Stacking PANCAKEZ: sPectroscopic Analysis with NirspeC stAcKs in the Epoch of reioniZation. Weak ISM Absorption and Implications for Ionizing Photon Escape at $z\sim7$
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 zspec=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 (vcen) of low-ionization species (LIS) absorption features, transmitted Lyman-alpha (Lyα) emission, and nebular emission lines. Overall, we find our sample has weaker LIS absorption lines (EW(LIS)≈1A˚), smaller vcen,LIS≈−20±50kms−1, and significantly suppressed Lyα emission (EW(Lyα)≈5A˚), 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 vcen,LIS indicates less prevalent or weaker outflows in z>6 galaxies. Stacking our sub-sample of Lyα emitters (LAEs), we find high EW(Hβ)≈170±4A˚ and a detection of nebular CIV emission suggesting higher ξion 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.
@article{arxiv.2504.21080,
title = {Stacking PANCAKEZ: sPectroscopic Analysis with NirspeC stAcKs in the Epoch of reioniZation. Weak ISM Absorption and Implications for Ionizing Photon Escape at $z\sim7$},
author = {Kelsey S. Glazer and Tucker Jones and Yuguang Chen and Ryan L. Sanders and Marusa Bradac and Anthony J. Pahl and Alice E. Shapley and Richard S. Ellis and Michael W. Topping and Naveen A. Reddy},
journal= {arXiv preprint arXiv:2504.21080},
year = {2025}
}