Fine-structure Line Atlas for Multi-wavelength Extragalactic Study (FLAMES) III: C II as Tracer, Crisis of SFR, O III/C II at High-z, New Answers and New Questions

In the final paper of this series, we discuss new perspectives and challenges in the study of interstellar medium (ISM), leveraging comprehensive catalogs and physical insights presented in our previous papers. We focus on key questions of far-infrared (FIR) fine-structure lines (FSLs): their origins, diagnostic value, and implications of correlations. Our analysis reveals a strong dependence on elemental abundance, so that FSL/Hα traces metallicity, N II/C II traces N/O, and 80% of C II emission arises from neutral gas without systematic variations. We conclude a coherence exists between the emissions from ionized and neutral gases regarding energy sources and distribution. We argue that C II is physically a metallicity-dependent star formation rate (SFR) tracer, while its correlations with atomic or molecular gas masses are secondary. Crucially, the C II ``deficit'' is only part of a universal ``deficit'' problem that shows in all neutral and ionized gas lines including extinction-corrected Hα, caused by infrared (IR) luminosities and characterized by a dichotomy in gas and dust behaviors. This universal ``deficit'' marks a breakdown of the obscuration-corrected star-formation rate (SFR) calibration and imperils SFR estimates. We argue that it is caused by either IR ``excess'' or ionized gas ``deficit'', and present possible scenarios. A renewed picture of ISM structure is needed to reconcile with ionized--neutral gas coherence, metallicity dependence, and gas--dust dichotomy. We also discuss differences of FIR FSL at high redshifts: the offset in ``deficit'' trends, the similar O III/C II in metal-poor galaxies, and elevated O III/C II in dusty galaxies.