| Description |
We propose to use the unique viewing geometry of the Galactic spiral arm tangents , which provide an ideal environment for studying the effects of density waves on spiral structure. We propose a well-sampled map of theC II 1.9 THz line emission along a 15-degree longitude region across the Norma-3kpc arm tangential, which includes the edge of the Perseus Arm. The COBE-FIRAS instrument observed the strongest C II and N II emission along these spiral arm tangencies. The Herschel Open Time Key Project Galactic Observations of Terahertz C+ (GOT C+), also detects the strongest CII emission near these spiral arm tangential directions in its sparsely sampled HIFI survey of CII in the Galactic plane survey. The C II 158-micron line is the strongest infrared line emitted by the ISM and is an excellent tracer and probe of both the diffuse gases in the cold neutral medium (CNM) and the warm ionized medium (WIM). Furthermore, as demonstrated in the GOTC+ results, C II is an efficient tracer of the dark H2 gas in the ISM that is not traced by CO or HI observations. Thus, taking advantage of the long path lengths through the spiral arm across the tangencies, we can use the C II emission to trace and characterize the diffuse atomic and ionized gas as well as the diffuse H2 molecular gas in cloud transitions from HI to H2 and C+ to C and CO, throughout the ISM. The main goal of our proposal is to use the well sampled (at arcmin scale) C II to study these gas components of the ISM in the spiral-arm, and inter-arm regions, to constrain models of the spiral structure and to understand the influence of spiral density waves on the Galactic gas and the dynamical interaction between the different components. The proposed HIFI observations will consist of OTF 15 degree longitude scans and one 2-degree latitude scan sampled every 40arcsec across the Norma- 3kpc Perseus Spiral tangency. |
