  The scaling of galaxy properties with halo mass suggests that feedback loops
regulate star formation, but there is no consensus yet about how those feedback
loops work. To help clarify discussions of galaxy-scale feedback, Paper I
presented a very simple model for supernova feedback that it called the
minimalist regulator model. This followup paper interprets that model and
discusses its implications. The model itself is an accounting system that
tracks all of the mass and energy associated with a halo's circumgalactic
baryons--the central galaxy's atmosphere. Algebraic solutions for the
equilibrium states of that model reveal that star formation in low-mass halos
self-regulates primarily by expanding the atmospheres of those halos,
ultimately resulting in stellar masses that are insensitive to the mass-loading
properties of galactic winds. What matters most is the proportion of supernova
energy that couples with circumgalactic gas. However, supernova feedback alone
fails to expand galactic atmospheres in higher-mass halos. According to the
minimalist regulator model, an atmospheric contraction crisis ensues, which may
be what triggers strong black-hole feedback. The model also predicts that
circumgalactic medium properties emerging from cosmological simulations should
depend largely on the specific energy of the outflows they produce, and we
interpret the qualitative properties of several numerical simulations in light
of that prediction.
