  Venemans et al. (2005) found evidence for an overdensity of Ly-alpha emission
line galaxies associated with the radio galaxy MRC 0316-257 at z=3.13
indicating the presence of a massive protocluster. Here, we present the results
of a search for additional star-forming galaxies and AGN within the
protocluster. Narrow-band infrared imaging was used to select candidate [O III]
emitters in a 1.1 x 1.1 Mpc^2 region around the radio galaxy. Thirteen
candidates have been detected. Four of these are among the previously confirmed
sample of Ly-alpha galaxies, and an additional three have been confirmed
through follow-up infrared spectroscopy. The three newly confirmed objects lie
within a few hundred km/s of each other, but are blueshifted with respect to
the radio galaxy and Ly-alpha emitters by ~2100 km/s. Although the sample is
currently small, our results indicate that the radio-selected protocluster is
forming at the centre of a larger, ~60 co-moving Mpc super-structure. On the
basis of an HST/ACS imaging study we calculate dust-corrected star-formation
rates and investigate morphologies and sizes of the [O III] candidate emitters.
From a comparison of the star formation rate derived from UV-continuum and [O
III] emission, we conclude that at least two of the [O III] galaxies harbour an
AGN which ionized the O+ gas.
