ALMA Confirmation of an Obscured Hyperluminous Radio-Loud AGN at $z=6.853$ Associated with a Dusty Starburst in the 1.5 deg${}^2$ COSMOS Field

Abstract

We present band 6 ALMA observations of a heavily-obscured radio-loud ($L_{1.4\mathrm{GHz}}=10^{25.4}$ W Hz${}^{-1}$) AGN candidate at $z_{\mathrm{phot}}=6.83\pm 0.06$ found in the 1.5 deg${}^2$ COSMOS field. The ALMA data reveal detections of exceptionally strong [CII]158$\mu$m ($z_{[\mathrm{CII}]}=6.8532$) and underlying dust continuum emission from this object (COS-87259), where the [CII] line luminosity, line width, and 158$\mu$m continuum luminosity are comparable to that seen from $z\sim 7$ sub-mm galaxies and quasar hosts. The 158$\mu$m continuum detection suggests a total infrared luminosity of $9\times 10^{12}$ $L_{\odot }$ with corresponding very large obscured star formation rate (1300 $M_{\odot }$/yr) and dust mass ($2\times 10^9$ $M_{\odot }$). The strong break seen between the VIRCam and IRAC photometry perhaps suggests that COS-87259 is an extremely massive reionization era galaxy with $M_{\ast }\approx 1.7\times 10^{11}$ $M_{\odot }$. Moreover, the MIPS, PACS, and SPIRE detections imply that this object harbors an AGN that is heavily obscured (${\tau }_{{}_{9.7\mu \mathrm{m}}}=2.3$) with a bolometric luminosity of approximately $5\times 10^{13}$ $L_{\odot }$. Such a very high AGN luminosity suggests this object is powered by an $\approx$1.6 $\times$ 10${}^9$ $M_{\odot }$ black hole if accreting near the Eddington limit, and is effectively a highly-obscured version of an extremely UV-luminous ($M_{1450}\approx -27.3$) $z\sim 7$ quasar. Notably, these $z\sim 7$ quasars are an exceedingly rare population ($\sim$0.001 deg${}^{-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 $z\gtrsim 7$ objects similar to COS-87259, providing richer insight into the occurrence of intense obscured star formation and supermassive black hole growth among this population.

12 pages, 7 figures, 1 table. Updated to accepted version (MNRAS)

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