The ultraviolet continuum slopes of high-redshift galaxies: evidence for the emergence of dust-free stellar populations at z > 10

We present an analysis of the ultraviolet (UV) continuum slopes ($\beta$) for a sample of $172$ galaxy candidates at $8 < z_{\mathrm{phot}} < 16$ selected from a combination of JWST NIRCam imaging and COSMOS/UltraVISTA ground-based near-infrared imaging. Focusing primarily on a new sample of $121$ galaxies at $\langle z \rangle \simeq 11$ selected from $\simeq 320$ arcmin$^2$ of public JWST imaging data across $15$ independent data sets, we investigate the evolution of $\beta$ in the galaxy population at $z \geq 9$. We find a significant trend between $\beta$ and redshift, with the inverse-variance weighted mean UV slope evolving from $\langle \beta \rangle = -2.17 \pm 0.06$ at $z = 9.5$ to $\langle \beta \rangle = -2.59 \pm 0.06$ at $z = 11.5$. Based on a comparison with stellar population models including nebular continuum emission, we find that at $z>10.5$ the average UV continuum slope is consistent with the intrinsic blue limit of dust-free stellar populations $(\beta_{\mathrm{int}} \simeq -2.6)$. These results suggest that the moderately dust-reddened galaxy population at $z < 10$ was essentially unattenuated at $z \simeq 11$. The extremely blue galaxies being uncovered at $z>10$ place important constraints on dust attenuation in galaxies in the early Universe, and imply that the already observed galaxy population is likely supplying an ionising photon budget capable of maintaining ionised IGM fractions of $\gtrsim 5$ per cent at $z\simeq11$.