of reionization activity. In short, reionization epoch galaxies light up the universe's dawn from darkness to light, a turning point in cosmic history. Their research connects the creation of the earliest light-emitting objects to the contemporary universe we observe.