Mapping the aliphatic hydrocarbon content of interstellar dust in the Galactic plane

TL;DR

This study introduces a new spectrophotometric imaging method to map aliphatic hydrocarbons in interstellar dust, revealing their distribution and abundance in the Galactic plane and center.

Contribution

The paper presents a novel observational technique for mapping aliphatic hydrocarbons in interstellar dust using the 3.4 μm absorption feature, extending previous measurements to new Galactic regions.

Findings

Optical depths are relatively uniform across observed fields.

Weak trend of increasing optical depth towards Galactic center.

Aliphatic hydrocarbons constitute 10-20% of interstellar carbon.

Abstract

We implement a new observational method for mapping the aliphatic hydrocarbon content in the solid phase in our Galaxy, based on spectrophotometric imaging of the 3.4 $\mu$m absorption feature from interstellar dust. We previously demonstrated this method in a field including the Galactic Centre cluster. We applied the method to a new field in the Galactic centre where the 3.4 $\mu$m absorption feature has not been previously measured and we extended the measurements to a field in the Galactic plane to sample the diffuse local interstellar medium, where the 3.4 $\mu$m absorption feature has been previously measured. We have analysed 3.4 $\mu$m optical depth and aliphatic hydrocarbon column density maps for these fields. Optical depths are found to be reasonably uniform in each field, without large source-to-source variations. There is, however, a weak trend towards increasing optical depth in a direction towards $b=0^{\circ}$ in the Galactic centre. The mean value of column densities and abundances for aliphatic hydrocarbon were found to be about several $\rm \times 10^{18} \, cm^{-2}$ and several tens $\times 10^{-6}$, respectively for the new sightlines in the Galactic plane. We conclude that at least 10-20% of the carbon in the Galactic plane lies in aliphatic form.