Stellar streams around a combined spherical+cylindrical source. This is a toy model representing the galaxy plus an elongated DM halo. Top row: formation of a stellar stream from an initial orbiting cluster, as in Fig. 4, with positions given at the times indicated in the legend. Lower left: three-dimensional view of the stream compared with a plane containing the origin and the initial stream’s position and velocity, showing that at subsequent times, the stream leaves that plane. Lower right: piecewise helicoidal fit. A nonvanishing torsion can be extracted for each of the short segments at different times, yielding a mean value of τ = 0.22 × 10⁻³ kpc⁻¹.