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 $\mu$~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~$10^5{M}_{\odot }$), 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 H$\alpha$ emission (EW${}_{\mathrm{rest}}\simeq 880$ Å, $\log {\xi }_{\mathrm{ion}}\simeq 25.3$), a blue UV slope (${\beta }_{\mathrm{UV}}\simeq -2.5$, down to $\simeq -3.1$ in the core), and a Balmer decrement consistent with no dust attenuation indicate a young ($\lesssim 10$ Myr), dust-free burst. [OIII]$\lambda$5007 is not detected in the integrated spectrum, implying [OIII]/H$\beta \le 0.61$ ($3\sigma$) and, using an extrapolated strong-line calibration, $12+\log (O/H)\le 6.4$ ($\lesssim 0.5\%Z_{\odot }$). This places LAP2 among the most metal-poor galaxies known. Faint [OIII] emission is, however, detected in the core (with $4\sigma$ significance), where we derive $12+\log (O/H)\simeq 6.6$ ($\sim 1\%Z_{\odot }$). This difference can be explained either by local self-enrichment, requiring only $\sim 0.2{\mathrm{M}}_{\odot }$ of newly synthesized oxygen, or by a higher ionization parameter in the core. The upper limit on HeII$\lambda$1640 (EW${}_{\mathrm{rest}}<39$ Å, $3\sigma$) does not exclude a contribution from metal-free stars. LAP2 extends the census of spectroscopically confirmed EMPGs to star-cluster masses and to $M_{\mathrm{UV}}\approx -13$. Its location in a low-density region supports a scenario in which underdense environments preserve nearly pristine gas well after reionization.

arXiv comment12 pages (7 figures, 1 table). Submitted to A&A