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Fig. 3.

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Motion of a star ensemble around around a perfectly cylindrical toy-model source. The initial velocity is vz = 90 km s−1, with the same conventions as in Fig. 1. Upper plots: a stream forms along a spiral trajectory. From a helicoidal fit to the cloud of points, as needs to be done on real data, with χ2/Ngdl = 1.16 we find a value for the torsion of τ ≃ 1.1 × 10−2 kpc−1 (lower plot). This is in agreement with the analytical estimate in the main text below Eq. (16).

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