Velocity maps of [O III]_5007 Å in the PJ308–21 system (top panel). The system shows a prominent velocity gradient: The western companion and the bridge connecting it to the quasar host galaxies show a blue shift of Δv ≈ −650 and −880 km s⁻¹ respectively compared to the system rest frame (z = 6.2342, based on the [C II] emission). The host galaxy shows a prominent velocity gradient from the south (Δv ≈ −700 km s⁻¹) to the north (Δv = +140 km s⁻¹). The eastern companion is redshifted with respect to the system’s rest frame, with Δv increasing at increasing distance from the quasar (up to Δv ≈ 650 km s⁻¹). The eastern companion shows a modest velocity dispersion (σ_line < 140 km s⁻¹).