Interpreting ALMA non-detections of JWST super-early galaxies

Recent attempts to detect [O iii] 88μm 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 [OIII]−SFR relation as a result of their low ionization parameter, Uion ≲ 10−3. Such low Uion 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 (⁠log[L[OIII]/L⊙]=8.4⁠, Uion ≈ 0.1) could be detected by ALMA in only 2.8 h.