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 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 (dβ/dMUV = −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 〈∆β〉 = −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.