Hubble détaille la transformation du voisinage des premières galaxies 1,4 milliard d'années après le Big Bang

Astronomers using NASA’s Hubble Space Telescope have uncovered a surprising phenomenon related to a distant galaxy that existed just 1.4 billion years after the Big Bang. Detailed visible-light images captured by Hubble show that multiple bursts of young stars in the galaxy MXDFz4.4 have cleared the surrounding space, allowing astronomers to observe features previously hidden by dense, neutral hydrogen gas. This discovery marks the first time such a galaxy has been observed during this specific era, offering new insights into the process known as cosmic reionization. MXDFz4.4 is located in a region where the universe was transitioning from being largely opaque to becoming transparent. This transformation occurred over the course of the first billion years following the Big Bang, during a period called the Era of Reionization. Prior to this phase, the intergalactic medium was filled with neutral hydrogen, which absorbed most ultraviolet light, making it difficult to observe distant objects. However, as the first stars and galaxies began forming, their intense ultraviolet radiation started to ionize the surrounding gas, gradually turning it into a transparent state. The study, published in the Astrophysical Journal on June 23, highlights the unique nature of MXDFz4.4. Lead researcher Ilias Goovaerts, a postdoctoral fellow at the Space Telescope Science Institute (STScI) in Baltimore, noted that observing such a galaxy was considered nearly impossible due to the thick veil of neutral hydrogen. “We expected the ‘fog’ of neutral hydrogen to obscure our view of the ionizing light,” he explained. Instead, Hubble managed to detect the ultraviolet light emitted by young, massive stars within the galaxy, revealing intricate details about its structure and activity. The ultraviolet light produced by these young stars is capable of ionizing hydrogen atoms, effectively clearing the surrounding space of the neutral gas. This light, which originated from MXDFz4.4, had to travel for over 12 billion years before reaching Earth. Due to the expansion of the universe, the light experienced redshift, shifting from ultraviolet wavelengths into the visible spectrum. Hubble’s advanced capabilities, including its high sensitivity and spatial resolution, allowed scientists to capture this rare glimpse of the early universe. MXDFz4.4 is significantly smaller in size compared to the Milky Way, spanning approximately 100 times less area. Despite its compact size, it is forming stars at a rate ten times greater than the Milky Way. This rapid star formation is concentrated in tight clusters, enhancing the galaxy’s ability to emit ionizing radiation. According to Goovaerts, the density of young, hot, and massive stars in such a confined space plays a crucial role in piercing through the opaque gas surrounding the galaxy. Researchers estimate that between 50% and 100% of the ionizing light generated by these stars escapes the surrounding medium. The brief lifespan of massive stars further contributes to this process. These stars typically exist for only a few million years before exploding as supernovas, releasing vast amounts of energy and creating large voids in the surrounding gas. These explosions help propagate the ionizing light even further, increasing the transparency of the intergalactic medium. To achieve these findings, Hubble worked in conjunction with other observational tools and surveys. While Hubble provided the detailed imaging necessary to detect the ultraviolet light, additional data from ground-based telescopes and other space observatories contributed to confirming the results. The collaboration among various observatories underscores the importance of multi-wavelength studies in understanding the complex processes shaping the early universe. As research continues, scientists aim to identify more galaxies exhibiting similar properties to MXDFz4.4. Such discoveries will provide further evidence regarding the timeline and mechanisms behind the reionization of the universe. Understanding this pivotal epoch in cosmic history remains essential for comprehending the evolution of galaxies and the distribution of matter throughout the cosmos. Future observations using upcoming telescopes, such as the James Webb Space Telescope, may offer even deeper insights into the conditions of the early universe and the role played by galaxies like MXDFz4.4 in shaping the cosmos we see today.