3D self-gravitating gas: rho_t+div m=0, m_t+div(mv)+grad p=−rho gradφ, e_t+div(ev)+p div v=nu|grad u|^2, lapφ=G rho, p=gamma e. 100³, isolated (φ=0 at walls). Two warm rotating over-densities (two co-rotating discs, spin Ω) on a prograde encounter (transverse speed v_orb) are pulled together by mutual self-gravity — they swing past, raise tidal arms, and merge into one larger rotating system (the IC 2163 / NGC 2207 picture). (z-slice = merger plane face-on; switch axis to x-slice for edge-on.) Tune G, Ω and v_orb for a head-on plunge vs a grazing spiral. Centre-row line plots on the z-slice (log scale): density, temperature, pressure, |potential|, |∇p|, ρ|∇φ|. Force balance: where ρ|∇φ| (gravity) overtakes |∇p| (pressure) the gas collapses. 3D threshold Γ>4/3 (gamma>1/3); no black hole (Newtonian).