Observed pixel-by-pixel Hα line widths (assumed equal to the shock velocities) as function of Hα surface brightnesses (corrected for cosmological surface brightness dimming) in a sub-sample of 5 representative galaxies in each of two redshift bins, z ~ 1.4 (+ symbols) and z ~ 2.3 ( × ). Lines correspond to Hα surface brightnesses derived using shock models from Raymond (1979) (black) and Allen et al. (2008) (blue). The numbers along each line indicate the surface accretion rates (in M_⊙ yr^-1 kpc^-2) which produce the Hα surface brightness. The 3 sets of lines are for pre-shock densities of 10, 100 and 1000 cm^-3, respectively, from left to right.