The ALPINE-ALMA [C II] Survey: [C II] 158 μm Emission Line Luminosity Functions at z ~ 4–6

Year: 2020 DOI: 10.3847/1538-4357/abc41c We present the [C II] 158 μm line luminosity functions (LFs) at z ~ 4–6 using the ALMA observations of 118 sources, which are selected to have UV luminosity M_(1500Å) < −20.2 and optical spectroscopic redshifts in COSMOS and ECDF-S. Of the 118 targets, 75 have significant [C II] detections and 43 are upper limits. This is by far the largest sample of [C II] detections, which allows us to set constraints on the volume density of [C II] emitters at z ~ 4–6. But because this is a UV-selected sample, we are missing [C II]-bright but UV-faint sources, making our constraints strict lower limits. Our derived LFs are statistically consistent with the z ~ 0 [C II] LF at 10^(8.25)–10^(9.75) L_⊙. We compare our results with the upper limits of the [C II] LF derived from serendipitous sources in the ALPINE maps. We also infer the [C II] LFs based on published far-IR and CO LFs at z ~ 4–6. Combining our robust lower limits with these additional estimates, we set further constraints on the true number density of [C II] emitters at z ~ 4–6. These additional LF estimates are largely above our LF at L_([CII]) > 10⁹ L_⊙, suggesting that UV-faint but [C II]-bright sources likely make significant contributions to the [C II] emitter volume density. When we include all the LF estimates, we find that available model predictions underestimate the number densities of [C II] emitters at z ~ 4–6. Finally, we set a constraint on the molecular gas mass density at z ~ 4–6, with ρ_(mol) ~ (2–7) × 10⁷ M_⊙ Mpc⁻³. This is broadly consistent with previous studies.