Constraining Star Formation and AGN in z~2 Massive Galaxies using High
  Resolution MERLIN Radio Observations
  We present high spatial resolution MERLIN 1.4GHz 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 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)~0.75", diameters ~12kpc) and SED
fitting to photometric points (optical through the mid-IR) reveals massive
(~5x10^(11) M_sun), old (a few Gyr) stellar populations. Both sources have low
flux densities at observed 24um and are undetected in observed 70um and 850um,
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~2.
