Ionized gas in the Scutum spiral arm as traced in [N II] and [C II]
  Determining the properties of the warm ionized medium (WIM) at the leading
edge of spiral arms is important for understanding its dynamics and cloud
formation. The inner edge of the Scutum arm tangency is a unique location in
which to disentangle the WIM from other components. We use high spectral
resolution [C II] 158 micron and [N II] 205 micron fine structure line
observations taken with the upGREAT and GREAT instruments on SOFIA, along with
auxiliary HI and 13CO observations. The observations were in and out of the
Galactic plane along 18 lines of sight between longitude 30deg and 32deg. We
detect strong [N II] emission throughout the Scutum tangency. At VLSR = 110 to
125 km/s, where there is little, if any, 13CO, we could disentangle the [N II]
and [C II] emission that arises from the WIM at the inner edge. We find an
average electron density, <n(e)> about 0.9 cm{-3} in the plane, and about 0.4
cm{-3} above the plane. For VLSR < 110 km/s there is [N II] emission tracing
highly ionized gas throughout the arm's molecular layer. This ionized gas has a
high density, n(e) ~ 30 cm{-3}, and a few percent filling factor. Thus, [N II]
and [C II] at the Scutum arm tangency reveal a highly ionized gas with <n(e)>
about 10 to 20 times those of the interarm WIM, which is best explained by a
model in which the interarm WIM is compressed as it falls into the potential
well of the arm. The widespread distribution of [N II] in the molecular layers
shows that high density ionized gas is distributed throughout the Scutum arm.
The n(e) derived from [N II] for these molecular cloud regions are about 30
cm{-3}, and probably arise in the ionized boundary layers of clouds. This [N
II] emission from the ionized boundary layers is probably the result of the
shock compression of the WIM as it impacts the arm's neutral gas, but some
could arise from extended HII regions.
