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

Glazer, Kelsey (author), Jones, Tucker (author), Chen, Yuguang (author), Sanders, Ryan L. (author), Bradač, Maruša (author), Pahl, Anthony J. (author), Shapley, Alice E. (author), Ellis, Richard Salisbury (author), Topping, Michael W. (author), Reddy, Naveen A. (author)

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.

• Language: English
• Keywords: astrophysics, galaxies
• Work type: Article
• Typology: 1.01 - Original Scientific Article
• Organization: FMF - Faculty of Mathematics and Physics
• Publication status: Published
• Publication version: Version of Record
• Year: 2025
• Number of pages: 19 str.
• Numbering: Vol. 992, no. 2, art. no. 191
• PID: 20.500.12556/RUL-176541
• UDC: 524.8
• ISSN on article: 1538-4357
• DOI: 10.3847/1538-4357/ae0194
• COBISS.SI-ID: 259763971
• Publication date in RUL: 03.12.2025

Record is a part of a journal

• Title: The Astrophysical journal
• Shortened title: Astrophys. j.
• Publisher: University of Chicago Press for the American Astronomical Society
• ISSN: 1538-4357
• COBISS.SI-ID: 515079705

Licences

• License: CC BY 4.0, Creative Commons Attribution 4.0 International
• Link: http://creativecommons.org/licenses/by/4.0/
• Description: This is the standard Creative Commons license that gives others maximum freedom to do what they want with the work as long as they credit the author.

Secondary language

• Language: Slovenian
• Keywords: astrofizika, galaksije