LATED: Ly$α$-anchored photometric selection of candidate metal-free and extremely metal-poor star formation from the end of reionisation to cosmic noon

Mingyu Li, Zheng Cai, Roberto Maiolino, Fuyan Bian, Sijia Cai, Francesco D'Eugenio, Qiao Duan, Eiichi Egami, Xiaohui Fan, Yuki Isobe, Xihan Ji, Gareth C. Jones, Maria Koller, Xiaojing Lin, Boyuan Liu, Christopher C. Lovell, Kimihiko Nakajima, Masami Ouchi, Robert G. Pascalau, Zijin Su, Jan Scholtz, Fengwu Sun, Sandro Tacchella, Hannah Übler, Yunjing Wu, Fujiang Yu, Zijian Zhang

First listed 2026-09-22 | Last updated 2026-09-21

Abstract

Cosmological simulations allow a low-level tail of Population III (Pop III) star formation to persist to $z=2-6$. Spectroscopic confirmation is expensive, so an efficient photometric pre-selection is needed. We present LATED (Lyman-Alpha Tomography of Extremely Metal-poor Domains), which selects metal-free and extremely metal-poor candidates from a Ly$α$-emitter parent sample using strong-line diagnostics. The method requires three bands and two colours, $x=m_{\rm OIII}-m_{{\rm H}α}$ and $y=m_{{\rm H}α}-m_{\rm cont}$, which trace oxygen abundance and the H$α$ equivalent width, respectively. Requiring that the filters simultaneously contain [O III]+H$β$ and H$α$, together with a Ly$α$ parent selection, defines five windows spanning $z=1.92-6.60$ (four JWST/NIRCam, one Roman/WFI). The criteria, $x\geq x_{\rm min}(z)$ and $y\leq y_{\rm max}(z)$, are set by the per-redshift extrema of a forward-modelled Pop III template locus. Ordinary metal-enriched star-forming and AGN templates fall outside the selection region, and possible contaminants such as little red dots are flagged by their multi-band colours. To check contamination empirically, we apply LATED to 1126 JADES spectroscopic galaxies, and no source is selected in the four NIRCam windows. We release a Python package which converts the same photometry into R3=[O III]/H$β$ as a measurement or upper limit, reproducing JADES spectroscopy with small 0.12 dex scatter. Applied to 85 archival MUSE Ly$α$ emitters in Abell 2744, LATED recovers the confirmed extremely metal-poor galaxy AMORE6 and reveals three new candidates at $z=3-5$. Photometry alone cannot establish a metal-free nature. LATED delivers prioritised candidates for spectroscopic follow-up, providing a scalable route toward a systematic census of late-time Pop III star formation.

Short digest

Li et al. introduce LATED, a Lyα-emitter–anchored three-band selection that uses the contrast between [O III]+Hβ, Hα, and nearby continuum emission to isolate metal-free and extremely metal-poor star-formation candidates across five windows at z=1.92–6.60. The method rejects ordinary enriched star-forming and AGN templates, flags little red dots through additional colours, selects none of 1,126 JADES spectroscopic galaxies in the NIRCam windows, and recovers the confirmed extremely metal-poor Abell 2744 source AMORE6 while identifying three new z=3–5 candidates. Its accompanying photometric line-inference package recovers spectroscopic R3=[O III]/Hβ with 0.12 dex scatter, making LATED a scalable way to prioritize targets for the spectroscopy still required to establish genuinely metal-free star formation.

Key figures to inspect

• Figure 1. This workflow figure succinctly shows the full LATED logic: a tomographic Lyα-emitter parent sample, redshift-window construction, the EW(Hα)–oxygen-abundance colour plane, little-red-dot contaminant flags, and photometric line-ratio inference feeding spectroscopic follow-up.
• Figure 5. The W 2.1 selection plane demonstrates the core physical discriminator in practice: strong Hα combined with weak [O III]+Hβ places Pop III-like templates in a distinct colour corner, while the mock catalogue shows how photometric noise affects contamination and recovery.
• Figure 10. This figure supplies the redshift-dependent x and y boundaries that operationalize LATED across all five windows, clarifying that the selection follows the extrema of the forward-modelled Pop III locus rather than imposing a single fixed colour cut.
• Figure 15. The JADES validation directly supports the empirical cleanliness claim: 1,126 spectroscopically confirmed galaxies populate the four NIRCam colour planes but none satisfy the appropriate redshift-dependent LATED criteria.
• Figure 20. This validation figure tests the released three-band inference machinery against simulations and NIRSpec measurements, showing how photometric [O III]/Hβ estimates and upper limits connect the candidate selection to a quantitative metallicity-sensitive diagnostic.