The ultraviolet continuum slopes ($\mathbfβ$) of galaxies at $\mathbf{z\simeq8-16}$ from JWST and ground-based near-infrared imaging

Abstract: We study the rest-frame ultraviolet (UV) continuum slopes ($\beta$) of galaxies at redshifts $8 < z < 16$, using a combination of JWST ERO and ERS NIRcam imaging and ground-based near-infrared imaging of the COSMOS field. The combination of JWST and ground-based imaging provides a wide baseline in both redshift and absolute UV magnitude ($-22.6 < M_{\rm UV} < 17.9$), sufficient to allow a meaningful comparison to previous results at lower redshift. Using a power-law fitting technique, we find that our full sample (median $M_{\rm UV}=-19.3\pm 1.3$) returns an inverse-variance weighted mean value of $\langle \beta \rangle = -2.10 \pm 0.05$, with a corresponding median value of $\beta=-2.29\pm 0.09$. These values imply that the UV colours of galaxies at $z>8$ are, on average, no bluer than the bluest galaxies in the local Universe. Moreover, we find evidence for a $\beta-M_{\rm UV}$ relation, such that brighter UV galaxies display redder UV slopes ($\rm{d}\beta/ \rm{d} M_{\rm UV} = -0.17 \pm 0.05$). Comparing to results at lower redshift, we find that the slope of our $\beta-M_{\rm UV}$ relation is consistent with the slope observed at $z\simeq 5$ and that, at a given $M_{\rm UV}$, our $8<z\<16$ galaxies are somewhat bluer than their $z\simeq 5$ counterparts, with an inverse-variance weighted mean offset of $\langle \Delta \beta \rangle = -0.38 \pm 0.09$. We do not find strong evidence that any objects in our sample display ultra-blue UV continuum slopes (i.e., $\beta\lesssim-3$) that would require their UV emission to be dominated by ultra-young, dust-free stellar populations with high Lyman-continuum escape fractions. Comparing our results to the predictions of theoretical galaxy formation models, we find that the galaxies in our sample are consistent with the young, metal-poor and moderately dust-reddened galaxies expected at $z\>8$.