Characterizing the Contribution of Dust-Obscured Star Formation at Z ≳ 5 Using 18 Serendipitously Identified [CII] 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 [CII] (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 [CII] 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 [CII] 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 [CII] emitters identified around SF galaxies have ≈63% of their SFR obscured, while [CII] 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 M_UV = -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.