  We investigate the consequences of applying different star formation laws in
the galaxy formation model GALFORM. Three broad star formation laws are
implemented: the empirical relations of Kennicutt and Schmidt and Blitz &
Rosolowsky and the theoretical model of Krumholz, McKee & Tumlinson. These laws
have no free parameters once calibrated against observations of the star
formation rate (SFR) and gas surface density in nearby galaxies. We start from
published models, and investigate which observables are sensitive to a change
in the star formation law, without altering any other model parameters. We show
that changing the star formation law (i) does not significantly affect either
the star formation history of the universe or the galaxy luminosity functions
in the optical and near-IR, due to an effective balance between the quiescent
and burst star formation modes; (ii) greatly affects the cold gas contents of
galaxies; (iii) changes the location of galaxies in the SFR versus stellar mass
plane, so that a second sequence of "passive" galaxies arises, in addition to
the known "active" sequence. We show that this plane can be used to
discriminate between the star formation laws.
