Searching for obscured AGN in z $\sim$ 2 submillimetre galaxies
  Submillimetre-selected galaxies (SMGs) at high redshift ($z$ $\sim$ 2) are
potential host galaxies of active galactic nuclei (AGN). If the local Universe
is a good guide, $\sim$ 50$\%$ of the obscured AGN amongst the SMG population
could be missed even in the deepest X-ray surveys. Radio observations are
insensitive to obscuration; therefore, very long baseline interferometry (VLBI)
can be used as a tool to identify AGN in obscured systems. A well-established
upper limit to the brightness temperature of 10$^5$ K exists in star-forming
systems, thus VLBI observations can distinguish AGN from star-forming systems
via brightness temperature measurements. We present 1.6 GHz European VLBI
Network (EVN) observations of four SMGs (with measured redshifts) to search for
evidence of compact radio components associated with AGN cores. For two of the
sources, e-MERLIN images are also presented. Out of the four SMGs observed, we
detect one source, J123555.14, that has an integrated EVN flux density of 201
$\pm$ 15.2 $\mu$Jy, corresponding to a brightness temperature of 5.2 $\pm$ 0.7
$\times$ 10$^5$ K. We therefore identify that the radio emission from
J123555.14 is associated with an AGN. We do not detect compact radio emission
from a possible AGN in the remaining sources (J123600.10, J131225.73, and
J163650.43). In the case of J131225.73, this is particularly surprising, and
the data suggest that this may be an extended, jet-dominated AGN that is
resolved by VLBI. Since the morphology of the faint radio source population is
still largely unknown at these scales, it is possible that with a $\sim$ 10 mas
resolution, VLBI misses (or resolves) many radio AGN extended on kiloparsec
scales.
