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

ABSTRACT The ultraviolet (UV) continuum slope of galaxies, $\beta$, 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 $\beta$ measurements for 395 spectroscopically confirmed galaxies at $5 \lt z \lt 14.3$ selected primarily from JADES, using high-quality JWST (James Webb Space Telescope) NIRSpec/PRISM spectra. We find a median $\beta = -2.15$, finding a mild increase in blueness of $\beta$ with increasing redshift and fainter UV magnitudes. Interestingly, we find evidence for reddening of the average $\beta$ at $z \gt 9.5$, deviating from the trend observed at $z \lt 9.5$. Using stacked spectra in bins of redshift and $\beta$, we derive trends between $\beta$ and dust attenuation, metallicity, ionization parameter, and stellar age indicators, finding a lack of dust attenuation to be the dominant driver of bluer $\beta$-values. We further report five galaxies with $\beta \le -2.9$, which show a range of spectroscopic properties and signs of significant LyC photon leakage. Finally, we show that the redder $\beta$-values at $z \gt 9.5$ 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 $n_e \gt 10\,000$ cm$^{-3}$ can reproduce the range of red $\beta$ that we see in our sample. Higher gas densities can also redden the nebular continuum emission, potentially explaining the observed $\beta$-values.