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

Abstract

We present band 6 ALMA observations of a heavily obscured radio-loud (L1.4 GHz = 1025.4 W Hz−1) active galactic nucleus (AGN) candidate at zphot = 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 (τ9.7 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. © 2023 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.