The Mass–Metallicity Relation at z ≃ 8: Direct-method Metallicity Constraints and Near-future Prospects

Abstract

Physical properties of galaxies at z>7 are of interest for understanding both the early phases of star formation and the process of cosmic reionization. Chemical abundance measurements offer valuable information on the integrated star formation history, and hence ionizing photon production, as well as the rapid gas accretion expected at such high redshifts. We use reported measurements of [OIII]88 μm emission and star formation rate to estimate gasphase oxygen abundances in five galaxies at z=7.1-9.1 using the direct T e method. We find typical abundances + 12 log O H ( )=7.9 (∼0.2 times the solar value) and an evolution of 0.9±0.5 dex in oxygen abundance at fixed stellar mass from z;8 to 0. These results are compatible with theoretical predictions, albeit with large (conservative) uncertainties in both mass and metallicity. We assess both statistical and systematic uncertainties to identify promising means of improvement with the Atacama Large Millimeter/submillimeter Array (ALMA) and the James Webb Space Telescope (JWST). In particular we highlight [OIII]52 μm as a valuable feature for robust metallicity measurements. Precision of 0.1-0.2 dex in T e -based O/H abundance can be reasonably achieved for galaxies at z≈5-8 by combining [OIII]52 μm with rest-frame optical strong lines. It will also be possible to probe gas mixing and mergers via resolved T e -based abundances on kiloparsec scales. With ALMA and JWST, direct metallicity measurements will thus be remarkably accessible in the reionization epoch.

Measuring the HI content of individual galaxies out to the epoch of reionization with [CII]

“…1). While the FMR is not well-established at z > 3, the average metallicities inferred for the high-redshift samples described below reproduces the expected decrease of ∆ log(Z/Z ) = −0.1 per unit redshift observed for starforming galaxies up to z ≈ 3.5 (Sanders et al 2020), and potentially beyond (Cucchiara et al 2015;Faisst et al 2016;Jones et al 2020).…”

High [OIII]/[CII] surface brightness ratios trace early starburst galaxies

“…• Gas metallicity and density are relatively high (Z = 0.1− 0.5Z , and n = 10 1−3 cm −3 ) in the nine sample galaxies, also in agreement with findings from Jones et al (2020); Yang & Lidz (2020).…”

“…Note that also several SED fitting analysis (e.g. Laporte et al 2017b; Jones et al 2020) and cosmological simulations (Pallottini et al 2017(Pallottini et al , 2019Arata et al 2020) point towards EoR sources being characterized by intermittent star formation histories with separated bursts of star formation. This highlights the importance of using multi-parameter MCMC approaches to derive the metallicity from [OIII]/[CII] ratios (see Figure 5) and represents an attempt in the direction suggested by Harikane et al (2020), who pointed out that the metallicity dependence of the [OIII]/[CII] ratio cannot be easily disentangled from variations 8 in the ionization parameter and n.…”

“…extracted from SERRA 5 . The sample extracted from SERRA is built by selecting central galaxies with stellar mass in the range log(M * /M ) = 8.0 − 10.1, comparable to that spanned by the observed z = 7−9 sources in our EoR sample(Roberts- Borsani et al 2020;Jones et al 2020). …”

Intensity mapping from the sky: synergizing the joint potential of [OIII] and [CII] surveys at reionization

“…Cross-correlations with the HI 21-cm line during the epoch of reionization (EoR) (Lidz et al 2011;Gong et al 2012;Dumitru et al 2018) are also an important motivation for focusing on [OIII] from EoR galaxies, given that as many consistency tests as possible will strengthen the reliability of the results, especially in view of the challenges such as foregrounds facing current 21-cm observations. Furthermore, measuring [CII] and [OIII] together provides a more complete picture of the ISM than either of the tracers taken separately, especially because [OIII] is associated with the HII regions while [CII] at high redshift originates primarily from photon dominated regions (PDRs; Vallini et al 2016;Lagache 2018).There are also prospects for measuring the [OIII] 52 𝜇m line with ALMA and JWST (Yang et al 2021;Jones et al 2020) which together will help break important degeneracies in physical processes governing galaxy formation at these epochs.…”