Tracing the bar with the quadrupole feature: distribution of stellar surface density (left panel), mean radial velocity, ⟨V_R⟩ (middle panel), and radial velocity dispersion, σ_R (right panel), calculated in the (x − y)-plane, for the rthick0.0 model, at the end of the simulation run (t = 9 Gyr). Dashed black lines denote the contours of constant surface density. The cyan dashed circle denotes the bar length, R_bar, and the magenta dashed circle denotes the extent of the quadrupole feature, R_quadrupole. The magenta points denote the spatial distribution of the phase-angle of the m = 4 Fourier moment (φ₄). The bar excites a prominent quadrupole pattern in the mean radial velocity field, and the orientation of the quadrupole pattern agrees fairly accurately with the orientation of the bar. We used a galactocentric cylindrical coordinate system (R, ϕ, z) to calculate ⟨V_R⟩ and σ_R.