The James Webb Space Telescope (JWST) has significantly advanced our exploration of the high-redshift universe. A crucial breakthrough for the high-redshift community stems from the exceptional capabilities of the Near Infrared Spectrograph (NIRSpec). This instrument enables precise investigations into the chemical enrichment of galaxies at redshifts greater than 4 by directly observing rest-frame optical emission lines.

We showcase the evolution of the mass-metallicity (MZ) relations at z=4-10, drawing insights from 138 identified galaxies in the JWST/NIRSpec data. These data are sourced from three significant public spectroscopy programs: Early Release Observations (ERO), Early Release Science (ERS) of GLASS, and CEERS, employing an enhanced NIRSpec data reduction procedure developed in-house. Additionally, our dataset includes a DDT program focusing on the RX J2129 cluster field. The top panel illustrates a compilation of 2D spectra from our NIRSpec sample, arranged in order of increasing redshift from bottom to top.

Our analysis reveals a modest evolution in the MZ relation from redshift z~2-3 to z=4-10 (2nd panel). When factoring in the star formation rate (SFR) dependence (3rd panel), galaxies at z=4-8 show minimal metallicity evolution compared to the average of galaxies at z=0-3. Notably, among the most distant galaxies at z=8-10, 6 out of the 7 galaxies in this highest-redshift subset  (4th panel) exhibit oxygen abundance levels roughly half or less than expected from the SFR-MZ relation at the >2sigma level. These findings suggest a rapid increase in oxygen abundance during the first 500-700 million years after the universe's birth, followed by a subsequent metallicity equilibrium maintained through processes such as star formation, inflow, and outflow, possibly persisting over the past 13 billion years.

The research findings, particularly emphasizing the "Rapid Increase in Oxygen in the Early Universe," were unveiled at a press conference in Japan on November 9, 2023. This announcement garnered attention from various media outlets, including television, newspapers, and online news platforms. Further details about the press release can be accessed on the web news pages of ICRR (Univ. Tokyo) and NAOJ.

I am chatting about this article here on the AAS channel on YouTube.

The summary of properties for the JWST/NIRSpec objects presented in this paper as Table D1 is available here. 

Forthcoming observational facilities such as James Webb Space Telescope and Extremely Large Telescopes will make the exploration of the early universe routine, likely probing large populations of galaxies at very low metallicities. It will therefore be important to have diagnostics that can solidly identify and distinguish different classes of objects in such low metallicity regimes. 

We use state-of-the-art photoionization models to develop diagnostic diagrams involving various nebular lines. We show that combinations of these diagrams allow the identification and discrimination of the following classes of objects in the early universe: PopIII and Direct Collapse Black Holes (DCBH) in pristine environments, PopIII and DCBH embedded in slightly enriched ISM (Z~10^-5 - 10^-4), (metal poor) PopII and AGN in enriched ISM. For example, the left figures present optical diagnostics to identify PopIII galaxies (top) and infer the chemical enrichment (bottom) that will be accessible with JWST.

The photoionization model results presented in this paper will be shared on reasonable request to us.

We are carrying out a program: "Extremely Metal-Poor Representatives Explored by the Subaru Survey" (EMPRESS) to search for galaxies with an extremely low metallicity (EMPGs) using the deep and wide multi-wavelength imaging data of Subaru/Hyper Suprime-Cam (HSC) Subaru Strategic Program (SSP), and examine the detailed properties with follow-up spectroscopic observations with Keck, Subaru and Magellan. Seven papers have been published so far in the project. 

In the new paper, we present optical-line gas metallicity diagnostics established by the combination of local SDSS galaxies and the largest compilation of EMPGs over the range of 12+log(O/H)=~6.9-8.9 corresponding to 0.02-2 solar metallicity. We confirm that R23-index, ([OIII]+[OII])/H-beta, is the most accurate metallicity indicator (top). The other metallicity indicators such as R3-index=[OIII]/H-beta and N2-index=[NII]/H-alpha show large scatters in the metal-poor range, because, unlike R23-index, they do not use a sum of singly and doubly ionized lines and cannot trace both low and high ionization gas. We find that the accuracy of the metallicity indicators is significantly improved, if one uses H-beta equivalent width measurements that tightly correlate with ionization states (an example (R3-index) in the bottom). These metallicity indicators and the recipes will be useful for forthcoming JWST spectroscopic studies. The metallicity-sensitive emission line ratios and the properties for the compiled 103 EMPGs are publicly available here.

The ionizing output of early star-forming galaxies is key to understanding whether they were responsible for the cosmic reionization. Major uncertainties include the number of ionizing photons per UV luminosity (ξion) and the fraction that escape, fesc. 

Since neither can be directly/easily observed at high-redshift, it is important to measure these parameters for suitable analogs at lower redshift. For this purpose, we have embarked on a project: "Lyman Continuum Escape Survey" (LACES) by examining a large sample of Subaru-selected z=3.1 Lyman alpha emitting galaxies (LAEs) whose intense nebular emission and young stellar population indicate they are useful analogs of early galaxies. Several important results emerged from the LACES team: 

Using measures of the ionizing radiation of the stellar population, characterized by ξion, the left top figure (a) shows that LAEs produce ionizing photons consistently more efficiently than inferred for continuum-selected galaxies (LBGs) at similar redshift. Similar hard radiation fields are being inferred for z>7 galaxies.

Secondly, LAEs have uniformly larger [OIII]5007/[OII]3727 line ratios compared to similar redshift LBGs (Fig. b) and so it has been conjectured that these systems may have higher values of fesc following a hypothesis proposed by Nakajima & Ouchi (2014). We argue that such a large line ratio arises from density-bound rather than ionization-bound HII regions, possibly achieved by the hard ionizing radiation. The associated porosity of the star-forming regions would therefore lead to significant LyC leakage and a high fesc.

To address this last point, the LACES team has secured deep (64 orbits) HST WFC3/UV F336W imaging for a sample of 54 z=3.1 LAEs to make individual measures of fesc. We stress z~3 is the highest redshift, thus the closest to the reionization era, where direct measures of fesc are possible. Whereas fesc is typically 5-8% for z=2-3 LBGs, LACES finds 20% of the sample of LAEs are strong LyC leakers with fesc=13-60% (Fig. c). Given the homogeneous nature of the sample, it provides a valuable basis for understanding whether their high redshift equivalents caused cosmic reionization. 

Moreover, we confirm with the LACES objects that a large [OIII]/[OII] line ratio is indeed a necessary condition for LyC leakage, strengthening our earlier claims (Fig. 4). However, not all LAEs with large [OIII]/[OII] line ratios are leakers and leaking radiation appears not to be associated with differences in other spectral diagnostics. This suggests the detection of leaking radiation is modulated by an additional property, possibly the viewing angle for porous H II regions.

To be fully completed soon.