Radio and Far-IR Emission Associated with a Massive Star-forming Galaxy
  Candidate at z$\simeq$6.8: A Radio-Loud AGN in the Reionization Era?
  We report the identification of radio (0.144-3 GHz), mid-IR, far-IR, and
sub-mm (24-850$\mu$m) emission at the position of one of 41 UV-bright
(M$_\mathrm{UV}^{}\lesssim-21.25$) $z\simeq6.6-6.9$ 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 $z\simeq6.8$. 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 $\beta=-0.59$) $z\simeq6.8$
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 ($\tau_{\mathrm{9.7\mu
m}}=7.7\pm2.5$) radio-loud (L$_{\mathrm{1.4\ GHz}}\approx10^{25.4}$ 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^{+0.22}_{-0.21}$) that flattens at lower frequencies
($\alpha=-0.86^{+0.22}_{-0.16}$ 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 $z\simeq6.6-6.9$, 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
$z\simeq6.8$, 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.
