On the origin of the fundamental metallicity relation and the scatter in
  galaxy scaling relations
  We present a simple toy model to understand what sets the scatter in star
formation and metallicity of galaxies at fixed mass. The scatter ultimately
arises from the intrinsic scatter in the accretion rate, but may be
substantially reduced depending on the timescale on which the accretion varies
compared to the timescale on which the galaxy loses gas mass. This model
naturally produces an anti-correlation between star formation and metallicity
at a fixed mass, the basis of the fundamental metallicity relation. We show
that observational constraints on the scatter in galaxy scaling relations can
be translated into constraints on the galaxy-to-galaxy variation in the mass
loading factor, and the timescales and magnitude of stochastic accretion onto
star-forming galaxies. We find a remarkably small scatter in the mass loading
factor, < 0.1 dex, and that the scatter in accretion rates is smaller than
expected from N-body simulations.
