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(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 < z > similar or equal to 11 selected from similar or equal to 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 >= 9. We find a significant trend between beta and redshift, with the inverse-variance weighted mean UV slope evolving from <beta > = -2.17 +/- 0.06 at z = 9.5 to <beta > = -2.59 +/- 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(int) similar or equal to -2.6). These results suggest that the moderately dust-reddened galaxy population at z < 10 was essentially unattenuated at z similar or equal to 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 ionizing photon budget capable of maintaining ionized intergalactic medium fractions of greater than or similar to 5 per cent at z similar or equal to 11.