Hitting the slopes: a spectroscopic view of UV continuum slopes of galaxies reveals a reddening at z > 9.5

The ultraviolet (UV) continuum slope of galaxies, (Formula presented), is a powerful diagnostic of the metallicity and ages of stars, nebular gas properties, dust content, and the escape of Lyman continuum (LyC) photons. In this study, we present (Formula presented) measurements for 395 spectroscopically confirmed galaxies at (Formula presented) selected primarily from JADES, using high-quality JWST (James Webb Space Telescope) NIRSpec/PRISM spectra. We find a median (Formula presented), finding a mild increase in blueness of (Formula presented) with increasing redshift and fainter UV magnitudes. Interestingly, we find evidence for reddening of the average (Formula presented) at (Formula presented), deviating from the trend observed at (Formula presented). Using stacked spectra in bins of redshift and (Formula presented), we derive trends between (Formula presented) and dust attenuation, metallicity, ionization parameter, and stellar age indicators, finding a lack of dust attenuation to be the dominant driver of bluer (Formula presented) -values. We further report five galaxies with (Formula presented), which show a range of spectroscopic properties and signs of significant LyC photon leakage. Finally, we show that the redder (Formula presented) -values at (Formula presented) may require rapid build-up of dust reservoirs in the very early Universe or a significant contribution from the nebular continuum emission to the observed UV spectra, with the nebular continuum fraction depending on the gas temperatures and densities. We show that in the absence of dust, nebular emission at (Formula presented)  cm(Formula presented) can reproduce the range of red (Formula presented) that we see in our sample. Higher gas densities can also redden the nebular continuum emission, potentially explaining the observed (Formula presented) -values.