Large-scale structure of Lyman break galaxies around a radio galaxy protocluster at z ∼ 4

We present broad-band imaging with the Subaru Telescope of a 25′ × 25′ field surrounding the radio galaxy TN J1338-1942 at redshift z = 4.1. The field contains excesses of Lyman-a emitters (LAEs) and Lyman break galaxies (LBGs) identified with a protocluster surrounding the radio galaxy. Our new wide-field images provide information about the boundary of the protocluster and its surroundings. There are 874 candidate LBGs within our field, having redshifts in the range z = 3.5-4.5. An examination of the brightest of these (with i′ < 25.0) shows that the most prominent concentration coincides with the previously discovered protocluster. The diameter of this galaxy overdensity corresponds to ∼2 Mpc at z = 4, consistent with the previous estimation using LAEs. Several other concentrations of LBGs are observed in the field, some of which may well be physically connected with the z = 4.1 protocluster. The observed structure in the smoothed LEG distribution can be explained as the projection of large-scale structure, within the redshift range z = 3.5-4.5, comprising compact overdensities and prominent larger voids. If the 5-8 observed compact overdensities are associated with protoclusters, the observed protocluster volume density is ∼5 × 10^-6 Mpc^-3, similar to the volume density of rich clusters in the local Universe.