Insights on Metal Enrichment and Environmental Effect at $z\approx5-7$ with JWST ASPIRE/EIGER and Chemical Evolution Model

TL;DR

The paper leverages JWST/NIRCam WFSS data from the ASPIRE and EIGER programs to measure metallicities and the MZR for 604 galaxies at $z\approx5-7$, distinguishing overdense (cluster) from field environments. By combining direct $T_e$-based metallicities from the $\textrm{O}\,4363$ auroral line with strong-line calibrations, the authors derive a MZR slope of $\gamma\approx0.26$ for the full sample and find a steeper slope in overdense regions ($\gamma_{\text{cluster}}\approx0.42$), with cluster galaxies systematically more metal-rich at high masses. They also show that the FMR at these redshifts is offset from the local relation by about $0.25$ dex on average, though cluster galaxies lie closer to the local FMR, which they interpret as evidence for accelerated, environmentally influenced metal enrichment. A simple bathtub chemical-evolution model attributes the observed trends to dilution effects from rapid gas accretion in a non-equilibrium regime, implying that proto-cluster environments promote earlier star formation and faster approach to chemical equilibrium. Together, these results highlight environmental modulation of early metal enrichment and the utility of JWST measurements in constraining fast gas–star cycles in the first gigayear of cosmic history.

Abstract

We present the mass-metallicity relation (MZR) for a parent sample of 604 galaxies at $z=5.34-6.94$ with \OIII\ doublets detected, using the deep JWST/NIRCam wide field slitless spectroscopic (WFSS) observations in 26 quasar fields. The sample incorporates the full observations of 25 quasar fields from the JWST Cycle 1 GO program ASPIRE and the quasar SDSS J0100+2802 from the JWST EIGER program. We identify 204 galaxies residing in overdense structures using the friends-of-friends (FoF) algorithm. We estimate the electron temperature of $2.0^{+0.3}_{-0.4}\times10^4$ K from the Hg and OIII4363 lines in the stacked spectrum, indicating a metal-poor sample with median gas phase metallicity 12+$\log(\mathrm{O/H})=7.65^{+0.26}_{-0.15}$. With the most up-to-date strong line calibration based on NIRSpec observations, we find that the MZR shows a metal enhancement of $\sim0.2$ dex at the high mass end in overdense environments. However, compared to the local Fundamental Metallicity Relation (FMR), our galaxy sample at $z>5$ shows a metal deficiency of $\sim0.2$ dex relative to FMR predictions. We explain the observed trend of FMR with a simple analytical model, favoring dilution from intense gas accretion over outflow to explain the metallicity properties at $z > 5$. Those high-redshift galaxies are likely in a rapid gas accretion phase, during which their metal and gas contents are in a non-equilibrium state. According to model predictions, the protocluster members are closer to the gas equilibrium state than field galaxies and thus have higher metallicity and are closer to the local FMR. Our results suggest that the accelerated star formation during protocluster assembly likely plays a key role in shaping the observed MZR and FMR, indicating a potentially earlier onset of metal enrichment in overdense environments at $z\approx5-7$.

Figures (11)

• Figure 1: Redshift distribution of the full sample in this work. [Oiii] emitters from ASPIRE and EIGER programs are marked by red and blue, respectively. The EIGER sample is stacked on top of the ASPIRE sample.
• Figure 2: Median stacked 1D rest-frame spectra for our full sample galaxies at $z>6.25$, with continuum subtracted. The fluxes are normalized by the peak of [Oiii]$_{5007}$ flux after stacking.
• Figure 3: Mass-excitation diagram for our sample galaxies. The blue circles represent individual measurements. The red and blue diamonds represent stacked measurements in cluster and field galaxies, respectively. The blue and orange curves indicate the lower and upper mass-excitation demarcation by Coil_15. The AGNs (star-forming galaxies) are above (below) the demarcation. The gray shaded region indicates the high-mass end, which we did not analyze due to uncertainties in the metallicity calibrations in this regime.
• Figure 4: The mass-metallicity relation for galaxies in protoclusters (red) and blank fields (blue), which are based on R3 calibration from Chakraborty_24. The Sanders_24 calibration provides similar results, which are listed in the Table. \ref{['tab:mzr_info']} for comparison. The results from other works in the literature are shown with different colors, which are also listed in the Table. \ref{['tab:mzr_info']} for reference. Higher redshift ($z>3$) measurements in literature are shown in solid lines, and lower redshift ($z\sim2-3$) measurements are shown in dashed lines.
• Figure 5: The metallicity offset between galaxies in overdense and field environments. The blue line shows the offset of MZR between cluster and field galaxies, with the shaded region representing $1\sigma$ confidence interval. The single blue diamond shows the offset between the mass-matched cluster and field samples. The measurements in literature Wang_22Kacprzak_15Sattari_21Kulas_13Chartab_21Shimakawa_15Valentino_15 are also shown for comparison, each marked in different colors. The error bars represent $1\sigma$ uncertainties.