Evolution of the UV slope of galaxies at cosmic morning (z > 4): The properties of extremely blue galaxies

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 ∆z = ±0.5 and ∆MUV = ±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.