Clustering of star-forming galaxies near a radio galaxy at z=5.2

We present HSTACS observations of the most distant radio galaxy known, TNJ0924-2201 at z = 5.2. This radio galaxy has six spectroscopically confirmed Ly alpha-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 similar to 4-5. The Ly alpha emitters are sub-L-* galaxies, with deduced star formation rates of 1-10 M-circle dot yr(-1). One of the Ly alpha emitters is only detected in Ly alpha. Based on the star formation rate of similar to 3 M-circle dot yr(-1) calculated from Ly alpha, the lack of continuum emission could be explained if the galaxy is younger than similar to 2 Myr and is producing its first stars. Observations in V(606)i(775)z(850) were used to identify additional Lyman break galaxies associated with this structure. In addition to the radio galaxy, there are 22 V-606 break (z similar to 5) galaxies with z(850) < 26.5 (5 sigma), two of which are also in the spectroscopic sample. We compare the surface density of similar to 2 arcmin(-2) to that of similarly selected V-606 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 suggests that the V-606 break objects are associated with a forming cluster around the radio galaxy.