JWST [OIII] Emitters at Cosmic Dawn

• JWST-selected [OIII] emitters are high-redshift galaxies characterized by strong nebular lines that signal vigorous, young star formation and low metallicity.
• They are efficiently identified using slitless spectroscopy and medium-/narrow-band photometry, allowing robust selection across the epoch of reionization.
• Their extreme ionization properties and spatial emission offsets suggest they play a pivotal role in creating large ionized bubbles during cosmic reionization.

JWST-selected [OIII]-emitting galaxies are extragalactic systems at high redshift, identified and characterized using the unique spectroscopic and photometric sensitivity of the James Webb Space Telescope (JWST). The oxygen forbidden lines—primarily [OIII] λλ4960, 5008 Å and in some cases [OIII] 88 μm—serve as powerful diagnostics of massive star formation, ionization conditions, metal enrichment, and feedback mechanisms in the early universe. Surveys and targeted campaigns with JWST have uncovered vast populations of such emitters across the epoch of reionization (z ≳ 5), revealing that [OIII] emission is a ubiquitous marker of intensely star-forming, low-mass, chemically young galaxies that play an outsized role in cosmic reionization and early galaxy assembly.

1. Identification and Selection of [OIII] Emitters

JWST has enabled the selection of [OIII] emitters at high redshift using several complementary methodologies:

• Slitless Spectroscopy: Wide-field slitless spectroscopic (WFSS) modes with NIRCam and NIRISS have efficiently identified [OIII] emission from z ≈ 5 up to z ≈ 9, yielding samples as large as N ≈ 117 spanning 5.3 < z < 6.9 in a single EIGER field (Kashino et al., 2022). Both doublet lines ([OIII] λ4960,5008) are required for robust confirmation in spectrally-resolved datasets (Kashino et al., 2022).
• Medium- and Narrow-Band Photometric Excess: Use of JWST/NIRCam medium- and narrow-band filters (e.g., F410M, F466N/F470N) enables color excess selections for strong emission line galaxies based on boosted flux relative to continuum bands. Flux excess criteria in, e.g., F410M–F444W > 0.25 (rest-frame EW₀ ≳ 660 Å) isolate Hβ+[OIII] emitters at z ≈ 7 (Daikuhara et al., 17 Jul 2025). Narrow-broad imaging selection in the JWST Emission Line Survey (JELS) exploits the overlap of [OIII] in specific filters to select emitters at z ≈ 8.3 to unprecedented depths (Duncan et al., 11 Oct 2024).
• Dual-Line and Extreme Emission-Line Galaxy (EELG) Selection: Combined selection of both [OIII]+Hβ and Hα excess in the appropriate filters (e.g., F277W and F410M) isolates dual-line emitters in narrowly-defined redshift windows (e.g., 5.03 ≤ z ≤ 5.26) (Guo et al., 28 Sep 2024).
• Spectroscopic Confirmation and SED Validation: Subsamples of photometric candidates are followed up spectroscopically with JWST/NIRSpec or NIRCam grism modes to confirm redshifts, measure line fluxes/EWs, and validate selection purity (Withers et al., 2023, Daikuhara et al., 17 Jul 2025).

These techniques collectively allow assembly of statistically significant, homogeneous samples of [OIII] emitters at high redshift, spanning a wide range in mass, SFR, equivalent width, and environment.

2. Physical Properties and Diversity

JWST-selected [OIII] emitters, especially at z ≳ 6, are characterized by extreme nebular line strengths, high specific star formation rates, low metallicities, and compact morphologies:

Property	Typical Value/Range	References
Stellar mass ($M_*$)	$\sim 3 \times 10^7$ – $10^{9} M_\odot$; median log(M*) ≈ 7.5–8.0	(Daikuhara et al., 17 Jul 2025, Withers et al., 2023)
EW(Hβ+[OIII])	420 Å $<$ EW$_0$ $<$ 6850 Å; median ≈ 1700 Å; EELG subset EW$_0$ > 3000 Å	(Daikuhara et al., 17 Jul 2025)
Metallicity ($Z/Z_\odot$)	Median $Z/Z_\odot \sim 0.1$–0.2; many systems 5–15% solar	(Schaerer et al., 2022, Withers et al., 2023)
UV slope $\beta_{UV}$	Typical $\beta_{UV} \sim -2.2$ (blue); some even bluer for young populations	(Daikuhara et al., 17 Jul 2025, Guo et al., 28 Sep 2024)
Dust attenuation	$A_V \sim 0.1$ mag (minimal)	(Daikuhara et al., 17 Jul 2025, Withers et al., 2023)
[OIII]/Hβ ratio	$\sim$6–7 (highly ionized ISM)	(Matthee et al., 2022)
Electron temperature	$T_e \sim 2\times10^4$ K from [OIII]4364	(Matthee et al., 2022)
Mass–Metallicity	Strong relation; high-z [OIII] emitters often lie below the z∼2 MZ relation	(Schaerer et al., 2022, Matthee et al., 2022)

The physical mechanisms behind the high equivalent widths are predominantly stellar photoionization by massive, young stars (burst populations), with only a minor AGN contribution even among the most extreme EELG outliers ($<$ 10%) (Daikuhara et al., 17 Jul 2025). High ionization parameters, low dust, and feedback-driven shocks combine to maximize [OIII] output per unit SFR at these epochs (Matthee et al., 2022).

