The UV Continuum Slopes of Early Star-Forming Galaxies in JADES

Abstract The power-law slope of the rest-UV continuum (fλ∝λβ) is a key metric of early star forming galaxies, providing one of our only windows into the stellar populations and physical conditions of z ≳ 10 galaxies. Expanding upon previous studies with limited sample sizes, we leverage deep imaging from JADES to investigate the UV slopes of 179 z ≳ 9 galaxies with apparent magnitudes of mF200W ≃ 26 − 31, which display a median UV slope of β = −2.4. We compare to a statistical sample of z ≃ 5 − 9 galaxies, finding a shift toward bluer rest-UV colors at all $\rm M_{\rm UV}$. The most UV-luminous z ≳ 9 galaxies are significantly bluer than their lower-redshift counterparts, representing a dearth of moderately-red galaxies within the first 500 Myr. At yet earlier times, the z ≳ 11 galaxy population exhibits very blue UV slopes, implying very low impact from dust attenuation. We identify a robust sample of 44 galaxies with β ≲ −2.8, which have SEDs requiring models of density-bounded HII regions and median ionizing photon escape fractions of 0.51 to reproduce. Their rest-optical colors imply that this sample has weaker emission lines (median mF356W − mF444W = 0.19 mag) than typical galaxies (median mF356W − mF444W = 0.39 mag), consistent with the inferred escape fractions. This sample consists of relatively low stellar masses (median $\log (\rm M/{\rm M}_{\odot })=7.5\pm 0.2$), and specific star-formation rates (sSFR; median $=79~\rm Gyr^{-1}$) nearly twice that of our full galaxy sample (median sSFR$=44~\rm Gyr^{-1}$), suggesting these objects are more common among systems experiencing a recent upturn in star formation. We demonstrate that the shutoff of star formation provides an alternative solution for modelling of extremely blue UV colors, making distinct predictions for the rest-optical emission of these galaxies. Future spectroscopy will be required to distinguish between these physical pictures.