The ultraviolet continuum slopes ($\beta$) of galaxies at $\mathbf{z}\simeq 8-16$ 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 ($-22.6<M_{\mathrm{UV}}<17.9$), 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 $M_{\mathrm{UV}}=-19.3\pm 1.3$) returns an inverse-variance weighted mean value of $⟨\beta ⟩=-2.10\pm 0.05$, 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 $\beta -M_{\mathrm{UV}}$ relation, such that brighter UV galaxies display redder UV slopes ($\mathrm{d}\beta /\mathrm{d}{\mathrm{M}}_{\mathrm{UV}}=-0.17\pm 0.05$). Comparing to results at lower redshift, we find that the slope of our $\beta -M_{\mathrm{UV}}$ relation is consistent with the slope observed at $z\simeq 5$ and that, at a given $M_{\mathrm{UV}}$, our $8<z<16$ galaxies are somewhat bluer than their $z\simeq 5$ counterparts, with an inverse-variance weighted mean offset of $⟨\Delta \beta ⟩=-0.38\pm 0.09$. 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 $z>8$.

9 pages, 5 figures, MNRAS accepted version

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