The ultraviolet continuum slopes ($\mathbf{\beta}$) of galaxies at $\mathbf{z\simeq8-16}$ from JWST and ground-based near-infrared imaging

Understand

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 ($-22.6 < M_{\rm UV} < 17.9$), 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 $M_{\rm UV}=-19.3\pm 1.3$) returns an inverse-variance weighted mean value of $\langle \beta \rangle = -2.10 \pm 0.05$, 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.