  The formation of galaxies is regulated by a balance between the supply of gas
and the rate at which it is ejected. Traditional explanations of gas ejection
equate the energy required to escape the galaxy or host halo to an estimate for
the energy yield from supernovae. This yield is usually assumed to be a
constant fraction of the total available from the supernova, or is derived from
the assumption of a consistent momentum yield. By applying these ideas in the
context of a cold dark matter cosmogony, we derive a 1st-order analytic
connection between these working assumptions and the expected relationship
between baryon content and galaxy circular velocity, and find that these quick
predictions straddle recent observational estimates. To examine the premises
behind these theories in more detail, we then explore their applicability to a
set of gasdynamical simulations of idealised galaxies. We show that different
premises dominate to differing degrees in the simulated outflow, depending on
the mass of the system and the resolution with which it is simulated. Using
this study to anticipate the emergent behaviour at arbitrarily high resolution,
we motivate more comprehensive analytic model which allows for the range of
velocities with which the gas may exit the system, and incorporates both
momentum and energy-based constraints on the outflow. Using a trial exit
velocity distribution, this is shown to be compatible with the observed baryon
fractions in intermediate-mass systems, but implies that current estimates for
low-mass systems can not be solely accounted for by supernova winds under
commonly-held assumptions.
