S22A0003

We propose deep Keck/MOSFIRE spectroscopy for a strongly lensed star-forming main-sequence galaxy (SFG) at z=6.072, RXCJ0600-z6, identified by the 100-hr ALMA Lensing Cluster Survey (ALCS). Thanks to the strong lensing effect (µ ≃ 20–30), RXCJ0600-z6 is apparently the brightest normal SFG identified to date at z>∼6 but intrinsically a sub-L^∗ galaxy with M_star ∼ 10⁹ M_⊙. We will observe the rest UV emission lines of Civ1549, Oiii]1665, and Ciii]1909 to investigate whether the ionization parameter and the C/O ratio can explain the high [Oiii]88µm/[Cii]158µm ratios in high-z SFGs revealed by recent ALMA observations. By combining the results of our already scheduled ∼70-hr deep JWST and ALMA follow-up observations for RXCJ0600-z6, we will thoroughly examine the proposed physical origins of the high [Oiii]/[Cii] ratios by simultaneously determining the ISM properties of the sub-L^∗ high-z SFG for the first time, which will enable an essential step in providing a representative view of early galaxy evolution at around the end of cosmic reionization.

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