We present a spectral stacking analysis of galaxies at $z\geq6$ observed with the James Webb Space Telescope (JWST). We curate a sample of $64$ galaxies spanning redshifts $z_{ \rm spec} = 6.0 - 9.4$ which have NIRSpec medium resolution data. The stacks achieve sufficient signal-to-noise to measure equivalent widths (EW) and velocity centroids ($v_{ \rm{ cen}}$) of low-ionization species (LIS) absorption features, transmitted Lyman-alpha ($\rm{ Ly\alpha}$) emission, and nebular emission lines. Overall, we find our sample has weaker LIS absorption lines ($\rm{ EW}(\rm{ LIS}) \approx 1 \r{ A}$), smaller $v_{ \rm{ cen,LIS}} \approx -20 \pm 50~ \rm{ km} \; \rm{ s}^{ -1}$, and significantly suppressed $\rm{ Ly\alpha}$ emission ($\rm{ EW}(\rm{ Ly\alpha}) \approx 5~\r{ A}$), compared to similar studies undertaken at lower redshift. The weaker LIS absorption may suggest a lower covering fraction of HI and larger escape fraction of ionizing photons from our sample. Additionally, the smaller blueshifted $v_{ \rm{ cen,LIS}}$ indicates less prevalent or weaker outflows in $z>6$ galaxies. Stacking our sub-sample of $\rm{ Ly\alpha}$ emitters (LAEs), we find high EW$(\rm{ H}\beta) \approx 170 \pm 4~\r{ A}$ and a detection of nebular $\rm{ C}\; \rm{ IV}$ emission suggesting higher $\xi_{ ion}$ 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.
Categories: astro-ph.GA