CECILIA: Gas-phase Physical Conditions and Multielement Chemistry at Cosmic Noon
Creators
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Northwestern University
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Carnegie Observatories
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Franklin & Marshall College
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California Institute of Technology
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University of Wisconsin–Madison
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Chinese University of Hong Kong
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Space Telescope Science Institute
Abstract
Galaxies at Cosmic Noon (z ∼ 2–3) are characterized by rapid star formation that will lead to significant metal enrichment in the interstellar medium (ISM). While much observational evidence suggests that these galaxies are chemically distinct from those in the local Universe, directly measuring the ISM chemistry in large samples of high-z galaxies is only now possible with the observational capabilities of JWST. In this first key paper of the CECILIA program, we present the direct-method physical conditions and multielement abundances in 20 galaxies at Cosmic Noon. Using a combination of archival Keck/MOSFIRE and new ∼30 hr NIRSpec spectroscopy, we measure multiple electron gas densities and the temperature structure from the O+ and S2+ ions. We find that ne[O ii] and ne[S ii] are comparable but elevated with respect to ne in local star-forming galaxies, and the simultaneous Te[O ii] and Te[S iii] generally agree with photoionization model Te scaling relations. The O abundances in the CECILIA galaxies range from 12 + log(O/H) = 7.76 to 8.81 (12%–131% solar O/H), representing some of the highest direct-method metallicities and lowest Te (Te[O ii] ≈ 6500 K) measured with JWST to date. The CECILIA galaxies exhibit significantly subsolar S/O and Ar/O, a signature of predominant enrichment from core-collapse supernovae. The N/O–O/H trends in the CECILIA galaxies generally agree with the abundance trends in local nebulae, but the large scatter in N/O could be sensitive to the star formation history. The CECILIA observations demonstrate that exceptionally deep JWST spectroscopy can unveil the multielement ISM abundance patterns in typical high-z galaxies.
Copyright and License
© 2026. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Acknowledgement
We thank the anonymous referee for their thoughtful review and useful feedback, which helped elucidate further detail in the analysis. We would also like to thank Bernie Rauscher for useful discussions concerning NSClean, as well as Gabe Brammer for the assistance with the msaexp reduction of the CECILIA NIRSpec data.
N.S.J.R. is supported by JWST-GO-02593.008-A, provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127. A.L.S., G.C.R., and R.F.T. acknowledge partial support from the JWST-GO-02593.008-A, JWST-GO-02593.004-A, and JWST-GO-02593.006-A grants, respectively. A.L.S. is also supported by the David and Lucile Packard Foundation (Packard Fellowship, grant ID 2024-77399) and performed aspects of the work contained in this Letter at the Aspen Center for Physics, which is supported by National Science Foundation grant PHY-2210452. R.F.T. also acknowledges support from the Pittsburgh Foundation (grant ID UN2021-121482) and the Research Corporation for Scientific Advancement (Cottrell Scholar Award, grant ID 28289). T.B.M. was supported by a CIERA Postdoctoral Fellowship.
This work is primarily based on observations made with NASA/ESA/CSA JWST, associated with PID 2593, which can be accessed via doi: DOI: 10.17909/x66z-p144. The data were obtained from the Mikulski Archive for Space Telescopes (MAST) at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. The ground-based spectroscopy included in the analysis were obtained at W. M. Keck Observatory, which is operated as a scientific partnership between the California Institute of Technology, the University of California, and NASA. Keck access was provided by NASA, the California Institute of Technology, and Northwestern University and the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). The Observatory was made possible by the generous financial support of the W. M. Keck Foundation, and the authors wish to recognize and acknowledge the significant cultural role and reverence that the summit of Maunakea has within the indigenous Hawaiian community.
Facilities
Keck:I - KECK I Telescope (MOSFIRE), JWST - James Webb Space Telescope (NIRSpec).
Software References
BPASSv2 (E. R. Stanway et al. 2016; J. J. Eldridge et al. 2017), CLOUDY (G. J. Ferland et al. 2013), GalDNA (A. L. Strom et al. 2018), JWST Calibration Pipeline (H. Bushouse et al. 2023), grizli (G. Brammer 2023b), msaexp (G. Brammer 2022), PyNeb (V. Luridiana et al. 2015).
Files
Rogers_2026_ApJL_997_L44.pdf
Additional details
Related works
- Is new version of
- Discussion Paper: arXiv:2509.18257 (arXiv)
- Is supplemented by
- Dataset: 10.17909/x66z-p144 (DOI)
Funding
- Space Telescope Science Institute
- JWST-GO-02593.008-A
- National Aeronautics and Space Administration
- NAS 5-03127
- Space Telescope Science Institute
- JWST-GO-02593.008-A
- Space Telescope Science Institute
- JWST-GO-02593.004-A
- Space Telescope Science Institute
- JWST-GO-02593.006-A
- David and Lucile Packard Foundation
- 2024-77399
- National Science Foundation
- PHY-2210452
- Pittsburgh Foundation
- UN2021-121482
- Research Corporation for Science Advancement
- Northwestern University
- CIERA Postdoctoral Fellowship -
Dates
- Submitted
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2025-09-19
- Accepted
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2025-12-08
- Available
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2026-01-28Published
Caltech Custom Metadata
- Caltech groups
- Astronomy Department , Division of Physics, Mathematics and Astronomy (PMA)
- Publication Status
- Published