HyGAL: Characterizing the Galactic ISM with observations of hydrides and other small molecules
Year: 2026
Type: article
Abstract: The HyGAL Stratospheric Observatory for Infrared Astronomy (SOFIA) legacy program aims to characterize the interstellar medium (ISM) in the Milky Way using hydrides, [C II ], and [O I ] absorption lines with the 2.7 m SOFIA telescope toward twenty-five submillimeter-bright Galactic star-forming regions. As part of HyGAL, we investigated correlations among the known H 2 tracers – CH and OH from SOFIA observations, and HCO + and CCH from ancillary absorption line data from ground-based telescopes. We also examined the abundance variation of neutral atomic oxygen, [O I ], observed in absorption. CH, OH, HCO + , and CCH all exhibit strong mutual correlations. OH in particular shows tight correlations with HCO + and CCH, reflecting their linked chemical and physical pathways. Column density ratios among these H 2 tracers are consistent with previous measurements in local diffuse clouds and remain uniform across Galactic environments and velocity intervals. The gas phase oxygen abundance relative to total hydrogen, ⟨ X (O)⟩ = N (O)/ N (H total ), is (3.09 ± 0.64) × 10 −4 , which is slightly below the elemental solar value but consistent with the previous observations measuring gas-phase abundances. We also find that N (H I ) decreases toward the regions where the molecular fraction exceeds f H 2 N ~ 0.5, marking the onset of the molecular phase. While the atomic oxygen abundance remains roughly constant, the abundances of OH, HCO + , and CCH increase with the molecular fraction. Gas traced by the HCO + absorption corresponds to higher molecular fractions than that traced by H I and hydride ions, highlighting density variations in the diffuse-to-translucent ISM along different lines of sight.
Source: Astronomy and Astrophysics
Institutions: University of Cologne, Max Planck Institute for Radio Astronomy, Johns Hopkins University, Centre National de la Recherche Scientifique, Université Paris Sciences et Lettres, Observatoire de Paris, Laboratoire d’Etudes du Rayonnement et de la Matière en Astrophysique et Atmosphères, University of Maryland, College Park, Nagoya University, École Normale Supérieure - PSL, Sorbonne Université, Laboratoire de Physique de l'Ecole Normale Supérieure, Jet Propulsion Laboratory, Technion – Israel Institute of Technology, Deutsches Zentrum für Astrophysik, Institute of Space Sciences, Institut d'Estudis Espacials de Catalunya, National Radio Astronomy Observatory
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,865
Funders: National Science Foundation, National Aeronautics and Space Administration, California Institute of Technology, Universities Space Research Association, European Commission, Deutsche Forschungsgemeinschaft, Israel Science Foundation, Universität Stuttgart, Jet Propulsion Laboratory, Agencia Estatal de Investigación
Awards: G1134310069, G1175115578, G2048821276, G210041235, G2262748287, G2469010403, G2601397997, G2650114919, G2676722403, G2756433129, G300979063, G34346518, G3480869486, G3648211085, G3681454997, G4042783231, G425668148, Collaborative Research: Greater New York (NY) Oxidant, Trace gas, Halogen, and Aerosol Airborne Mission (GOTHAAM), G4442677895, G4526231891, G464105488, G5080475149, G5195188938, G5274026675, G59165571, G617610489, G7084143925, G7515182164, G7823692103, G830101876
OpenAlex ID: W7161690944