  We present HST/ACS observations of the most distant radio galaxy known, TN
J0924-2201 at z=5.2. This radio galaxy has 6 spectroscopically confirmed Lya
emitting companion galaxies, and appears to lie within an overdense region. The
radio galaxy is marginally resolved in i_775 and z_850 showing continuum
emission aligned with the radio axis, similar to what is observed for lower
redshift radio galaxies. Both the half-light radius and the UV star formation
rate are comparable to the typical values found for Lyman break galaxies at
z~4-5. The Lya emitters are sub-L* galaxies, with deduced star formation rates
of 1-10 Msun/yr. One of the Lya emitters is only detected in Lya. Based on the
star formation rate of ~3 Msun/yr calculated from Lya, the lack of continuum
emission could be explained if the galaxy is younger than ~2 Myr and is
producing its first stars.
  Observations in V_606, i_775, and z_850 were used to identify additional
Lyman break galaxies associated with this structure. In addition to the radio
galaxy, there are 22 V-break (z~5) galaxies with z_850<26.5 (5sigma), two of
which are also in the spectroscopic sample. We compare the surface density of
2/arcmin^2 to that of similarly selected V-dropouts extracted from GOODS and
the UDF Parallel fields. We find evidence for an overdensity to very high
confidence (>99%), based on a counts-in-cells analysis applied to the control
field. The excess is suggestive of the V-break objects being associated with a
forming cluster around the radio galaxy.
