Evolution of the UV slope of galaxies at cosmic morning (<i>z</i> > 4): The properties of extremely blue galaxies
D. Dottorini, A. Calabr, L. Pentericci, Sara Mascia, M. Llerena, Lorenzo Napolitano, P. Santini, G. Roberts-Borsani, M. Castellano, R. Amorín, M. Dickinson, A. Fontana, Nimish P. Hathi, Michaela Hirschmann, A. M. Koekemoer, R. A. Lucas, E. Merlin, Anthony J. Morales, Fabio Pacucci, Stephen M. Wilkins, Pablo Arrabal Haro, Micaela B. Bagley, S. L. Finkelstein, J. Kartaltepe, Casey Papovich, N. Pirzkal
Abstract
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 M UV , we find an overall decreasing trend, described by β = ( − 0.055 ± 0.017) M UV + ( − 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 1 σ . 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 Δ z = ±0.5 and Δ M UV = ±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 f esc > 10% in at least 25% of the XBGs, whereas all the red sources exhibit much lower f esc 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.