Metal enrichment of galaxies in a massive node of the Cosmic Web at $z \sim 3$

Abstract

We present the mass-metallicity relation for star-forming galaxies in the MUSE Quasar Nebula 01 (MQN01) field, a massive cosmic web node at $z \sim 3.245$, hosting one of the largest overdensities of galaxies and AGNs found so far at $z > 3$. Through James Webb Space Telescope (JWST) Near Infrared Spectrograph (NIRSpec) spectra and images from JWST and Hubble Space Telescope (HST), we identify a sample of 9 star-forming galaxies in the MQN01 field with detection of nebular emission lines ($\rm Hβ$, [OIII], $\rm Hα$, [NII]), covering the mass range of $\rm 10^{7.5}M_\odot - 10^{10.5}M_\odot$. We present the relations of the emission-line flux ratios versus stellar mass for the sample and derive the gas-phase metallicity based on the strong line diagnostics of [OIII]$\lambda5008$/$\rm Hβ$ and [NII]$\lambda6585$/$\rm Hα$. Compared to the typical, field galaxies at similar redshifts, MQN01 galaxies show relatively higher [NII]$\lambda6585$/$\rm Hα$ and lower [OIII]$\lambda5008$/$\rm Hβ$ at the same stellar mass, which implies a higher metallicity by about $0.25\pm 0.07$ dex with respect to the field mass-metallicity relation. These differences are decreased considering the ``Fundamental Metallicity Relation'', i.e. if the galaxies' Star Formation Rates (SFR) are also taken into account. We argue that these results are consistent with a scenario in which galaxies in overdense regions assemble their stellar mass more efficiently (or, equivalently, start forming at earlier epochs) compared to field galaxies at similar redshifts.

Figures (14)

• Figure 1: Slit design of the JWST Program GO 1835 (PI: Cantalupo) targeting the MQN01 protocluster field (see text for details on slit location selection). The composite false-color image of the field is created with HST F814W (0.8$\rm \mu m$; blue), JWST F150W2 (1.5$\rm \mu m$; green), and JWST F322W2 (3.2$\rm \mu m$; red). The dashed contours show the overdensity of galaxies Galbiati2025. The distribution of samples identified from MUSE observations Galbiati2025, Chandra X-ray observations Travascio2025 and ALMA Pensabene2024 are also included. The sample of identified star-forming galaxies detected with emission lines is marked with cyan circles. Galaxies with X-ray emission are marked in orange circles. A quiescent galaxy serendipitously found with the JWST NIRSpec observations (Wang et al., in prep.) is marked with a red circle.
• Figure 2: Integrated 1D spectra and composite false-color images of two galaxies in the sample, with slit IDs 3001 and 1135. The flux density is in unit of $10^{-17} \rm erg/s/cm^2/$Å, and the wavelength is in rest-frame Angstroms. The slit regions where the 1D spectra are extracted are overplotted on the galaxy images as light pink boxes. Emision lines are modeled with single Gaussians. For galaxy 3001, which shows road emission-line features, an additional Gaussian component is included (shown by the green dotted line). Flux of the narrow Gaussian components are adopted in the analysis for the galaxy 3001. The filters used to create the color images are HST F814W (0.8$\rm \mu m$; blue), JWST F150W2 (1.5$\rm \mu m$; green), and JWST F322W2 (3.2$\rm \mu m$; red). Spectra and images for the remaining galaxies are given in Fig. \ref{['spec_image_full']}.
• Figure 3: The BPT diagram for the MQN01 galaxy sample. The classification curve is from Kewley2013 with $z = 3.245$. Galaxies are shown as blue stars. For one galaxy (ID: 6005) with no $\rm H\beta$ detection, a 3$\sigma$ upper limit is adopted, resulting in a lower limit for $\rm [OIII]\lambda5008/H\beta$, shown as a blue triangle. The galaxy whose [OIII] falls within the spectra detection gap (ID: 3201, see Fig.\ref{['spec_image_full']}) is shown as the vertical dashed line at its $\rm [NII]\lambda 6585/H\alpha$ value. The distribution of the stacked CEERS $2.7 < z < 4.0$ sample Shapley2023, stacked MOSDEF $z \sim 2.2$Sanders2021, and the best fit relation for the KBSS-MOSFIRE sample at $z\sim 2.3$Steidel2014, are also included.
• Figure 4: Star formation rate (SFR) and specific Star formation Rate (SSFR) versus stellar mass for the MQN01 sample. SFR is calculated by $\rm H\alpha$ emission with aperture and dust correction. The galaxy sample is shown as blue stars. For the galaxy with no $\rm H\beta$ detection (ID: 6005), the SFR without dust correction is adopted as a lower limit, shown as a blue triangle. For the galaxy with no JWST photometry (ID: 3201), aperture correction calculated from HST F814W is adopted and is shown as a blue pentagon. SFR and stellar mass derived from SED fitting for a larger sample of MQN01 galaxies Galbiati2025 are shown as gray circles. The histograms on the top and right in both panels show the distribution of the SFR, SSFR and stellar mass for MQN01 sample in this work (blue) and that from Galbiati2025 (gray). For comparison, the best-fit SFR-$\rm M_*$ relation for the MOSDEF sample with median $z \sim 3.3$Sanders2021 is shown as the orange line, with the shaded-region indicating the 1$\sigma$ uncertainty of the fitting parameters. SFR and SSFR versus stellar mass from Li2023 are shown as purple circles. SFR, SSFR-$\rm M_*$ from Speagle2014 and Popesso2023 at $z = 3.245$ are shown as the green lines and purple lines, with 1$\sigma$ uncertainties of the fitting parameters shown as the shaded regions. Compared with reference SFMS, the MQN01 sample shows a median offset of $\sim$ 0.07 dex above the referred star formation main sequence Speagle2014.
• Figure 5: Emission line flux ratio versus stellar mass for the MQN01 sample. The sample is marked as blue stars. Left: $\log$ [NII]$\lambda$6585/$\rm H\alpha$ versus stellar mass. Reference data from stacked spectra of the CEERS sample with $2.7 < z < 4.0$Shapley2023 are shown as green squares. Right: $\log$ [OIII]$\lambda5008$/$\rm H\beta$ versus stellar mass. The galaxy without $\rm H\beta$ detection is shown as a triangle at the lower limit of [OIII]$\lambda5008$/$\rm H\beta$ based on the 3$\sigma$ detection. The datasets from Onodera2016Sanders2021Li2023Shapley2023 are included for comparison. The individual data points are shown in filled circles, with stacked values shown as squares. Compared with the reference datasets, the MQN01 galaxies show on average higher [NII]$\lambda6585$/$\rm H\alpha$ and lower [OIII]$\lambda5008$/$\rm H\beta$.