SIMBAD references

We present a spectral stacking analysis of galaxies at z ≥ 6 observed with the James Webb Space Telescope (JWST). We curate a sample of 64 galaxies spanning redshifts z_spec = 6.0-9.4, which have NIRSpec medium-resolution data. The stacks achieve sufficient signal-to-noise ratios to measure equivalent widths (EWs) and velocity centroids (v_cen) of low-ionization species (LIS) absorption features, transmitted Lyα emission, and nebular emission lines. Overall, we find our sample has weaker LIS absorption lines (EW(LIS) ≈ 1 Å), smaller v_cen,LIS ≈ -20 ± 50 km s^–1, and significantly suppressed Lyα (EW(Lyα) ≈ 5 Å), compared to similar studies undertaken at lower redshift. The weaker LIS absorption may suggest a lower covering fraction of H I and larger escape fraction of ionizing photons from our sample. Additionally, the smaller blueshifted v_cen,LIS indicates less prevalent or weaker outflows in z > 6 galaxies. Stacking our subsample of Lyα emitters (LAEs), we find high EW(Hβ) ≈ 170 ± 4 Å and a detection of nebular C IV emission suggesting higher ξ_ion in LAEs at z > 6. This work showcases the enormous potential for stacked JWST spectra revealing properties of galaxies and their diffuse interstellar medium in the epoch of reionization.