Star formation is mainly determined by the observation of H$alpha$ radiation which is related to the presence of short lived massive stars. Disc galaxies show a strong cutoff in H$alpha$ radiation at a certain galactocentric distance which has led to the conclusion that star formation is suppressed in the outer regions of disc galaxies. This is seemingly in contradiction to recent UV observations (Boissier et al., 2007) that imply disc galaxies to have star formation beyond the Halpha cutoff and that the star-formation-surface density is linearly related to the underlying gas surface density being shallower than derived from Halpha luminosities (Kennicutt, 1998). In a galaxy-wide formulation the clustered nature of star formation has recently led to the insight that the total galactic Halpha luminosity is non-linearly related to the galaxy-wide star formation rate (Pflamm-Altenburg et al., 2007d). Here we show that a local formulation of the concept of clustered star formation naturally leads to a steeper radial decrease of the Halpha surface luminosity than the star-formation-rate surface density in quantitative agreement with the observations, and that the observed Halpha cutoff arises naturally.
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