3. Spatial Structure, Morphology, and Environmental Context

Measurements of spatially resolved [OIII] emission using JWST’s high angular resolution demonstrate:

• Spatial Offsets: In at least 13 sources, there is a significant spatial displacement (>0.3 kpc) between the Hβ+[OIII] emission and the UV or stellar continuum peaks (Daikuhara et al., 17 Jul 2025). Such offsets are consistent with strong, radiative shocks driven by stellar feedback, causing ionized gas to be displaced from the older stars.
• Compact Morphology: The highest-EW emitters tend to have more compact emission-line sizes than continuum sizes (Davis et al., 2023). This suggests a contribution from nuclear activity or highly centralized starbursts, though evidence for significant AGN dominance is limited.
• Overdensity and Bubble Formation: Discovery of a significant overdensity of extreme EELGs in a JWST field (e.g., JADES GOODS-S) points toward the formation of large ionized bubbles, with local densities >50× field value. Such “star-bursting” over-dense regions are plausible sites of early ionization structure formation (Daikuhara et al., 17 Jul 2025, Kashino et al., 2022).

4. Line Luminosity Functions and Evolution

The [OIII] luminosity function (LF) has been extended to new depths (faint-end) with medium/narrow band and lensing field surveys:

• The [OIII]$\lambda$5008 LF at z ≈ 7–8 is fit by a single power law, with a faint-end slope $\alpha \approx -2.07$ and no evidence for a turnover down to log(L/erg s⁻¹)=41.1 (Wold et al., 26 Jul 2024).
• There is little evolution in the [OIII] LF between z ≈ 3 to z ≈ 7, despite strong evolution in the UV LF, indicating a rapid shift to more extreme nebular properties at earlier epochs (Barros et al., 2019, Sun et al., 2022).
• Simulations such as FLARES reproduce the broad shape and normalization for “typical” [OIII] emitters, but underpredict the tail of extreme-EW sources (Wilkins et al., 2023).
• The bright end of the [OIII] LF matches the observed Lyman-α LF, suggesting a strong connection between [OIII] emitters and the population of Lyman-α emitters at cosmic dawn (Wold et al., 26 Jul 2024).

5. Role in Cosmic Reionization

JWST-selected [OIII] emitters are directly implicated as major contributors to the reionization of the intergalactic medium (IGM):

• Ionizing Efficiencies: Typical ionizing photon production efficiencies reach $$\log_{10}(\xi_{\rm ion} [\mathrm{Hz\, erg}^{-1}]) \sim 25.3$$–26.3, much higher than canonical values (Matthee et al., 2022, Barros et al., 2019). High EWs and blue colors reflect efficient Lyman-continuum photon output.
• IGM Transmission Correlations: Cross-correlation of [OIII] emitter positions with the Lyα forest in background QSO spectra demonstrates a statistically significant excess Lyα transmission (highly ionized bubbles ≳50 cMpc) around such galaxies at $z=5.86$ (Kakiichi et al., 10 Mar 2025). This supports a scenario in which clustered [OIII] emitters and associated faint galaxies generate large ionized regions as reionization completes.
• Relation to Lyman-α Escape: The observed properties (e.g., O32 and Ne3O2 ratios, high EWs) in both composites and individual EELGs indicate extreme ionization states consistent with Lyman-continuum leakage and enhanced Lyman-α visibility (Tang et al., 2018).

6. AGN Contribution and the Faint AGN Census

Deep JWST surveys reach the faint end of the [OIII] and Hα LFs, revealing a hitherto unseen population of compact, dual-line emitters interpreted as faint AGN:

• In the CEERS field at z ≈ 5.1, ≈22% of dual Hα + [OIII] emitters (58/261) have compact morphologies and inferred AGN bolometric luminosities $$2 \times 10^{43} \lesssim L_{\rm bol} \lesssim 3 \times 10^{44}$$ erg s⁻¹, a number density ≈100× the extrapolated abundance of UV-selected luminous quasars (Guo et al., 28 Sep 2024).
• Modeling requires an AGN duty cycle of ≲10% among early galaxies to match number densities (Guo et al., 28 Sep 2024).
• While most extreme EELGs at z ≳ 7 are not AGN-dominated, JWST’s sensitivity has unveiled a trailing population of low-mass black holes rapidly growing in early galaxies.

7. Future Prospects and Impact

Ongoing and planned JWST spectroscopic programs, in conjunction with deep, wide-area narrow- and medium-band imaging, are poised to:

• Further map the faint and bright ends of the [OIII] LF with high completeness and purity (Duncan et al., 11 Oct 2024, Wold et al., 26 Jul 2024).
• Resolve the spatial structure and feedback processes in EELGs at sub-kpc scales (Daikuhara et al., 17 Jul 2025).
• Test and calibrate theoretical models regarding the efficiency of burst-driven feedback, bubble growth, and the faint-end slope of the AGN population (Wilkins et al., 2023, Kakiichi et al., 10 Mar 2025).
• Enable robust, unbiased census-taking of the galaxy and AGN populations responsible for the completion of reionization.

This comprehensive body of JWST-driven research converges on a scenario where compact, chemically young, vigorously star-forming galaxies—identified by their extreme [OIII] emission—dominate the photon budget for reionization, structure the earliest ionized bubbles in the IGM, and serve as the local analogues for studies of feedback, chemical enrichment, and AGN-galaxy co-evolution at cosmic dawn.