The Survey of Water and Ammonia in the Galactic Center (SWAG): Molecular
  Cloud Evolution in the Central Molecular Zone
  The Survey of Water and Ammonia in the Galactic Center (SWAG) covers the
Central Molecular Zone (CMZ) of the Milky Way at frequencies between 21.2 and
25.4 GHz obtained at the Australia Telescope Compact Array at $\sim 0.9$ pc
spatial and $\sim 2.0$ km s$^{-1}$ spectral resolution. In this paper, we
present data on the inner $\sim 250$ pc ($1.4^\circ$) between Sgr C and Sgr B2.
We focus on the hyperfine structure of the metastable ammonia inversion lines
(J,K) = (1,1) - (6,6) to derive column density, kinematics, opacity and kinetic
gas temperature. In the CMZ molecular clouds, we find typical line widths of
$8-16$ km s$^{-1}$ and extended regions of optically thick ($\tau > 1$)
emission. Two components in kinetic temperature are detected at $25-50$ K and
$60-100$ K, both being significantly hotter than dust temperatures throughout
the CMZ. We discuss the physical state of the CMZ gas as traced by ammonia in
the context of the orbital model by Kruijssen et al. (2015) that interprets the
observed distribution as a stream of molecular clouds following an open
eccentric orbit. This allows us to statistically investigate the time
dependencies of gas temperature, column density and line width. We find heating
rates between $\sim 50$ and $\sim 100$ K Myr$^{-1}$ along the stream orbit. No
strong signs of time dependence are found for column density or line width.
These quantities are likely dominated by cloud-to-cloud variations. Our results
qualitatively match the predictions of the current model of tidal triggering of
cloud collapse, orbital kinematics and the observation of an evolutionary
sequence of increasing star formation activity with orbital phase.
