Stacking PANCAKEZ: sPectroscopic Analysis with NirspeC stAcKs in the Epoch of reioniZation. Weak ISM Absorption and Implications for Ionizing Photon Escape at $z\sim 7$

Abstract

We present a spectral stacking analysis of galaxies at $z\ge 6$ 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 to measure equivalent widths (EW) and velocity centroids ($v_{\mathrm{cen}}$) of low-ionization species (LIS) absorption features, transmitted Lyman-alpha ($\mathrm{Ly}\hat{\mathrm{I}}\pm$) emission, and nebular emission lines. Overall, we find our sample has weaker LIS absorption lines ($\mathrm{EW}(\mathrm{LIS})\approx 1\tilde{\mathrm{A}}$), smaller $v_{\mathrm{cen},\mathrm{LIS}}\approx -20\pm 50\mathrm{km}{\mathrm{s}}^{-1}$, and significantly suppressed $\mathrm{Ly}\hat{\mathrm{I}}\pm$ emission ($\mathrm{EW}(\mathrm{Ly}\hat{\mathrm{I}}\pm )\approx 5\tilde{\mathrm{A}}$), 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 $v_{\mathrm{cen},\mathrm{LIS}}$ indicates less prevalent or weaker outflows in $z>6$ galaxies. Stacking our sub-sample of $\mathrm{Ly}\hat{\mathrm{I}}\pm$ emitters (LAEs), we find high EW$(\mathrm{H}{\hat{\mathrm{I}}}^2)\approx 170\pm 4\tilde{\mathrm{A}}$ and a detection of nebular $\mathrm{C}\mathrm{IV}$ emission suggesting higher $\hat{I}{¾}_{ion}$ 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.

23 pages, 13 figures, 5 tables