Astro2020 Science White Paper: Physical Conditions in the Cold Gas of
  Local Galaxies
  We describe a next major frontier in observational studies of galaxy
evolution and star formation: linking the physical conditions in the cold,
star-forming interstellar medium to host galaxy and local environment. The
integrated gas content of galaxies has been surveyed extensively over the last
decades. The link between environment and cold gas density, turbulence,
excitation, dynamical state, and chemical makeup remain far less well
understood. We know that these properties do vary dramatically and theoretical
work posits a strong connection between the state of the gas, its ability to
form stars, and the impact of stellar feedback. A next major step in the field
will be to use sensitive cm-, mm-, and submm-wave spectroscopy and high
resolution spectroscopic imaging to survey the state of cold gas across the
whole local galaxy population. Such observations have pushed the capabilities
of the current generation of telescopes. We highlight three critical elements
for progress in the next decade: (1) robust support and aggressive development
of ALMA, (2) the deployment of very large heterodyne receiver arrays on single
dish telescopes, and (3) development of a new interferometric array that
dramatically improves on current capabilities at cm- and mm-wavelengths (~
1-115 GHz).
