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 density (SFRD) 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 characterise the fraction of obscured star formation rate (SFR). The advantage of a [C ii] selection is that our sample is SFR-selected, in contrast to an ultraviolet (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 quasi-stellar objects (QSOs) – three of which we identify for the first time – using previous literature and archival Atacama Large Millimeter/submillimeter Array data. 18 of these serendipitously identified galaxies have sufficiently deep rest-UV data and are used to characterise 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 per cent of their SFR obscured, while [C ii] emitters around QSOs have ≈93 per cent of their SFR obscured. By forward modelling existing wide-area UV luminosity function (LF) determinations, we derive the intrinsic UV LF using our characterisation of the obscured SFR. Integrating the intrinsic LF to MUV = $-$20, we find that the obscured SFRD contributes to > 3~\ per\ cent and > 10~\ per\ cent of the total SFRD at z ~ 5 and z ~ 6 based on our sample of companions galaxies near SF galaxies and QSOs, respectively. Our results suggest that dust obscuration is not negligible at z 5, further underlining the importance of far-infrared observations of the z 5 Universe.