Internal structure of spiral arms traced with [CII]: Unraveling the WIM,
  HI, and molecular emission lanes
  The spiral arm tangencies are ideal lines of sight in which to determine the
distribution of interstellar gas components in the spiral arms and study the
influence of spiral density waves on the interarm gas in the Milky Way. We
present a large scale (~15deg) position-velocity map of the Galactic plane in
[CII] from l = 326.6 to 341.4deg observed with Herschel HIFI. We use [CII] l-v
maps along with those for Hi and 12CO to derive the average spectral line
intensity profiles over the longitudinal range of each tangency. Using the VLSR
of the emission features, we locate the [CII], HI, and 12CO emissions along a
cross cut of the spiral arm. In the spectral line profiles at the tangencies
[CII] has two emission peaks, one associated with the compressed WIM and the
other the molecular gas PDRs. When represented as a cut across the inner to
outer edge of the spiral arm, the [CII]-WIM peak appears closest to the inner
edge while 12CO and [CII] associated with molecular gas are at the outermost
edge. HI has broader emission with an intermediate peak located nearer to that
of 12CO. The velocity resolved spectral line data of the spiral arm tangencies
unravel the internal structure in the arms locating the emission lanes within
them. We interpret the excess [CII] near the tangent velocities as shock
compression of the WIM induced by the spiral density waves and as the innermost
edge of spiral arms. For the Norma and Perseus arms, we estimate widths of ~250
pc in [CII]-WIM and ~400 pc in 12CO and overall spiral arm widths of ~500 pc in
[CII] and 12CO emissions. The electron densities in the WIM are ~ 0.5 cm^-3,
about an order of magnitude higher than the average for the disk. The enhanced
electron density in the WIM is a result of compression of the WIM by the spiral
density wave potential.
