tokens: that gravitational instabilities drive the formation of molecular clouds within the Galactic Centre (hereafter, GC) gas stream. 2 DATA This Letter makes use of data from the Mopra CMZ survey, first published by Jones et al. ( 2012 ). We focus on the J = 1 → 0 transition of N 2 H + (the rest frequency of the main J, F 1 , F = 1, 2, 3 → 0, 1, 2 hyperfine component is ∼93.174 GHz), the emission from which is both bright and contiguous over the region of interest (Henshaw et al. 2016 ). The presented data cubes have an effective spatial resolution of ∼60 arcsec (corresponding to ∼2.4 pc), a pixel size of 30 arcsec × 30 arcsec (1.2 pc × 1.2 pc), a spectral resolution of 2 km s −1 , and the typical rms noise level is ∼0.01–0.02 K. Fig. 1 is a map of the inner CMZ. The colour-scale represents the Herschel -derived column density (Battersby et al., in preparation). The present investigation focuses on the region −07 l 00, −01 b 01, which is indicated by the black dashed rectangle. The black contours highlight integrated emission of N 2 H + . The data enclosed within the black dashed rectangle were analysed using the Semi-automated multi-COmponent Universal Spectral-line fitting Engine ( scouse 1 ; Henshaw et al. 2016 ). 3 RESULTS Panel A of Fig. 2 provides a close-up view of the Herschel -derived column density map (Fig. 1 ). It focuses on the location of the upstream velocity field (depicted by the blue dots). To identify the main structures we use dendrograms (Rosolowsky et al. 2008 ). Each contour in panel A represents a leaf, the highest level in the dendrogram hierarchy. 2 Red contours depict structures that are spatially coincident with the velocity pattern. The column density map of Battersby et al. (in preparation) is used to derive physical properties of each leaf, including the peak column density ( N ), total mass ( M ), equivalent number density and corresponding equivalent free-fall time ( n eq and t ff,eq ). 3 These properties are presented in Table 1 . Panel B of Fig. 2 showcases the velocity pattern first identified by Henshaw et al. ( 2016 ), which appears as a corrugation in { l , v LSR } space. Each data point refers to the location ( l ) and centroid velocity ( v 0 ) of a Gaussian component extracted using scouse . We corroborated these values using the hyperfine structure (HFS) fitting functionality available in class . 4 This method accounts for the effects of opacity on the line-profiles (whereas Gaussian fitting does not), leading to a more accurate measure of the velocity dispersion and line centroid where the opacity is high. We find that the velocity dispersions derived using scouse are systematically larger than those obtained using class (). However, the derived centroid velocities are very similar, km s −1 . Both the scouse and class derived kinematic properties are included in Table 1 . Figure 2. Galactic longitude versus: (A) Galactic latitude. A column density map of the CMZ focusing on the region of interest
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