New Detections of Galactic Molecular Absorption Systems toward ALMA Calibrator Sources

TL;DR

This study reports new detections of Galactic molecular absorption lines toward ALMA calibrator sources, revealing insights into the diffuse interstellar medium's chemistry and demonstrating the potential of calibrators for ISM research.

Contribution

First detection of multiple Galactic molecular absorption systems toward ALMA calibrators, including HCO lines, expanding the understanding of diffuse ISM chemistry.

Findings

Detection of three new Galactic absorption systems.

HCO absorption lines nearly double existing samples.

High HCO/H13CO+ ratios indicate photodissociation regions.

Abstract

We report on Atacama Large Millimeter/submillimeter Array (ALMA) detections of molecular absorption lines in Bands 3, 6 and 7 toward four radio-loud quasars, which were observed as the bandpass and complex gain calibrators. The absorption systems, three of which are newly detected, are found to be Galactic origin. Moreover, HCO absorption lines toward two objects are detected, which almost doubles the number of HCO absorption samples in the Galactic diffuse medium. In addition, high HCO to H13CO+ column density ratios are found, suggesting that the interstellar media (ISM) observed toward the two calibrators are in photodissociation regions, which observationally illustrates the chemistry of diffuse ISM driven by ultraviolet (UV) radiation. These results demonstrate that calibrators in the ALMA Archive are potential sources for the quest for new absorption systems and for detailed investigation of the nature of the ISM.