the rotation curve to be spatially resolved the projection of this distance on the slit had to be greater than some limiting value, which we took as $r_{lim}=0.4$arcsec. If the simulation passes the spatial cut we add a velocity error drawn at random from an appropriate Gaussian distribution and then check if the observation meets the velocity cut, $V_{min}$. Rotation velocities were only recorded by Pettini et al. (2001) and Erb et al. (2003) if a velocity gradient was clearly visible, corresponding to a rotation velocity greater than twice the typical velocity error (Erb, private communication). Therefore, in our simulation, we disregard rotation velocities smaller than $V_{min}=25$[kms$^{-1}$]{} for the ISAAC data, and $V_{min}=50$[kms$^{-1}$]{} for the NIRSPEC data. We chose a nominal scale length $R_0=0.3$arcsec, and found that a value $\beta=2.6$ reproduced the average projected radius of the seven measurements. This scale radius is about twice as large as the typical value measured for LBGs (Marleau & Simard, 1998); nevertheless, given that rotation velocities were secured for only seven of the 32 LBGs considered here, it may be true that the light profiles of these seven galaxies are more extended on average.

In summary, our model is fully specified by the 