Evolution of the UV slope of galaxies at cosmic morning (z > 4): The properties of extremely blue galaxies Dottorini, D. Calabrò, A. Pentericci, L. Mascia, Sara Llerena, M. Napolitano, L. Santini, P. Roberts-Borsani, G. Castellano, M. Amorin, R. Dickinson, M. Fontana, A. Hathi, N. Hirschmann, M. Koekemoer, A. M. Lucas, R. A. Merlin, E. Morales, A. Pacucci, F. Wilkins, S. Arrabal Haro, P. Bagley, M. Finkelstein, S. L. Kartaltepe, J. Papovich, C. Pirzkal, N. ddc:520 We present an analysis of the UV continuum slope, β, using a sample of 726 galaxies with z > 4, selected from a mixture of JWST ERS, GTO, and GO observational programs. We considered only spectroscopic data obtained with the low-resolution (R ∼ 30 − 300) PRISM/CLEAR NIRSpec configuration. Studying the correlation between β and MUV, we find an overall decreasing trend, described by β = ( − 0.055 ± 0.017)MUV + ( − 2.98 ± 0.34). This is consistent with previous studies, where brighter galaxies show redder β values. However, when analyzing the trend in separate redshift bins, we find that at high redshift the relation becomes much flatter and is consistent with a flat slope within 1σ. Furthermore, we find that β tends to decrease with redshift, following β = ( − 0.075 ± 0.010)z + ( − 1.496 ± 0.056). This is consistent with most recent results showing a steepening of the spectra at higher z. We selected a sample of galaxies with extremely blue slopes (i.e., β < −2.6). Such slopes are steeper than predicted by stellar evolution models – even for dust-free, young, metal-poor populations – when the contribution of nebular emission is included. We selected 44 extremely blue galaxies (XBGs) and investigated the possible physical origin of their steep slopes by comparing them to a subsample of redder galaxies (matched in Δz = ±0.5 and ΔMUV = ±0.2). We find that XBGs have younger stellar populations, stronger ionization fields, lower dust attenuation, and lower but not pristine metallicity (∼10% Z⊙) compared to red galaxies. However, these properties alone cannot explain the extreme β values. Using indirect inference of Lyman continuum escape with the most recent models, we estimated the escape fraction fesc > 10% in at least 25% of the XBGs, whereas all the red sources exhibit much lower fesc values. A reduced nebular continuum contribution – resulting from either a high escape fraction or a bursty star formation history – is likely the origin of the extremely blue slopes. EDP Sciences 2025 info:eu-repo/semantics/article doc-type:article text http://purl.org/coar/resource_type/c_2df8fbb1 https://research-explorer.ista.ac.at/record/19930 https://research-explorer.ista.ac.at/download/19930/19932 Dottorini D, Calabrò A, Pentericci L, et al. Evolution of the UV slope of galaxies at cosmic morning (z &#62; 4): The properties of extremely blue galaxies. <i>Astronomy &#38; Astrophysics</i>. 2025;698. doi:<a href="https://doi.org/10.1051/0004-6361/202453267">10.1051/0004-6361/202453267</a> eng info:eu-repo/semantics/altIdentifier/doi/10.1051/0004-6361/202453267 info:eu-repo/semantics/altIdentifier/issn/0004-6361 info:eu-repo/semantics/altIdentifier/e-issn/1432-0746 info:eu-repo/semantics/altIdentifier/wos/001510826300019 info:eu-repo/semantics/altIdentifier/arxiv/2412.01623 info:eu-repo/semantics/openAccess