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 $z geqslant 6$ observed with the James Webb Space Telescope (JWST). We curate a sample of 64 galaxies spanning redshifts $z_{mathrm{spec}} = 6.0-9.4$, which have NIRSpec medium-resolution data. The stacks achieve sufficient signal-to-noise ratios to measure equivalent widths (EWs) and velocity centroids ($nu_{mathrm{cen}}$) of low-ionization species (LIS) absorption features, transmitted Ly$alpha$ emission, and nebular emission lines. Overall, we find our sample has weaker LIS absorption lines (EW(LIS) $ approx$ 1 Å), smaller $nu_{mathrm{cen,LIS}}approx -20 pm 50$ km s$^{-1}$, and significantly suppressed Ly$alpha$ (EW(Ly$alpha$) $approx 5$ 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 $nu_{mathrm{cen,LIS}}$ indicates less prevalent or weaker outflows in $z > 6$ galaxies. Stacking our subsample of Ly$alpha$ emitters (LAEs), we find high EW(H$beta$) $approx 170 pm$ 4 Å and a detection of nebular C IV emission suggesting higher $xi_{mathrm{ion}}$ 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.