Characterising the contribution of dust-obscured star formation at ≳ 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 |$z \gtrsim 5$| that includes the contribution of dust-obscured star formation. For this purpose, we use a [C  ii ] (158 |$\mu$| m) selected sample of galaxies serendipitously identified in the fields of known |$z\gtrsim 4.5$| 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 |$\gtrsim 30\ \text{M}_{\odot } \ \text{yr}^{-1}$|⁠. We find that [C  ii ] emitters identified around SF galaxies have |$\approx$| 63 per cent of their SFR obscured, while [C  ii ] emitters around QSOs have |$\approx$| 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 |$M_{\mathrm{ UV}}$| = |$-$| 20, we find that the obscured SFRD contributes to |$\gt 3~{{\ \rm per\ cent}}$| and |$\gt 10~{{\ \rm per\ cent}}$| of the total SFRD at |$z \sim 5$| and |$z \sim 6$| based on our sample of companions galaxies near SF galaxies and QSOs, respectively. Our results suggest that dust obscuration is not negligible at |$z\gtrsim 5$|⁠ , further underlining the importance of far-infrared observations of the |$z\gtrsim 5$| Universe. [ABSTRACT FROM AUTHOR]