Response of the halo test particles to an analytical spiral potential (Cox & Gómez 2002), rotating at a constant pattern speed. From left to right, we show the radially normalized density (δρ_r), the mean radial and azimuthal velocities (⟨V_R⟩,⟨V_ϕ⟩), and the vertical breathing motion (V_{Z, br}). In black and white, we show the 0 and 0.5 ρ_max density contours of the potential, respectively. The response of the halo is compatible with the results shown in Figs. 1 and 4.