Spatial variation in Toomre Q parameter within the gas disk in ALESS 73.1. The main panel shows the azimuthally integrated Q as marked with dashed lines in the inset. We see that Q declines with radius into the outer, gas-dominated, parts of the galaxy. This behaviour is similar to that inferred for less actively star-forming galaxies at z ~ 2 from Hα kinematic studies (Genzel et al. 2011; Swinbank et al. 2012a). However, the entire disk has Q < 1, meaning it is unstable throughout.