Relative location and motion of a galaxy, j, at distance r_j from an observer with respect to the centre of mass at distance r_c. The vector v_j represents the line-of-sight velocity of galaxy j, while v_c is the line-of-sight velocity of the barycentre. The angle θ is measured between r_j and r_c. The prerequisites for the two infall models to accurately describe the radial velocity, v_rad, are listed in the table.