The Central 300 pc of the Galaxy probed by infrared spectra of H3+ and
  CO: II. Expansion and morphology of the warm diffuse gas
  Velocity profiles of a line of H$_3^+$ at 3.7 $\mu$m produced in warm diffuse
gas have been observed toward 18 stars in the Central Molecular Zone (CMZ) of
the Galaxy. Their longitude-velocity diagram indicates that the gas is radially
expanding within the CMZ at speeds up to a maximum of $\sim$150 km s$^{-1}$.
The current momentum and energy in the gas are $\sim 5 \times 10^8 M_\odot$ km
s$^{-1}$ and $\sim 5\times 10^{53}$ erg. The motion is similar to that of the
Expanding Molecular Ring (EMR) discovered in 1972 by Kaifu et al. and by
Scoville. We propose that the expanding gas seen in H$_3^+$ is part of the same
phenomenon, in spite of differences in estimates of density, morphology, and
degree of rotation. The outward motion suggests that one or more ejection
events occurred near the center of the CMZ (0.5$-$1) $\times$ 10$^6$ years ago,
which may be related to creation of the recently observed microwave bubble.
These observations revive the circular face-on view of the CMZ proposed in
1972, which fell out of favor after 1991 when Binney et al. proposed that a
face-on view of the CMZ would show it to have an elliptical shape, with high
eccentricity. While that model may apply on kiloparsec scales, we argue that it
is incorrect to apply it to the much smaller CMZ. We discuss the fate of the
expanding gas, which appears to be eventual infall into the center, leading to
episodes of star formation and violent events associated with accretion onto
Sgr~A$^\ast$.
