(Top) Comparison of size versus velocity dispersion in the helix stream (red dots) with values of Galactic centre clouds from Oka et al. (2001b, green ×), the Orion A and B giant molecular clouds (magenta stars; Caselli & Myers 1995) and other Galactic giant molecular clouds (blue crosses; Heyer et al. 2009). The dotted line corresponds to the best-fit result from Solomon et al. (1987), σ = 0.7 R^0.5. The dashed line corresponds to the best-fit result for the helix stream σ = 2.97 R^0.70. (Bottom) Heyer’s relation (surface density, ∑, versus σ/R^0.5) for different Galactic clouds. Dots correspond to the structures within the helix stream, squares correspond to structures within the helix stream where follow-up molecular observations are carried out, crosses represent Galactic giant molecular clouds (Heyer et al. 2009), and triangles represent high-density cores in the GC ‘dust-ridge’ clouds (Barnes et al. 2019). Black curves indicate solutions of pressure-bounded virial equilibrium and dotted lines correspond to α_vir=1.0, 3.0, and 10.0, where there is negligible external pressure.