Radio and Far-IR Emission Associated with a Massive Star-forming Galaxy Candidate at z$\simeq$6.8: A Radio-Loud AGN in the Reionization Era?

Abstract

We report the identification of radio (0.144-3 GHz), mid-IR, far-IR, and sub-mm (24-850$\mu$m) emission at the position of one of 41 UV-bright (M${}_{\mathrm{UV}}^{}\lesssim -21.25$) $z\simeq 6.6-6.9$ Lyman-break galaxy candidates in the 1.5 deg${}^2$ COSMOS field. This source, COS-87259, exhibits a sharp flux discontinuity (factor $>$3) between two narrow/intermediate bands at 9450 and 9700 Angstroms and is undetected in all nine bands blueward of 9600 Angstroms, as expected from a Lyman-alpha break at $z\simeq 6.8$. The full multi-wavelength (X-ray through radio) data of COS-87529 can be self-consistently explained by a very massive (M${}_{\ast }=10^{10.8}$ M${}_{\odot }$) and extremely red (rest-UV slope $\beta =-0.59$) $z\simeq 6.8$ galaxy with hyperluminous infrared emission (L${}_{\mathrm{IR}}=10^{13.6}$ L${}_{\odot }$) powered by both an intense burst of highly-obscured star formation (SFR$\approx$1800 M${}_{\odot }$ yr${}^{-1}$) and an obscured (${\tau }_{9.7\mu \mathrm{m}}=7.7\pm 2.5$) radio-loud (L${}_{1.4\mathrm{GHz}}\approx 10^{25.4}$ W Hz${}^{-1}$) AGN. The radio emission is compact (1.04$\pm$0.12 arcsec) and exhibits an ultra-steep spectrum between 1.32-3 GHz ($\alpha =-1.57_{-0.21}^{+0.22}$) that flattens at lower frequencies ($\alpha =-0.86_{-0.16}^{+0.22}$ between 0.144-1.32 GHz), consistent with known $z>4$ radio galaxies. We also demonstrate that COS-87259 may reside in a significant (11$\times$) galaxy overdensity at $z\simeq 6.6-6.9$, as common for systems hosting radio-loud AGN. Nonetheless, a spectroscopic redshift will ultimately be required to establish the true nature of COS-87259 as we cannot yet completely rule out low-redshift solutions. If confirmed to lie at $z\simeq 6.8$, the properties of COS-87259 would be consistent with a picture wherein AGN and highly-obscured star formation activity are fairly common among very massive (M${}_{\ast }>10^{10}$ M${}_{\odot }$) reionization-era galaxies.

14 pages, 6 figures. Accepted in MNRAS with minor revisions. This accepted version considers very recent data from LOFAR and MeerKAT which improve our analyses on the radio slope and luminosity

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