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Arxiv: Astraeus X: Indications of a top-heavy initial mass function in highly star-forming galaxies from JWST observations at z>10 Published: 10/1/2024 5:17:44 PM Updated: 10/1/2024 5:17:44 PM

Paper abstract: The James Webb Space Telescope (JWST) has uncovered an abundance of z>10galaxies bright in the ultraviolet (UV) that has challenged traditionaltheoretical models at high redshifts. Recently, various new models have emergedto address this discrepancy by refining their description of star formation.Here we investigate whether modifications to the stellar initial mass function(IMF) alone can reproduce the z>10 UV luminosity functions (UV LFs) when thestar formation rate is used as a proxy for the fraction of massive stars. Weincorporate an Evolving IMF into the {\sc astraeus} galaxy evolution andreionisation simulation framework, which becomes increasingly top-heavy as thegas density in a galaxy rises above a given threshold. Our implementationaccounts for the IMF's effects on supernova (SN) feedback, metal enrichment,and the UV and ionising emissivities. For this Evolving IMF model, we find that(i) the maximum UV luminosity enhancement is twice as large in massive galaxies(\Delta M_\mathrm{UV}~eq2.6) than those where star formation is stronglylimited by SN feedback (\Delta M_\mathrm{UV}~eq1.3); (ii) it successfullyreproduces the observed UV LFs at z=5-15; (iii) galaxies with top-heavy IMFsexhibit the highest star formation rates, driven by their location in localdensity peaks, which facilitates higher gas accretion rates; (iv) the 1\sigmavariances in the UV luminosity are only slightly higher compared to whenassuming a Salpeter IMF, but the 2\sigma variances are significantlyincreased by a factor of 1.4-2 boosting the abundance of UV-bright galaxiesat z>10; (v) reionisation begins earlier with more extended large ionisedregions and fewer smaller ones during its initial stages, though thesedifferences diminish at lower redshifts, leading to a similar end ofreionisation at z~eq5.6.}