The Hubble Space Telescope has detected an exceptionally dense cluster of hot, massive stars within the ancient galaxy MXDFz4.4, which existed just 1.4 billion years after the Big Bang. This discovery has brought scientists closer to solving one of the early Universe’s greatest mysteries: how the Epoch of Reionization concluded.
During the first few hundred million years following the Big Bang, the Universe was filled with a thick cloud of neutral hydrogen. This kind of ‘cosmic fog’ absorbed ultraviolet radiation, making space opaque to much of the light. For a long time, astronomers debated what exactly dispersed this gas and rendered the Universe transparent: the first generations of stars or active supermassive black holes.
Hubble’s observations have provided strong evidence in favor of the first scenario. The telescope recorded a powerful stream of ionizing ultraviolet radiation emanating from a compact star cluster in the galaxy MXDFz4.4, highlighting the crucial role of young, massive stars in the reionization process.
The galaxy was initially discovered as part of the ultra-deep MXDF survey using the MUSE spectrograph on the Very Large Telescope (VLT) in Chile. Its designation z4.4 corresponds to a redshift of 4.4, meaning we are observing the object as it appeared roughly 12.4 billion years ago.
During its journey to Earth, the ultraviolet radiation stretched due to the expansion of the Universe and shifted into the visible spectrum, allowing Hubble to detect it. According to researchers, astronomers had previously found galaxies from this era, but MXDFz4.4 is the first object from which ionizing photons have been directly captured.
Although this galaxy is about a hundred times smaller than the Milky Way, it forms stars ten times faster. The bulk of this star formation is concentrated within a single, extremely compact cluster.
By combining data from Hubble with observations from the James Webb Space Telescope, researchers have concluded that the stars in this cluster formed through a series of powerful bursts. Each such wave gave rise to new generations of hot, massive stars, whose radiation gradually pierced the surrounding hydrogen ‘fog’, creating channels for the ultraviolet light to escape.
Supernova explosions also played an additional role. A few million years after their birth, massive stars ended their lives with powerful blasts, pushing away surrounding gas and carving out vast cavities in the interstellar medium. Through these ‘windows’, ionizing radiation could spread even more effectively.
Scientists believe that similar processes likely occurred throughout the early Universe, gradually clearing the cosmos of its primordial hydrogen ‘fog’. The search for further objects like MXDFz4.4 will help reconstruct a more precise picture of how the Universe became transparent to light.
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