  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.
