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A z ∼ 3 radio galaxy and its protocluster: evidence for a superstructure?




We present spectroscopic follow-up observations of Lyman-break galaxies (LBGs) selected in the field surrounding the radio galaxy MRC 0316−257 at z ∼ 3.13 (0316). Robust spectroscopic redshifts are determined for 20 out of 24 objects. Three of the spectroscopically confirmed galaxies have 3.12 < z < 3.13 indicating that these objects reside in a protocluster structure previously found around the radio galaxy. An additional five objects are found 1600 km s−1 blue shifted with respect to the main protocluster structure. This is in addition to three [O iii] emitters found at this redshift in a previous study. This is further evidence that a structure exists directly in front of the 0316 protocluster. We estimate that the foreground structure is responsible for half of the surface overdensity of LBGs found in the field as a whole. The foreground structure is associated with a strong surface density peak 1.4 Mpc to the north-west of the radio galaxy, and a 2D Kolmogorov–Smirnov test indicates that the spatial distributions of the 0316 and foreground galaxies differ at the 3σ level. In addition, we compare the properties of protocluster, foreground structure and field galaxies, but we find no significant differences. In terms of the nature of the two structures, a merger scenario is a possible option. Simple merger dynamics indicates that the observed relative velocity of 1600 km s−1 can be reproduced if the two structures have masses of ∼5 × 1014 M and have starting separations of around 2.5–3 Mpc. It is also possible that the foreground structure is unrelated to the 0316 protocluster in which case the two structures will not interact before z = 0.

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