2023MNRAS.520L..16K

Recent attempts to detect [O III] 88$\, \mu$m emission from super-early (z > 10) galaxy candidates observed by JWST have been unsuccessful. Non-detections can be either due to wrong photometric redshifts or to the faintness of the line in such early systems. By using zoom-in simulations, we show that if redshifts of these galaxies are confirmed, they are faint and mostly fall below the local metal-poor $\rm [O\, {\small III}]-SFR$ relation as a result of their low ionization parameter, U_ion ≲ 10^–3. Such low U_ion values are found in galaxies that are in an early assembly stage, and whose stars are still embedded in high-density natal clouds. However, the most luminous galaxy in our sample ($\rm {log}[L_{\rm {[O\, {\small III}]}}/\mathrm{L}_\odot ] = 8.4$, U_ion ≈ 0.1) could be detected by ALMA in only 2.8 h.