The Excitation State of Galactic Diffuse Molecular Gas, Investigated with ALMA Observations of Multi-transition Absorption Lines

We report Atacama Large Millimeter/submillimeter Array (ALMA) observations of 243-263 GHz absorption lines from Galactic diffuse molecular gas seen against three extragalactic radio-loud objects, J1717-337, J1625-254, and NRAO530. No significant absorption features were detected in our new sensitive ALMA Band 6 (λ 1.2 mm) spectra (1 2-4 mJy beam−1 with a velocity resolution of 1 km s−1), despite the fact that the obtained spectra cover the line frequencies of c-C3H2, CS, H13CN, HCO, H13CO+, SiO, and C2H, and that the lower transition absorption lines of these molecular species have been detected in ALMA Band 3 (λ 3 mm) observations toward these three Galactic molecular absorption systems. By combining the upper limits of the higher transitions that we obtain in this study and the archival detections of the lower transitions, we constrain the excitation temperatures of these species in the Galactic diffuse medium to be below 10 K, in spite of their PDR-like chemical states, where elevated column density ratios of HCO to H13CO+ are previously reported. These constraints are in line with the widely accepted view of the diffuse molecular clouds, where the molecular excitation is almost in equilibrium with the cosmic microwave background, although its kinetic temperature can be a few tens of Kelvin. Molecular abundance patterns seem to be similar in the Galactic diffuse gas and nearby active galaxies observed with a kpc-scale beam, suggesting the importance of observations with a parsec-scale resolution, which can resolve individual molecular clouds.