Origin and z-distribution of Galactic diffuse [C II] emission⋆
Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
Received: 6 June 2014
Accepted: 4 September 2014
Context. The [C ii] emission is an important probe of star formation in the Galaxy and in external galaxies. The GOT C+ survey and its follow up observations of spectrally resolved 1.9 THz [C ii] emission using Herschel HIFI provides the data needed to quantify the Galactic interstellar [C ii] gas components as tracers of star formation.
Aims. We determine the source of the diffuse [C ii] emission by studying its spatial (radial and vertical) distributions by separating and evaluating the fractions of [C ii] and CO emissions in the Galactic ISM gas components.
Methods. We used the HIFI [C ii] Galactic survey (GOT C+), along with ancillary H i, 12CO, 13CO, and C18O data toward 354 lines of sight, and several HIFI [C ii] and [C i] position-velocity maps. We quantified the emission in each spectral line profile by evaluating the intensities in 3 km s-1 wide velocity bins, “spaxels”. Using the detection of [C ii] with CO or [C i], we separated the dense and diffuse gas components. We derived 2D Galactic disk maps using the spaxel velocities for kinematic distances. We separated the warm and cold H2 gases by comparing CO emissions with and without associated [C ii].
Results. We find evidence of widespread diffuse [C ii] emission with a z-scale distribution larger than that for the total [C ii] or CO. The diffuse [C ii] emission consists of (i) diffuse molecular (CO-faint) H2 clouds and (ii) diffuse H i clouds and/or WIM. In the inner Galaxy we find a lack of [C ii] detections in a majority (~62%) of H i spaxels and show that the diffuse component primarily comes from the WIM (~21%) and that the H i gas is not a major contributor to the diffuse component (~6%). The warm-H2 radial profile shows an excess in the range 4 to 7 kpc, consistent with enhanced star formation there.
Conclusions. We derive, for the first time, the 2D [C ii] spatial distribution in the plane and the z-distributions of the individual [C ii] gas component. From the GOT C+ detections we estimate the fractional [C ii] emission tracing (i) H2 gas in dense and diffuse molecular clouds as ~48% and ~14%, respectively, (ii) in the H i gas ~18%, and (iii) in the WIM ~21%. Including non-detections from H i increases the [C ii] in H i to ~27%. The z-scale distributions FWHM from smallest to largest are [C ii] sources with CO, ~130 pc, (CO-faint) diffuse H2 gas, ~200 pc, and the diffuse H i and WIM, ~330 pc. When combined with [C ii], CO observations probe the warm-H2 gas, tracing star formation.
Key words: ISM: structure / Galaxy: structure
© ESO, 2014