The Structure of the Interstellar Medium of Star Forming Galaxies
  We present numerical methods for including stellar feedback in galaxy-scale
simulations. We include heating by SNe (I & II), gas recycling and
shock-heating from O-star & AGB winds, HII photoionization, and radiation
pressure from stellar photons. The energetics and time-dependence are taken
directly from stellar evolution models. We implement these in simulations with
pc-scale resolution, modeling galaxies from SMC-like dwarfs and MW analogues to
massive z~2 starburst disks. Absent feedback, gas cools and collapses without
limit. With feedback, the ISM reaches a multi-phase steady state in which GMCs
continuously form, disperse, and re-form. Our primary results include: (1) Star
forming galaxies generically self-regulate at Toomre Q~1. Most of the volume is
in diffuse hot gas with most of the mass in dense GMC complexes. The phase
structure and gas mass at high densities are much more sensitive probes of
stellar feedback physics than integrated quantities (Toomre Q or gas velocity
dispersion). (2) Different feedback mechanisms act on different scales:
radiation & HII pressure are critical to prevent runaway collapse of dense gas
in GMCs. SNe and stellar winds dominate the dynamics of volume-filling hot gas;
however this primarily vents out of the disk. (3) The galaxy-averaged SFR is
determined by feedback. For given feedback efficiency, restricting star
formation to molecular gas or modifying the cooling function has little effect;
but changing feedback mechanisms directly translates to shifts off the
Kennicutt-Schmidt relation. (4) Self-gravity leads to marginally-bound GMCs
with an ~M^-2 mass function with a cutoff at the Jeans mass; they live a few
dynamical times before being disrupted by stellar feedback and turn ~1-10% of
their mass into stars (increasing from dwarfs through starburst galaxies).
Low-mass GMCs are preferentially unbound.
