Error in the spectral alignment for the TNG50 galaxy 520885 under two different CGM and IGM kinematics assumptions. Left panel: the residuals in terms of |mom₁ − v|, where mom₁ is the noiseless moment 1 map and v the assumed kinematics, when v = v_sys. Light-blue corresponds to |mom₁ − v|< 20 km s⁻¹, yellow to 20 < |mom₁ − v|< 50 km s⁻¹ and red is |mom₁ − v|> 50 km s⁻¹. Right panel: same as left panel but when v is given by assuming the gas is co-rotating with the galaxy with a flat rotation curve.