[O iii] emitters in the field of the MRC 0316–257 protocluster

Venemans et al. found evidence for an overdensity of Lyα 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 active galactic nucleus (AGN) within the protocluster. Narrow-band infrared imaging was used to select candidate [O iii] emitters in a 1.1 × 1.1 Mpc^2 region around the radio galaxy. 13 candidates have been detected. Four of these are among the previously confirmed sample of Lyα 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^−1 of each other, but are blueshifted with respect to the radio galaxy and Lyα emitters by ∼2100 km s−1. Although the sample is currently small, our results indicate that the radio-selected protocluster is forming at the centre of a larger, ∼60 comoving Mpc superstructure. On the basis of a Hubble Space Telescope/Advanced Camera for Surveys (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