Physical Conditions of the Earliest Phases of Massive Star Formation:
  Single-Dish and Interferometric Observations of Ammonia and CCS in Infrared
  Dark Clouds
  Infrared Dark Clouds (IRDCs) harbor the earliest phases of massive star
formation, and many of the compact cores in IRDCs, traced by millimeter
continuum or by molecular emission in high critical density lines, host massive
young stellar objects (YSOs). We used the Robert C. Byrd Green Bank Telescope
(GBT) and the Karl G. Jansky Very Large Array (VLA) to map NH$_{3}$ and CCS in
nine IRDCs to reveal the temperature, density, and velocity structures and
explore chemical evolution in the dense ($>10^{22}$ cm$^{-2}$) gas. Ammonia is
an excellent molecular tracer for these cold, dense environments. The internal
structure and kinematics of the IRDCs include velocity gradients, filaments,
and possibly colliding clumps that elucidate the formation process of these
structures and their YSOs. We find a wide variety of substructure including
filaments and globules at distinct velocities, sometimes overlapping at sites
of ongoing star formation. It appears that these IRDCs are still being
assembled from molecular gas clumps even as star formation has already begun,
and at least three of them appear consistent with the morphology of
``hub-filament structures'' discussed in the literature. Furthermore, we find
that these clumps are typically near equipartition between gravitational and
kinetic energies, so these structures may survive for multiple free-fall
times.Keywords: molecular data -- ISM: clouds -- (ISM:) dust, extinction --
ISM: molecules -- Stars: formation -- radio lines: ISM
