article published yes D. Dottorini author A. Calabrò author L. Pentericci author Sara Mascia author edaf889c-c7cd-11ef-ab1b-bb28c431bd29 M. Llerena author L. Napolitano author P. Santini author G. Roberts-Borsani author M. Castellano author R. Amorin author M. Dickinson author A. Fontana author N. Hathi author M. Hirschmann author A. M. Koekemoer author R. A. Lucas author E. Merlin author A. Morales author F. Pacucci author S. Wilkins author P. Arrabal Haro author M. Bagley author S. L. Finkelstein author J. Kartaltepe author C. Papovich author N. Pirzkal author JoMa department 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. https://research-explorer.ista.ac.at/download/19930/19932/2025_AstronomyAstrophysics_Dottorini.pdf application/pdfno https://creativecommons.org/licenses/by/4.0/ EDP Sciences2025 eng 0004-6361 1432-0746 2412.01623 00151082630001910.1051/0004-6361/202453267 698 Dottorini, D., 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>, vol. 698, A234, EDP Sciences, 2025, doi:<a href="https://doi.org/10.1051/0004-6361/202453267">10.1051/0004-6361/202453267</a>. D. Dottorini <i>et al.</i>, “Evolution of the UV slope of galaxies at cosmic morning (z &#62; 4): The properties of extremely blue galaxies,” <i>Astronomy &#38; Astrophysics</i>, vol. 698. EDP Sciences, 2025. D. Dottorini, A. Calabrò, L. Pentericci, S. Mascia, M. Llerena, L. Napolitano, P. Santini, G. Roberts-Borsani, M. Castellano, R. Amorin, M. Dickinson, A. Fontana, N. Hathi, M. Hirschmann, A.M. Koekemoer, R.A. Lucas, E. Merlin, A. Morales, F. Pacucci, S. Wilkins, P. Arrabal Haro, M. Bagley, S.L. Finkelstein, J. Kartaltepe, C. Papovich, N. Pirzkal, Astronomy &#38; Astrophysics 698 (2025). Dottorini, D., Calabrò, A., Pentericci, L., Mascia, S., Llerena, M., Napolitano, L., … Pirzkal, N. (2025). Evolution of the UV slope of galaxies at cosmic morning (z &#62; 4): The properties of extremely blue galaxies. <i>Astronomy &#38; Astrophysics</i>. EDP Sciences. <a href="https://doi.org/10.1051/0004-6361/202453267">https://doi.org/10.1051/0004-6361/202453267</a> Dottorini D, Calabrò A, Pentericci L, Mascia S, Llerena M, Napolitano L, Santini P, Roberts-Borsani G, Castellano M, Amorin R, Dickinson M, Fontana A, Hathi N, Hirschmann M, Koekemoer AM, Lucas RA, Merlin E, Morales A, Pacucci F, Wilkins S, Arrabal Haro P, Bagley M, Finkelstein SL, Kartaltepe J, Papovich C, Pirzkal N. 2025. Evolution of the UV slope of galaxies at cosmic morning (z &#62; 4): The properties of extremely blue galaxies. Astronomy &#38; Astrophysics. 698, A234. 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> Dottorini, D., A. Calabrò, L. Pentericci, Sara Mascia, M. Llerena, L. Napolitano, P. Santini, 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>. EDP Sciences, 2025. <a href="https://doi.org/10.1051/0004-6361/202453267">https://doi.org/10.1051/0004-6361/202453267</a>. 199302025-06-29T22:01:15Z2026-02-16T12:11:39Z