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The initial mass function (IMF) is essential for understanding the formation and evolution of stars and galaxies. However, our understanding of the IMF in the early universe is not comprehensive. Recent advancements in numerical simulations indicate that stellar masses in the early universe were significantly larger, often reaching several hundred times the mass of our sun, a stark contrast to what we observe in the modern universe. This suggests a potential shift in the IMF over the course of cosmic evolution. To explore this, we conducted three-dimensional hydrodynamics simulations to track the formation of star clusters across various metallicities and redshifts. These simulations are detailed enough to capture the formation of protostars down to ~au scales, providing insights into the IMF. We discovered that both metallicity and redshift are pivotal in shaping the IMF. Metallicity influences cooling efficiency, and redshift alters the baseline temperature by modifying the cosmic microwave background temperature. Our findings reveal that the IMF becomes more top-heavy in environments with metallicity Z/Zsun < 0.01 or with a redshift greater than 10. These insights may elucidate the recent observations by the James Webb Space Telescope (JWST) of an unexpectedly high number of bright, high-redshift galaxies. Our results suggest that a top-heavy IMF in these galaxies could lead to increased UV luminosity at a consistent stellar mass, thereby elevating the number density of UV-bright galaxies. |
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