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

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: vthiel@mpifr-bonn.mpg.de
² Departments of Chemistry and Astronomy, University of Virginia, Charlottesville, VA 22904, USA
³ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany

Received: 3 July 2017
Accepted: 15 August 2017

Abstract

Context. 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.

Aims. 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).

Methods. 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 H ii 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.

Results. 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.

Conclusions. 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.