We present high spatial resolution MERLIN 1.4GHz radio observations of two high redshift ( z \sim 2 ) sources , RG J123623 ( HDF147 ) and RG J123617 ( HDF130 ) , selected as the brightest radio sources from a sample of submillimetre-faint radio galaxies .
They have starburst classifications from their rest-frame UV spectra .
However , their radio morphologies are remarkably compact ( < 80mas and < 65mas respectively ) , demanding that the radio luminosity be dominated by Active Galactic Nuclei ( AGN ) rather than starbursts .
Near-IR imaging ( HST NICMOS F160W ) shows large scale sizes ( R _ { 1 / 2 } \sim 0.75″ , diameters \sim 12kpc ) and SED fitting to photometric points ( optical through the mid-IR ) reveals massive ( \sim 5 \times 10 ^ { 11 } M _ { \sun } ) , old ( a few Gyr ) stellar populations .
Both sources have low flux densities at observed 24 \mu mand are undetected in observed 70 \mu m and 850 \mu m , suggesting a low mass of interstellar dust .
They are also formally undetected in the ultra-deep 2Ms 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 onto massive black holes ( 10 ^ { 8.6 } M _ { \sun } ) whose radio luminosity is weakly beamed ( by factors of a few ) .
A cluster-like environment has been identified near HDF130 by an over-density 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 \sim 2 .