ALMA confirmation of an obscured hyperluminous radio-loud AGN at z = 6.853 associated with a dusty starburst in the 1.5 deg² COSMOS field

We present band 6 ALMA observations of a heavily obscured radio-loud (L_{1.4 GHz} = 10^25.4 W Hz⁻¹) active galactic nucleus (AGN) candidate at z_phot = 6.83 ± 0.06 found in the 1.5 deg² COSMOS field. The ALMA data reveal detections of exceptionally strong [C II]158 μm (z_[C II_] = 6.8532) and underlying dust continuum emission from this object (COS-87259), where the [C II] line luminosity, line width, and 158 μm continuum luminosity are comparable to those seen from z ∼ 7 sub-mm galaxies and quasar hosts. The 158 μm continuum detection suggests a total infrared luminosity of 9 × 10¹² L_☉ with corresponding very large obscured star formation rate (1300 M_☉ yr⁻¹) and dust mass (2 × 10⁹ M_☉). The strong break seen between the VIRCam and IRAC photometry perhaps suggests that COS-87259 is an extremely massive reionization-era galaxy with M_∗ ≈ 1.7 × 10¹¹ M_☉. Moreover, the MIPS, PACS, and SPIRE detections imply that this object harbours an AGN that is heavily obscured (τ_{9.7 μm} = 2.3) with a bolometric luminosity of approximately 5 × 10¹³ L_☉. Such a very high AGN luminosity suggests that this object is powered by an ≈1.6 × 10⁹ M_☉ black hole if accreting near the Eddington limit, and is effectively a highly obscured version of an extremely ultraviolet (UV)-luminous (M₁₄₅₀ ≈ −27.3) z ∼ 7 quasar. Notably, these z ∼ 7 quasars are an exceedingly rare population (∼0.001 deg⁻²), while COS-87259 was identified over a relatively small field. Future very wide area surveys with e.g. Roman and Euclid have the potential to identify many more extremely red yet UV-bright z ≳ 7 objects similar to COS-87259, providing richer insight into the occurrence of intense obscured star formation and supermassive black hole growth among this population.