How Cosmic Rays Mediate the Evolution of the Interstellar Medium
  We explore the impact of diffusive cosmic rays (CRs) on the evolution of the
interstellar medium (ISM) under varying assumptions of supernova explosion
environment. In practice, we systematically vary the relative fractions of
supernovae (SN) occurring in star-forming high-density gas and those occurring
in random locations decoupled from star-forming gas to account for SN from
run-away stars or explosions in regions that have been cleared by prior SN,
stellar winds, or radiation. We explore various mixed models by adjusting these
fractions relative to each other. We find that in the simple system of a
periodic stratified gas layer the ISM structure will evolve to one of two
solutions: a "peak driving" state where warm gas is volume filling or a
"thermal runaway" state where hot gas is volume filling. CR pressure and
transport are important factors that strongly influence the solution state the
ISM reaches and have the ability to flip the ISM between solutions. Observable
signatures such as gamma ray emission and HI gas are explored. We find that
gamma ray luminosity from pion decay is largely consistent with observations
for a range of model parameters. The thickness of the HI gas layer may be too
compact, however, this may be due to a large cold neutral fraction of midplane
gas. The volume fraction of hot gas evolves to stable states in both solutions,
but neither settles to a Milky Way-like configuration, suggesting that
additional physics which is omitted here (e.g. a cosmological circum-galactic
medium, radiation transport, or spectrally resolved and spatially varying CR
transport) may be required.
