Moment 1 map of the two TNG50 galaxies presented in this paper: 520855 is on the left and 555013 is on the right. Black solid-contours denote the receding (50, 100, 150) km s⁻¹ velocities with respect to the systemic velocity. Black dashed-contours instead refer to the approaching ( − 150, −100, −50) km s⁻¹ velocities. The maps are clipped at the 10¹⁶ cm⁻² column density level, as this is the noise level we expected to achieve via stacking.