The scale height of gas traced by [C ii] in the Galactic plane

Abstract

Context. The distribution of various interstellar gas components and the pressure in the interstellar medium (ISM) is a result of the interplay of different dynamical mechanisms and energy sources on the gas in the Milky Way. The scale heights of the different gas tracers, such as H i and CO, are a measure of these processes. The scale height of [C ii] emission in the Galactic plane is important for understanding those ISM components not traced by CO or H i.
Aims: We determine the average distribution of [C ii] perpendicular to the plane in the inner Galactic disk and compare it to the distributions of other key gas tracers, such as CO and H i.
Methods: We calculated the vertical, z, distribution of [C ii] in the inner Galactic disk by adopting a model for the emission that combines the latitudinal, b, spectrally unresolved BICE survey, with the spectrally resolved Herschel Galactic plane survey of [C ii] at b = 0°. Our model assumed a Gaussian emissivity distribution vertical to the plane, and related the distribution in z to that of the latitude b using the spectrally resolved [C ii] Herschel survey as the boundary solution for the emissivity at b = 0°.
Results: We find that the distribution of [C ii] perpendicular to the plane has a full-width half-maximum of 172 pc, larger than that of CO, which averages ~110 pc in the inner Galaxy, but smaller than that of H i, ~230 pc, and is offset by - 28 pc.
Conclusions: We explain the difference in distributions of [C ii], CO, and H i as due to [C ii] tracing a mix of ISM components. Models of hydrostatic equilibrium of clouds in the disk predict different scale heights, for the same interstellar pressure. The diffuse molecular clouds with [C ii] but no CO emission likely have a scale height intermediate between the low density atomic hydrogen H i clouds and the dense CO molecular clouds.

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.