Lyman Break Galaxies, Lyα Emitters, and a Radio Galaxy in a Protocluster at z = 4.1

We present deep HST ACS observations in g475r625i775z850 toward the z = 4.1 radio galaxy TN J1338–1942 and its overdensity of >30 spectroscopically confirmed Lyα emitters (LAEs). We select 66 g475 band dropouts to z850,5 σ = 27, 6 of which are also LAEs. Although our color-color selection results in a relatively broad redshift range centered on z = 4.1, the field of TN J1338–1942 is richer than the average field at the >5 σ significance, based on a comparison with GOODS. The angular distribution is filamentary with about half of the objects clustered near the radio galaxy, and a small, excess signal (2 σ) in the projected pair counts at separations of θ < 10″ is interpreted as being due to physical pairs. The LAEs are young (a few times 107 yr), small (⟨ rhl⟩ = 0.13″) galaxies, and we derive a mean stellar mass of ~108-109 M☉ based on a stacked Ks band image. We determine star formation rates, sizes, morphologies, and color-magnitude relations of the g475-dropouts and find no evidence for a difference between galaxies near TN J1338–1942 and in the field. We conclude that environmental trends as observed in clusters at much lower redshift are either not yet present or washed out by the relatively broad selection in redshift. The large galaxy overdensity, its corresponding mass overdensity, and the subclustering at the approximate redshift of TN J1338–1942 suggest the assemblage of a >1014 M☉ structure, confirming that it is possible to find and study cluster progenitors in the linear regime at z≳ 4.