Abstract
We present HST ACS observations of the most distant radio galaxy known, TN J0924-2201 at z = 5.2. This radio galaxy has six spectroscopically confirmed Lyα-emitting companion galaxies and appears to lie within an overdense region. The radio galaxy is marginally resolved in i <sub>775</sub> and z <sub>850</sub> , 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 atz 4-5. The Lyα emitters are sub-L* galaxies, with deduced star formation rates of 1-10 M⊙ yr <sup>-1</sup> . One of the Lyα emitters is only detected in Lyα. Based on the star formation rate of ∼3 M⊙ yr <sup>-1</sup> calculated from Lyα, 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 <sub>606</sub> i <sub>775z850</sub> were used to identify additional Lyman break galaxies associated with this structure. In addition to the radio galaxy, there are 22 V <sub>606</sub> break (z ∼ 5) galaxies with z <sub>850</sub> < 26.5 (5 σ), two of which are also in the spectroscopic sample. We compare the surface density of ∼2 arcmin <sup>-2</sup> to that of similarly selected V <sub>606</sub> 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 <sub>606</sub> break objects are associated with a forming cluster around the radio galaxy. © 2006. The American Astronomical Society. All rights reserved.