2009MNRAS.395.1249C

We present high spatial resolution Multi-Element Radio-Linked Interferometer Network (MERLIN) 1.4-GHz radio observations of two high-redshift (z ∼ 2) sources, RGJ123623 (HDF147) and RGJ123617 (HDF130), selected as the brightest radio sources from a sample of submillimetre-faint radio galaxies. They have starburst classifications from their rest-frame ultraviolet spectra. However, their radio morphologies are remarkably compact (<80 and <65mas, respectively), demanding that the radio luminosity be dominated by active galactic nuclei (AGN) rather than starbursts. Near-infrared (IR) imaging [Hubble Space Telescope Near Infrared Camera and Multi-Object Spectrometer (NICMOS) F160W] shows large-scale sizes (R_1/2∼ 0.75arcsec, diameters ∼12kpc) and spectral energy distribution (SED) fitting to photometric points (optical through the mid-IR) reveals massive (∼5x10¹¹M_☉), old (a few Gyr) stellar populations. Both sources have low flux densities at observed 24 µm and are undetected in observed 70 µm and 850 µm, suggesting a low mass of interstellar dust. They are also formally undetected in the ultradeep 2 Ms Chandra data, suggesting that any AGN activity is likely intrinsically weak. We suggest both galaxies have evolved stellar populations, low star formation rates and low accretion rates on to massive black holes (10^8.6M_☉) whose radio luminosity is weakly beamed (by factors of a few). A cluster-like environment has been identified near HDF130 by an overdensity of galaxies at z = 1.99, reinforcing the claim that clusters lead to more rapid evolution in galaxy populations. These observations suggest that high-resolution radio (MERLIN) can be a superb diagnostic tool of AGN in the diverse galaxy populations at z ∼ 2.