Interferometric imaging of the high-redshift radio galaxy, 4C 60.07: An SMA, Spitzer and VLA study reveals a binary AGN/starburst

High-resolution submillimetre (submm) imaging of the high-redshift radio galaxy (HzRG), 4C 60.07, at z = 3.8, has revealed two dusty components of roughly equal integrated flux. Spitzer imaging shows that one of these components (‘B’) is coincident with an extremely red active galactic nucleus (AGN), offset by ∼4 arcsec (∼30 kpc) from the HzRG core. The other submm component (‘A’) – resolved by our synthesised beam and devoid of emission at 3.6–8.0-μm – lies between ‘B’ and the HzRG core. Since the radio galaxy was discovered via its extremely young, steep-spectrum radio lobes and the creation of these lobes was likely triggered by the interaction, we argue that we are witnessing an early-stage merger, prior to its eventual equilibrium state. The interaction is between the host galaxy of an actively-fueled black hole (BH), and a gas-rich starburst/AGN (‘B’) marked by the compact submm component and coincident with broad CO(4–3) emission. The second submm component (‘A’) is a plume of cold, dusty gas, associated with a narrow (∼150 km s) CO feature, and may represent a short-lived tidal structure. It has been claimed that HzRGs and submm-selected galaxies (SMGs) differ only in the activity of their AGNs, but such complex submm morphologies are seen only rarely amongst SMGs, which are usually older, more relaxed systems. Our study has important implications: where a galaxy’s gas reservoir is not aligned with its central BH, CO may be an unreliable probe of dynamical mass, affecting work on the co-assembly of BHs and host spheroids. Our data support the picture wherein close binary AGN are induced by mergers. They also raise the possibility that some supposedly jet-induced starbursts may have formed co-evally (yet independently of) the radio jets, both triggered by the same interaction. Finally, we note that the HzRG host would have gone unnoticed without its jets and its companion, so there may be many other unseen BHs at high redshift, lost in the sea of ∼ 5 × 10 similarly bright IRAC sources – sufficiently massive to drive a >10-W radio source, yet practically invisible unless actively fueled.