tokens: such as the Schmidt–Kennicutt relationship between the surface density and the star formation rate (Schmidt 1959 ; Kennicutt 1998 ) or the density threshold for star formation (Lada, Lombardi Alves 2010 ). In particular, the observed star formation rates for several of these clouds are an order of magnitude below those predicted by these relationships between the density and the rate (Longmore et al. 2013a ). A case in point is the cloud GCM-0.02-0.07 ( M ∼ 10 5 M ⊙ ; Johnston et al. 2014 ) nicknamed the Brick because of its high optical depth in the infrared and lack of internal infrared emission from star formation. Only 3–4 per cent of the mass of this cloud is in dense star-forming regions often called high-mass cores. The elliptical ring also includes the massive star-forming region Sgr B2 with some of the highest rates of star formation in the galaxy and the region Sgr C rich with H ii regions indicating earlier active star formation. Fig. 5 shows the locations of the clouds on their elliptical orbit. Figure 5. Schematic of the clouds in elliptical orbit in the Galactic Centre adapted from Kruijssen et al. ( 2014 ). The view is of the plane of the Galaxy from above. [Kruijssen, Dale Longmore ( 2015 ) suggest that the orbit is not exactly elliptical in that the path from Sgr C may not close back to the location of the Brick.] The clouds on the ring orbit in the direction from the Brick toward Sgr B2. The Brick and other clouds with low star formation rates are near the pericentre, 45 pc from the supermassive black hole Sgr A* (Johnston et al. 2014 ), while Sgr B2 is at the apocentre, twice as distant at 100 pc. Longmore et al. ( 2013b ) suggest that the Brick and nearby clouds are currently undergoing tidal compression that will result in intense star formation by the time the clouds arrive at the location of Sgr B2. The mini-starburst Sgr B2 would then be the result of compression in its earlier passage through the pericentre of the orbit. Observations indicate that the clouds on the orbit between the Brick and Sgr B2, the dust ridge clouds, may be just beginning to fragment (Walker et al. 2018 ). Alternatively, fragmentation and ultimately the star formation rate may be controlled by the turbulent-entropic instability. If orbital shear slows the effective turbulent dissipation rate by supplying fresh turbulent energy, then the difference in orbital shear along the elliptical orbit from pericentre to apocentre may prevent star formation near the pericentre and allow it at the apocentre. Following Kruijssen et al. ( 2014 ), we derive a rotation curve, v circ = 23.5 r 0.38 km s −1 from the mass distribution measured around the Galactic Centre by Launhardt, Zylka Mezger ( 2002 ). At the radial distance from the Brick to the Galactic Centre, 45 pc, the orbital shear across a 10 pc diameter cloud is Δ v = 8.4 km s −1 , while the one-dimensional turbulent velocity dispersion in the Brick is 4.3 km s −1 (Johnston et al. 2014 ). At the
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