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 Lya-emitting companion galaxies and appears to ...lie within an overdense region. The radio galaxy is marginally resolved in i sub(775) and z sub(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 6 4-5. The Lya emitters are sub-L sub(*) galaxies, with deduced star formation rates of 1-10 M sub( )yr super(-1). One of the Lya emitters is only detected in Lya. Based on the star formation rate of 63 M sub( )yr super(-1) calculated from Lya, the lack of continuum emission could be explained if the galaxy is younger than 62 Myr and is producing its first stars. Observations in V sub(606)i sub(775)z sub(850) were used to identify additional Lyman break galaxies associated with this structure. In addition to the radio galaxy, there are 22 V sub(606) break (z 6 5) galaxies with z sub(850) < 26.5 (5 s), two of which are also in the spectroscopic sample. We compare the surface density of 62 arcmin super(-2) to that of similarly selected V sub(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 sub(606) break objects are associated with a forming cluster around the radio galaxy.
We present deep optical imaging of the z = 4.1 radio galaxy TN J1338-1942, obtained using the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope, as well as ground-based ...near-infrared imaging data from the European Southern Observatory (ESO) Very Large Telescope (VLT). The radio galaxy is known to reside within a large galaxy overdensity (both in physical extent and density contrast). There is good evidence that this "protocluster" region is the progenitor of a present-day rich galaxy cluster. TN J1338 is the dominant galaxy in the protocluster in terms of size and luminosity (in both the optical and near-infrared) and therefore seems destined to evolve into the brightest cluster galaxy. The high spatial resolution ACS images reveal several kiloparsec-scale features within and around the radio galaxy. The continuum light is aligned with the radio axis and is resolved into two clumps in the i sub(775) and z sub(850) bands. These components have luminosities 610 super(9) L sub(z) and sizes of a few kpc. The estimated nebular continuum, scattered light, synchrotron- and inverse Compton-scattering contributions to the aligned continuum light are only a few percent of the observed total, indicating that the observed flux is likely dominated by forming stars. The estimated star formation rate for the whole radio galaxy is 6200 M sub(z) yr super(-1). A simple model in which the jet has triggered star formation in these continuum knots is consistent with the available data. A striking, but small, linear feature is evident in the z sub(850) aligned light and may be indicative of a large-scale shock associated with the advance of the radio jet. The rest of the aligned light also seems morphologically consistent with star formation induced by shocks associated with the radio source, as seen in other high-z radio galaxies (e.g., 4C 41.17). An unusual feature is seen in Ly a emission. A wedge-shaped extension emanates from the radio galaxy perpendicularly to the radio axis. This "wedge" naturally connects to the surrounding asymmetric, large-scale (6100 kpc) Lya halo. We posit that the wedge is a starburst-driven superwind associated with the first major epoch of formation of the brightest cluster galaxy. The shock and wedge are examples of feedback processes due to both active galactic nucleus and star formation in the earliest stages of massive galaxy formation.
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