Stacking PANCAKEZ: Spectroscopic Analysis with NIRSpec Stacks in the Epoch of Reionization. Weak Interstellar Medium Absorption and Implications for Ionizing Photon Escape at z ∼ 7

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 Ly α 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 Ly α (EW(Ly α ) ≈ 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 Ly α emitters (LAEs), we find high EW(H β ) ≈ 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.