Herschel observations of extra-ordinary sources: Detecting spiral arm clouds by CH absorption lines

TL;DR

This study uses Herschel/HIFI to observe CH absorption lines towards Sgr B2(M), revealing complex spiral arm cloud structures with multiple velocity components and indicating CH depletion in denser cloud regions.

Contribution

First high-resolution Herschel observations of CH absorption lines across multiple spiral arm clouds, showing internal cloud structures and variations in CH-H2 relationship at different extinctions.

Findings

Detected 31 CH absorption features with diverse velocities.

Identified spiral arm clouds with multiple velocity components.

Found CH depletion in denser cloud cores.

Abstract

We have observed CH absorption lines ($J=3/2, N=1 \leftarrow J=1/2, N=1$) against the continuum source Sgr~B2(M) using the \textit{Herschel}/HIFI instrument. With the high spectral resolution and wide velocity coverage provided by HIFI, 31 CH absorption features with different radial velocities and line widths are detected and identified. The narrower line width and lower column density clouds show `spiral arm' cloud characteristics, while the absorption component with the broadest line width and highest column density corresponds to the gas from the Sgr~B2 envelope. The observations show that each `spiral arm' harbors multiple velocity components, indicating that the clouds are not uniform and that they have internal structure. This line-of-sight through almost the entire Galaxy offers unique possibilities to study the basic chemistry of simple molecules in diffuse clouds, as a variety of different cloud classes are sampled simultaneously. We find that the linear relationship between CH and H$_2$ column densities found at lower $A_V$ by UV observations does not continue into the range of higher visual extinction. There, the curve flattens, which probably means that CH is depleted in the denser cores of these clouds.