HST/ACS observations of Lyman-break galaxies and Lyα emitters associated with radio galaxies at z>4

Distant radio galaxies may pinpoint overdense regions in the early universe. We have collected data with HST/ACS towards several overdensities of Ly alpha emitters associated with radio galaxies discovered by Venemans et al. Using the Lyman break selection technique we find statistical evidence for additional galaxies associated with the radio galaxies TN J1338-1942 at z = 4.1 and TN J0924-2201 at z = 5.2. In the case of TN J1338-1942, the angular distribution of candidate Lyman break galaxies is highly filamentary across the similar to 12 arcmin(2) field, with more than half of the objects clustered in a 4.4 arcmin(2) region that includes the radio galaxy. Both fields appear to be significantly richer in Lyman break galaxies than the Great Observatories Origins Deep Survey (GOODS) fields, suggesting that the radio galaxies are embedded in galaxy groups or (forming) clusters. The Lyman break galaxies have mild to moderate star formation rates and relatively blue UV continuum colours. Except for their high equivalent width Lya, the properties of spectroscopically confirmed Ly alpha emitters associated with these radio galaxies are consistent with those of normal Lyman break galaxies at relatively low luminosities. The two radio galaxies have some intriguing properties: TN J1338-1942 is extremely bright in the rest-frame UV, and has a highly disturbed morphology presumed to arise from interactions between the jet and the surrounding medium, and a starburst-driven superwind. The UV star formation rate and (projected) size of TN J0924-2201 are typical of relatively faint Lyman break galaxies at z similar to 3-5. Yet it is a luminous, radio-loud AGN, suggesting the presence of a supermassive black hole that may have acquired its mass before the host galaxy produced the bulk of its stars.