Astro2020: The Cycling of Matter from the Interstellar Medium to Stars
  and back
  Understanding the matter cycle in the interstellar medium of galaxies from
the assembly of clouds to star formation and stellar feedback remains an
important and exciting field in comtemporary astrophysics. Many open questions
regarding cloud and structure formation, the role of turbulence, and the
relative importance of the various feedback processes can only be addressed
with observations of spectrally resolved lines. We here stress the importance
of two specific sets of lines: the finestructure lines of atomic carbon as a
tracer of the dark molecular gas and mid-J CO lines as tracers of the warm,
active molecular gas in regions of turbulence dissipation and feedback. The
observations must cover a wide range of environments (i.e., physical
conditions), which will be achieved by large scale surveys of Galactic
molecular clouds, the Galactic Center, the Magellanic clouds, and nearby
galaxies. To date, such surveys are completely missing and thus constitute an
important science opportunity for the next decade and beyond. For the
successful interpretation of the observations, it will be essential to combine
them with results from (chemical) modelling and simulations of the interstellar
medium.
