  We discuss the occurrence of subcritical star formation in the outskirts of
some galactic discs and across LSB galaxies, contrary to the picture that star
formation happens only when the gas surface density is above a critical
threshold density. This raises the question of whether the Toomre Q-criterion
for gas alone is valid for correlating the gas distribution to star formation
or rather some effective Q-parameter, taking account of components such as
stars, dark matter and magnetic fields, is more representative of the
correlation between gas instability and star formation activity. As a potential
candidate, we investigate the role of dark matter in triggering subcritical
star formation, particularly at the outskirts where dark matter is dominant.
Indeed, our axisymmetric analysis favours the picture that dark matter
contributes to disc instability at the subcritical regime where gas seems
stable when dark matter is neglected.
