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

ABSTRACT We study the rest-frame ultraviolet (UV) continuum slopes (β) 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 MUV −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 MUV = −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 − MUV relation, such that brighter UV galaxies display redder UV slopes (d / d M ₔₕ = -0. 17 0. 05). Comparing to results at lower redshift, we find that the slope of our β − MUV relation is consistent with the slope observed at z ≃ 5 and that, at a given MUV, 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.