The ultraviolet continuum slopes () of galaxies at z ? 8-16 from JWST and ground-based near-infrared imaging

We study the rest-frame ultraviolet (UV) continuum slopes (beta) of galaxies at redshifts 8 < z < 16 ((z) = 10), 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-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-UV = -19.3 +/- 1.3) returns an inverse-variance weighted mean value of (beta) = -2.10 +/- 0.05, with a corresponding median value of beta= -2.29 +/- 0.09. These values imply that the UV colours of galaxies at z > 8 are, on avera g e, no bluer than the bluest galaxies in the local universe (e.g. NGC 1705; beta= -2.46). We find evidence for a beta- M-UV relation, such that brighter UV galaxies display redder UV slopes (d beta/ dM(UV) = -0 . 17 +/- 0 . 05). Comparing to results at lower redshift, we find that the slope of our beta-MUV relation is consistent with the slope observed at z similar or equal to 5 and that, at a given M-UV, our 8 < z < 16 galaxies are bluer than their z <^> 5 counterparts, with an inverse-variance weighted mean offset of (Delta beta) = -0.38 +/- 0.09. We do not find strong evidence that any objects in our sample display ultra-blue UV continuum slopes (i.e. beta less than or similar to -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.