Ionized gas at the edge of the Central Molecular Zone
  To determine the properties of the ionized gas at the edge of the CMZ near
Sgr E we observed a small portion of the edge of the CMZ near Sgr E with
spectrally resolved [C II] 158 micron and [N II] 205 micron fine structure
lines at six positions with the GREAT instrument on SOFIA and in [C II] using
Herschel HIFI on-the-fly strip maps. We use the [N II] spectra along with a
radiative transfer model to calculate the electron density of the gas and the
[C II] maps to illuminate the morphology of the ionized gas and model the
column density of CO-dark H2. We detect two [C II] and [N II] velocity
components, one along the line of sight to a CO molecular cloud at -207 km/s
associated with Sgr E and the other at -174 km/s outside the edge of another CO
cloud. From the [N II] emission we find that the average electron density is in
the range of about 5 to 25 cm{-3} for these features. This electron density is
much higher than that of the warm ionized medium in the disk. The column
density of the CO-dark H$_2$ layer in the -207 km/s cloud is about 1-2X10{21}
cm{-2} in agreement with theoretical models. The CMZ extends further out in
Galactic radius by 7 to 14 pc in ionized gas than it does in molecular gas
traced by CO. The edge of the CMZ likely contains dense hot ionized gas
surrounding the neutral molecular material. The high fractional abundance of N+
and high electron density require an intense EUV field with a photon flux of
order 1e6 to 1e7 photons cm{-2} s{-1}, and/or efficient proton charge exchange
with nitrogen, at temperatures of order 1e4 K, and/or a large flux of X-rays.
Sgr E is a region of massive star formation which are a potential sources of
the EUV radiation that can ionize the gas. In addition X-ray sources and the
diffuse X-ray emission in the CMZ are candidates for ionizing nitrogen.
