Cosmic rays across the star-forming galaxy sequence. II: Stability
  limits and the onset of cosmic ray-driven outflows
  Cosmic rays (CRs) are a plausible mechanism for launching winds of cool
material from the discs of star-forming galaxies. However, there is no
consensus on what types of galaxies likely host CR-driven winds, or what role
these winds might play in regulating galaxies' star formation rates. Using a
detailed treatment of the transport and losses of hadronic CRs developed in the
previous paper in this series, here we develop a semi-analytic model that
allows us to assess the viability of using CRs to launch cool winds from
galactic discs. In particular, we determine the critical CR fluxes -- and
corresponding star formation rate surface densities -- above which hydrostatic
equilibrium within a given galaxy is precluded because CRs drive the gas off in
a wind or otherwise render it unstable. We show that, for star-forming galaxies
with lower gas surface densities typical of the Galaxy and local dwarfs, the
locus of this CR stability curve patrols the high side of the observed
distribution of galaxies in the Kennicutt-Schmidt parameter space of star
formation rate versus gas surface density. However, hadronic losses render CRs
unable to drive winds in galaxies with higher surface densities. Our results
show that quiescent, low surface density galaxies like the Milky Way are poised
on the cusp of instability, such that small changes to ISM parameters can lead
to the launching of CR-driven outflows, and we suggest that, as a result, CR
feedback sets an ultimate limit to the star formation efficiency of most modern
galaxies.
