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-α 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 th e 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 physicall y connected with the z = 4.1 protocluster. The observed structure in the smoothed LBG distribution can be explained as the projection of large-scale structure, within the redshift r ange 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 U niverse.