Complex organic molecules in diffuse clouds along the line of sight to Sgr B2

TL;DR

This study detects complex organic molecules in diffuse interstellar clouds using ALMA, revealing chemical similarities across different galaxies and suggesting cyclical cloud processes contribute to molecular complexity.

Contribution

First detection of several COMs in Galactic diffuse clouds with high-resolution ALMA data, showing chemical similarities with distant galaxies.

Findings

Detection of CH3OH, CH3CN, CH3CHO, HC3N, NH2CHO in Galactic diffuse clouds

Chemical composition similar to that in a z=0.89 spiral galaxy

COMs may form through cyclical cloud contraction and expansion processes

Abstract

Up to now, mostly relatively simple molecules have been detected in interstellar diffuse molecular clouds in our galaxy, but more complex species have been reported in the diffuse/translucent medium of a z = 0.89 spiral galaxy. We aim at searching for complex organic molecules (COMs) in diffuse molecular clouds along the line of sight to Sgr B2(N), taking advantage of the high sensitivity and angular resolution of the Atacama Large Millimeter/submillimeter Array (ALMA). We use data acquired as part of the EMoCA survey performed with ALMA. To analyse the absorption features of the molecules detected towards the ultracompact HII region K4 in Sgr B2(N), we calculate synthetic spectra for these molecules and fit their column densities, line widths, centroid velocities, and excitation temperatures. We report the detection of CH3OH, CH3CN, CH3CHO, HC3N, and NH2CHO in Galactic center (GC) diffuse clouds and CH3OH and CH3CN in a diffuse cloud in the Scutum arm. The chemical composition of one of the diffuse GC clouds is found to be similar to the one of the diffuse/translucent medium of the z=0.89 spiral galaxy. The chemical processes leading to chemical complexity in the diffuse molecular ISM appear to have remained similar since z=0.89. As proposed in previous studies, the presence of COMs in diffuse molecular clouds may result from a cyclical interstellar process of cloud contraction and expansion between diffuse and dense states.