Herschel Galactic plane survey of [NII] fine structure emission
  We present the first large scale high angular resolution survey of ionized
nitrogen in the Galactic Plane through emission of its two fine structure
transitions ([NII]) at 122 $\mu$m and 205 $\mu$m. The observations were largely
obtained with the PACS instrument onboard the Herschel Space Observatory. The
lines-of-sight were in the Galactic plane, following those of the Herschel OTKP
project GOT C+. Both lines are reliably detected at the 10$^{-8}$ - 10$^{-7}$
$W$m$^{-2}$sr$^{-1}$ level over the range -60$^{o}$ $\leq$ $l$ $\leq$ 60$^{o}$.
The $rms$ of the intensity among the 25 PACS spaxels of a given pointing is
typically less than one third of the mean intensity, showing that the emission
is extended. [NII] is produced in gas in which hydrogen is ionized, and
collisional excitation is by electrons. The ratio of the two fine structure
transitions provides a direct measurement of the electron density, yielding
$n(e)$ largely in the range 10 to 50 cm$^{-3}$ with an average value of 29
cm$^{-3}$ and N$^+$ column densities 10$^{16}$ to 10$^{17}$ cm$^{-2}$. [NII]
emission is highly correlated with that of [CII], and we calculate that between
1/3 and 1/2 of the [CII] emission is associated with the ionized gas. The
relatively high electron densities indicate that the source of the [NII]
emission is not the Warm Ionized Medium (WIM), which has electron densities
more than 100 times smaller. Possible origins of the observed [NII] include the
ionized surfaces of dense atomic and molecular clouds, the extended low density
envelopes of HII regions, and low-filling factor high-density fluctuations of
the WIM.
