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

We study the rest-frame ultraviolet (UV) continuum slopes (β) of galaxies at redshifts 8 < z < 16 (〈z〉 = 10), using a combination of James Webb Space Telescope (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 〈β〉 = −2.10 ± 0.05, with a corresponding median value of β = −2.29 ± 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 (e.g., NGC 1705; β = −2.46). We find evidence for a β − M_UV relation, such that brighter UV galaxies display redder UV slopes (⁠dβ/dMUV=−0.17±0.05⁠). Comparing to results at lower redshift, we find that the slope of our β − M_UV relation is consistent with the slope observed at z ≃ 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 〈Δβ〉 = −0.38 ± 0.09. We do not find strong evidence that any objects in our sample display ultra-blue UV continuum slopes (i.e., β ≲ −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.