A Herschel HIFI View of the Galactic Diffuse Interstellar Medium along the Spiral Arm Tangencies traced by [CII] 158 µm Line Emission
J. L. Pineda, W. D. Langer, K. Willacy, P.F. Goldsmith
We present the distribution and characteristics of the [CII] 158 µm emission in the direction of the Scutum -Centaurus spiral arm tangencies at l= 302º−313º and l = 24º−35º. We focus on these regions as they are known for several distinct anomalies, including excess star counts, and their unique viewing geometry provides an ideal testing ground for studying Galactic spiral structure and the effects of density waves. The [CII] intensities, observed by FIRAS on COBE, show local maxima in these regions. The unprecedented high spatial and velocity resolution of the HIFI [CII] spectra provide new insight by resolving the components over the longest path lengths along the spiral arms. These observations are part of the Herschel Open Time Key Project Galactic Observations of Terahertz C+ (GOTC+), a survey devoted to study the [CII] 158 µm (1.9 THz) line over different conditions in the Galactic plane with the Herschel HIFI instrument. Together with ancillary HI, 12CO and 13CO data, the [CII] emission reveals new details on the distribution and characteristics of the warm, dense and atomic and molecular components, including the so-called “dark H2 gas” of the interstellar medium. The [CII], HI, and CO intensities in well resolved velocity components allow us to map in 3-D the mass fractions of gases in these phases of the ISM, as well as the thermal pressures, and FUV radiation fields. This research was conducted at the Jet Propulsion Laboratory, California Institute of Technology under contract with the National Aeronautics and Space Administration.