Characterizing the contribution of dust-obscured star formation at z ≳ 5 using 18 serendipitously identified [C II] emitters

We present a new method to determine the star formation rate (SFR) density of the Universe at z ≳ 5 that includes the contribution of dust-obscured star formation. For this purpose, we use a [C II] (158 μm) selected sample of galaxies serendipitously identified in the fields of known z ≳ 4.5 objects to characterize the fraction of obscured SFR. The advantage of a [C II] selection is that our sample is SFR-selected, in contrast to a UV-selection that would be biased towards unobscured star formation. We obtain a sample of 23 [C II] emitters near star-forming (SF) galaxies and QSOs - three of which we identify for the first time - using previous literature and archival ALMA data. 18 of these serendipitously identified galaxies have sufficiently deep rest-UV data and are used to characterize the obscured fraction of the star formation in galaxies with SFRs ≳ 30 M⊙ yr-1. We find that [C II] emitters identified around SF galaxies have ≈63% of their SFR obscured, while [C II] emitters around QSOs have ≈93% of their SFR obscured. By forward modeling existing wide-area UV luminosity function (LF) determinations, we derive the intrinsic UV LF using our characterization of the obscured SFR. Integrating the intrinsic LF to MUV = -20 we find that the obscured SFRD contributes to >3 % and >10 % of the total SFRD at z ~ 5 and z ~ 6 based on our sample of companions galaxies near SFGs and QSOs, respectively. Our results suggest that dust obscuration is not negligible at z ≳ 5, further underlining the importance of far-IR observations of the z ≳ 5 Universe.