CECILIA: Gas-Phase Physical Conditions and Multi-Element Chemistry at Cosmic Noon

TL;DR

CECILIA addresses the chemical evolution of galaxies during Cosmic Noon ($z\sim2$--$3$) by obtaining direct-method ISM temperatures and densities and measuring multi-element abundances in a large sample of SFGs. The study combines ultra-deep JWST/NIRSpec spectroscopy with Keck/MOSFIRE rest-optical data to derive $n_e$, $T_e$, and abundances for O, N, S, and Ar, using a three-zone ionization model and ICFs. It finds $12+\log(O/H)$ in the range [$7.76$, $8.81$], with several galaxies near solar metallicity, and sub-solar $S/O$ and $Ar/O$ indicating CCSNe-dominated enrichment; N/O shows local-like trends but with substantial scatter. This work demonstrates that exceptionally deep JWST spectroscopy can reveal detailed ISM abundance patterns in typical high-$z$ galaxies, enabling robust tests of chemical evolution and the roles of CCSNe and Type Ia enrichment across cosmic time.

Abstract

Galaxies at Cosmic Noon (z$\sim$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 multi-element abundances in twenty galaxies at Cosmic Noon. Using a combination of archival Keck/MOSFIRE and new $\sim$30-hr NIRSpec spectroscopy, we measure multiple electron gas densities and the temperature structure from the O$^+$ and S$^{2+}$ ions. We find that n$_e$[O II] and n$_e$[S II] are comparable but elevated with respect to n$_e$ in local star-forming galaxies, and the simultaneous T$_e$[O II] and T$_e$[S III] generally agree with photoionization model T$_e$ scaling relations. The O abundances in the CECILIA galaxies range from 12+log(O/H)$=$7.76-8.81 (12-131% solar O/H), representing some of the highest direct-method metallicities and lowest T$_e$ (T$_e$[O II]$\approx$6500 K) measured with JWST to date. The CECILIA galaxies exhibit significantly sub-solar 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 multi-element ISM abundance patterns in typical high-z galaxies.

Figures (16)

• Figure 1: NIRSpec spectrum of BX523, a star-forming galaxy at z$=$ 2.2708$\pm$0.0001 in the CECILIA sample. The top two panels plot the 2D spectra from each grating, where the global background and bar shadow removal procedures have resulted in a featureless background with no self subtraction in the strong emission lines. The optimally-extracted 1D spectra, shifted to the rest-frame of BX523, are plotted in the main panel. The G235M spectrum (black) has been resampled to match the G395M data (red) for plotting purposes only. We highlight the high-S/N detections of the [S3]$\lambda$6314 and [O2]$\lambda\lambda$7322,32 T$_e$-sensitive auroral lines in the inset panels (plotted at the native sampling of G235M). The depth of the CECILIA observations reveal many other faint emission lines in both the 2D and 1D spectra.
• Figure 2: Comparison of NIRSpec and MOSFIRE line ratios: log([N2]$\lambda$6585/H$\alpha$) and log([O3]$\lambda$5008/H$\beta$) in the top and bottom panels, respectively. There is good consistency between the independent measurements of the line ratios in the CECILIA sample, where only two MOSFIRE line ratios are inconsistent with the same ratio in NIRSpec at $>$2$\sigma$. The similar flux ratios observed in the two detectors allows for a combination of the ground-based strong lines (e.g., from [O2] and [O3]) with the faint emission lines detected from JWST/NIRSpec.
• Figure 3: General properties of CECILIA and KBSS, the CECILIA galaxies' parent sample. Left: The star-forming main sequence, log(SFR) vs. log(M$_*$). The SFMS observed in KBSS is represented in blue contours (at 5%, 25%, 50%, 75%, and 95% densities), the CECILIA galaxies are denoted with white circles, where red circles represent the subsample of galaxies with at least one direct T$_e$. We also provide fits to the SFMS from speagle2014, popesso2023, and clarke2024 as solid green, dot-dashed brown, and dashed black lines, respectively. The T$_e$ sample presented here is both representative of the average properties of KBSS and typical galaxies at Cosmic Noon. Right: The N2-BPT, log([O3]/H$\beta$) vs. log([N2]/H$\alpha$). The ratios plotted on the horizontal axis are measured from NIRSpec. [O3]/H$\beta$ is measured from NIRSpec when available, otherwise they are measured from the archival MOSFIRE data. The green dotted and blue dashed lines represent the kewley2001 and kauffmann2003 demarcations for the line ratios predicted from an extreme starburst and AGN ionization, respectively. The line ratios measured in KBSS are represented as the blue contours, and the average trend derived by strom2017 is plotted in solid pink. The average N2-BPT line ratio trend for z$\sim$0 SFGs in SDSS is plotted as a black line. The T$_e$ sample spans almost 1 dex in [N2]/H$\alpha$ and follow the ionization trends predicted from the full KBSS sample.
• Figure 4: Density trends in the CECILIA galaxies (red circles) and local EELGs (light blue squares). Each point represents the simultaneous measurement of n$_e$[O2] and n$_e$[S2], and the dotted line reflects equivalent densities. The dark red and light blue stars are the average n$_e$ measured in CECILIA and at z$=$0, respectively. The top panel plots the histogram distribution of 23 n$_e$[S2] measurements in the CECILIA sample, where the red bins represent the galaxies with simultaneous n$_e$[O2]. In agreement with previous findings, the direct measurements of n$_e$[S2] and n$_e$[O2] from the CECILIA galaxies indicate that the ISM density at Cosmic Noon is elevated with respect to the n$_e$ measured in the local Universe. Furthermore, n$_e$[S2] and n$_e$[O2] show good agreement, as may be expected for two ions primarily originating in the same ionization zone.
• Figure 5: Simultaneous T$_e$[O2] and T$_e$[S3] measured in the CECILIA sample (red circles, 3$\sigma$ T$_e$[S3] upper limits provided with black arrows), local EELGs (blue squares), and the metal-rich CHAOS H2 regions (gray diamonds). We also include direct T$_e$ from the AURORA sanders2025 and MARTA surveys cataldi2025curti2025 as purple and white triangles, respectively. The theoretical T$_e$ scaling relation from garnett1992 is plotted in green, while the black dotted line indicates equivalent T$_e$[O2] and T$_e$[S3]. The histogram distribution of the 17 T$_e$[O2] and 9 T$_e$[S3] measurements in CECILIA are provided in the right and top panels, respectively. The high-z T$_e$ are generally consistent with those measured in the local Universe and from photoionization model predictions, but there is currently insufficient data to assess the scatter in T$_e$[O2]-T$_e$[S3] at high-z.