Time evolution of the variance in the logarithmic surface density, σ_η, where η is defined in Equation 8. The panel on the left is for runs with varying ⟨Q⟩ and fixed ℳ_c and the panel on the right for runs with varying ℳ_c but fixed ⟨Q⟩, as indicated in the legends. The solid lines represent the simulation data and the dotted lines are piecewise fits with a constant and an exponential part. We see that the ⟨Q⟩> 1 runs are stable, while there is exponential growth in all the runs with ⟨Q⟩ = 1. For the unstable runs, we see that growth is faster for higher ℳ_c. We also observe a decrease in the time of the onset of the instability with increasing ℳ_c. We do not provide an analytical fit to the ℳ_c = 14 case because of its complex behaviour.