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 $\mu$m (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 $\mu$m continuum luminosity are comparable to those seen from z ~ 7 sub-mm galaxies and quasar hosts. The 158 $\mu$m continuum detection suggests a total infrared luminosity of $9\times 10^{12}\, \mathrm{ L}_\odot$ with corresponding very large obscured star formation rate (1300 M_⊙ yr^-1) and dust mass ($2\times 10^9\, \mathrm{ M}_\odot$). The strong break seen between the VIRCam and IRAC photometry perhaps suggests that COS-87259 is an extremely massive reionization-era galaxy with $M_\ast \approx 1.7\times 10^{11}\, \mathrm{ M}_\odot$. Moreover, the MIPS, PACS, and SPIRE detections imply that this object harbours an AGN that is heavily obscured ($\tau _{_{\mathrm{9.7\,\mu m}}}=2.3$) with a bolometric luminosity of approximately $5\times 10^{13}\, \mathrm{ L}_\odot$. Such a very high AGN luminosity suggests that this object is powered by an ≈1.6 × 10$^9\, \mathrm{ M}_\odot$ 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.