Clustering of Star‐forming Galaxies Near a Radio Galaxy atz = 5.2

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 Ly?-emitting companion galaxies and appears to lie within an overdense region. The radio galaxy is marginally resolved in i775 and z850, 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 Ly? emitters are sub-L* galaxies, with deduced star formation rates of 1-10 M? yr-1. One of the Ly? emitters is only detected in Ly?. Based on the star formation rate of ~3 M? yr-1 calculated from Ly?, the lack of continuum emission could be explained if the galaxy is younger than ~2 Myr and is producing its first stars. Observations in V606i775z850 were used to identify additional Lyman break galaxies associated with this structure. In addition to the radio galaxy, there are 22 V606 break (z ~ 5) galaxies with z850 99%), based on a counts-in-cells analysis applied to the control field. The excess suggests that the V606 break objects are associated with a forming cluster around the radio galaxy.