A pocket of oxygen in a near-pristine galaxy at z=4.2: chemical inhomogeneity at 10-parsec scales in LAP2

Abstract

We aim to constrain the oxygen abundance, the ionizing properties, and the spatial structure of LAP2, a strongly lensed (with a magnification μ~50) oxygen-deficient galaxy at z=4.2 behind the galaxy cluster Abell 2744. We present new JWST/NIRSpec IFU PRISM observations (~13.6 hr) of LAP2, which we combine with archival JWST/NIRCam imaging. LAP2 is intrinsically extremely faint (M_UV=-12.7) and has a low stellar mass (M~10⁵ M_⊙), comparable to that of massive star clusters. It consists of a compact core (R_eff~7 pc) embedded in a more diffuse emission (with R_eff~20 pc). Strong Hα emission (EW_rest≃880 Å, logξ_ion≃25.3), a blue UV slope (β_UV≃−2.5, down to ≃−3.1 in the core), and a Balmer decrement consistent with no dust attenuation indicate a young (≲10 Myr), dust-free burst. [OIII]λ5007 is not detected in the integrated spectrum, implying [OIII]/Hβ≤0.61 (3σ) and, using an extrapolated strong-line calibration, 12+log(O/H)≤6.4 (≲0.5% Z_⊙). This places LAP2 among the most metal-poor galaxies known. Faint [OIII] emission is, however, detected in the core (with 4σ significance), where we derive 12+log(O/H)≃6.6 (∼1% Z_⊙). This difference can be explained either by local self-enrichment, requiring only ∼0.2 M_⊙ of newly synthesized oxygen, or by a higher ionization parameter in the core. The upper limit on HeIIλ1640 (EW_rest<39 Å, 3σ) does not exclude a contribution from metal-free stars. LAP2 extends the census of spectroscopically confirmed EMPGs to star-cluster masses and to M_UV≈−13. Its location in a low-density region supports a scenario in which underdense environments preserve nearly pristine gas well after reionization.

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Cite

BibTeX RIS

BibTeX

@misc{messa2026pocket,
  title = {{A pocket of oxygen in a near-pristine galaxy at z=4.2: chemical inhomogeneity at 10-parsec scales in LAP2}},
  author = {M. Messa and E. Vanzella and S. Rojas-Ruiz and T. Treu and P. Bergamini and G. Di Rosa and M. Lombardi and P. Rosati and A. Zanella and A. Adamo and A. Bolamperti and F. Calura and M. Castellano and R. Gilli and C. Grillo and U. Meštrić},
  howpublished = {arXiv},
  year = {2026},
  month = nov,
  doi = {10.48550/arxiv.2611.05487},
  eprint = {2611.05487},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2611.05487},
}

RIS

TY  - UNPB
TI  - A pocket of oxygen in a near-pristine galaxy at z=4.2: chemical inhomogeneity at 10-parsec scales in LAP2
AU  - M. Messa
AU  - E. Vanzella
AU  - S. Rojas-Ruiz
AU  - T. Treu
AU  - P. Bergamini
AU  - G. Di Rosa
AU  - M. Lombardi
AU  - P. Rosati
AU  - A. Zanella
AU  - A. Adamo
AU  - A. Bolamperti
AU  - F. Calura
AU  - M. Castellano
AU  - R. Gilli
AU  - C. Grillo
AU  - U. Meštrić
PY  - 2026
DA  - 2026/11/03
PB  - arXiv
DO  - 10.48550/arxiv.2611.05487
UR  - https://arxiv.org/abs/2611.05487
AB  - We aim to constrain the oxygen abundance, the ionizing properties, and the spatial structure of LAP2, a strongly lensed (with a magnification μ~50) oxygen-deficient galaxy at z=4.2 behind the galaxy cluster Abell 2744. We present new JWST/NIRSpec IFU PRISM observations (~13.6 hr) of LAP2, which we combine with archival JWST/NIRCam imaging. LAP2 is intrinsically extremely faint (M_UV=-12.7) and has a low stellar mass (M~10^5 M_⊙), comparable to that of massive star clusters. It consists of a compact core (R_eff~7 pc) embedded in a more diffuse emission (with R_eff~20 pc). Strong Hα emission (EW_rest≃880 Å, logξ_ion≃25.3), a blue UV slope (β_UV≃−2.5, down to ≃−3.1 in the core), and a Balmer decrement consistent with no dust attenuation indicate a young (≲10 Myr), dust-free burst. [OIII]λ5007 is not detected in the integrated spectrum, implying [OIII]/Hβ≤0.61 (3σ) and, using an extrapolated strong-line calibration, 12+log(O/H)≤6.4 (≲0.5% Z_⊙). This places LAP2 among the most metal-poor galaxies known. Faint [OIII] emission is, however, detected in the core (with 4σ significance), where we derive 12+log(O/H)≃6.6 (∼1% Z_⊙). This difference can be explained either by local self-enrichment, requiring only ∼0.2 M_⊙ of newly synthesized oxygen, or by a higher ionization parameter in the core. The upper limit on HeIIλ1640 (EW_rest<39 Å, 3σ) does not exclude a contribution from metal-free stars. LAP2 extends the census of spectroscopically confirmed EMPGs to star-cluster masses and to M_UV≈−13. Its location in a low-density region supports a scenario in which underdense environments preserve nearly pristine gas well after reionization.
ER  -

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