Small-scale Physical and Chemical Structure of Diffuse and Translucent Molecular Clouds along the Line of Sight to the Galactic Center

Abstract

Diffuse and translucent molecular clouds constitute a large part of the molecular interstellar medium (ISM) in our Galaxy as well as in other spiral galaxies. In particular, the envelopes of giant molecular clouds (GMCs), which host star forming regions, consist of lower density diffuse and translucent media. Hence, observational studies of diffuse and translucent molecular clouds help to improve our knowledge about their important role in the formation of dense clouds and their interaction with the rest of the ISM.<br /> For this project, we used absorption data from various molecules obtained by the Exploring Molecular Complexity with ALMA (EMoCA) interferometric spectral line survey performed with the Atacama Large Millimeter/submillimeter Array towards the prominent GMC Sagittarius B2 (Sgr B2). This survey allows us to investigate the variations in abundance and isotopic composition for many molecules as a function of galactocentric radius from the region surrounding the Galactic center out to the Solar circle. It also allows us to probe the kinematical properties of individual clouds. This project improves our understanding of the small-scale physical and chemical structure of a sample of diffuse and translucent molecular clouds.<br /> On the basis of data from HCO⁺, we conclude that most line-of-sight clouds towards Sgr B2 are translucent. We also find that CCH and CH are good probes of H₂ in both diffuse and translucent clouds.<br /> The chemical content of diffuse and translucent molecular clouds is similar. We find that the column densities of CH₃OH, SiO, and SO are well correlated and we propose that these molecules trace shocked gas. A second group of correlated species (CCH, HCN, HNC, CS, and CN) likely traces quiescent components.<br /> The spatial structure of the line-of-sight clouds is found to be relatively homogeneous on scales smaller than the extent of the background continuum source (15 arc seconds, that is 0.08 to 0.6 parsec depending on the distance). Our analysis of the column density maps suggests that the driving of clouds’ turbulence is mainly solenoidal, which produces small density contrasts and could explain the spatial homogeneity of these clouds.<br /> For many molecules, we also detect lines from some of their less abundant isotopologues. In particular, we report a direct measurement of the ¹⁴N/¹⁵N isotopic ratio in the envelope of Sgr B2. So far, the Galactic center value of this ratio had been poorly known. The value we determine, about 350, is consistent with that extrapolated from previous measurements at larger galactocentric radii.<br /> We report the detection of four complex organic molecules (COMs) in clouds of the Galactic Center (GC) region and of two COMs in the translucent clouds associated with the Scutum arm of the Galaxy. The COM composition of one of the translucent GC clouds is found to be similar to that of the extremely well-studied diffuse/translucent ISM of a spiral galaxy at redshift z=0.89 seen in absorption in front of PKS 1830–211. This suggests that the processes leading to chemical ISM complexity have remained similar for more than half the age of the Universe. The presence of COMs in the translucent molecular clouds may result from a cyclical interstellar process of cloud contraction and expansion between diffuse and dense states.