Left: zoom-in of the sky distribution of all galaxies in the redshift range of the same protostructure that is shown in Fig. 3 (Cl J0227-0421). This protostructure is also the most significantly detected protostructure in photometric redshift overdensity. The photometric density map, generated using the methodology of Sect. 3, is shown in the background, with the scale bar denoting the photometric redshift galaxy density. As in Fig. 3, the dashed circle designates 3 Mpc from the adopted center of the protostructure. Blue and red symbols show galaxies at the redshift of the protostructure differentiated by their M_NUV − M_r′ colors (where the delineation point was set roughly at the color of a 200 Myr old stellar population, see Sect. 4.1.2) and are logarithmically scaled (log ₄) by their stellar mass (see Sect. 4.1). Filled circles denote galaxies with secure spectroscopic redshifts, while Xs denote those galaxies with less secure spectroscopic redshifts. Plotted in the background are all galaxies with z_spec> 1.5. Several massive, redder galaxies are observed in the bounds of the protostructure and extended filamentary structure can be seen to the west-southwest of the central protostructure galaxy concentration. Bottom right: spectroscopic redshift distribution of galaxies within 3 Mpc of the protostructure center. The shaded histogram displays only those galaxies with secure spectroscopic redshifts, while the histogram plotted with a solid black line also includes those galaxies with less secure spectral redshifts. The redshift bounds defining membership are marked by magenta dot-dashed lines.