Spiral arms in the Iso-NBody simulation, at t = 7 Gyr. Left: surface density plot of the stellar component. Middle: radially normalized density (δρ_r) of the stellar component. The contours represent the regions where δρ_r = 0 (in black), and δρ_r = ±0.2 (in white). Right: δρ_r of the DM particles around the stellar disc (|Z| < 4 kpc). The contours are the same as the middle panel. We observe a clear spiral arm pattern in the DM halo, strongly correlated with the stellar spiral arms.