Galaxy overdensity HPC1001 at z ≈ 3.7. Left: VISTA K_s image overlaid with SCUBA2 850 μm contours in 3–5σ levels. Galaxies at 3.5 < z < 3.9 are marked with red circles and labeled with photo-z. The dashed circle shows the virial radius of a dark matter halo M_DM = 10¹³ M_⊙. Right: HSC i and VISTA JK_s color image for the core region, overlaid by ALMA 1.2 mm continuum contours in yellow (3–5σ). The ALMA beam (0.53″ × 0.44″, PA = −25.8°) is shown with a yellow ellipse in the bottom left. Candidate members are labeled with text in orange.