HyGAL: Characterizing the Galactic ISM with observations of hydrides and other small molecules
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 N H2 ∼ 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.
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BibTeX
@article{kim2026hygal,
title = {{HyGAL: Characterizing the Galactic ISM with observations of hydrides and other small molecules}},
author = {W-J Kim and A M Jacob and D A Neufeld and P Schilke and H Wiesemeyer and M Gerin and M G Wolfire and V Ossenkopf-Okada and V Valdivia and E Falgarone and D C Lis and S Bialy and M R Rugel and Á Sánchez-Monge and M Busch and T Möller and F Wyrowski and D Seifried and K M Menten and A Saintonge},
journal = {Astronomy \& Astrophysics - A\&A},
year = {2026},
month = sep,
doi = {10.1051/0004-6361/202658863},
url = {https://hal.science/hal-05753723v1},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - HyGAL: Characterizing the Galactic ISM with observations of hydrides and other small molecules
AU - W-J Kim
AU - A M Jacob
AU - D A Neufeld
AU - P Schilke
AU - H Wiesemeyer
AU - M Gerin
AU - M G Wolfire
AU - V Ossenkopf-Okada
AU - V Valdivia
AU - E Falgarone
AU - D C Lis
AU - S Bialy
AU - M R Rugel
AU - Á Sánchez-Monge
AU - M Busch
AU - T Möller
AU - F Wyrowski
AU - D Seifried
AU - K M Menten
AU - A Saintonge
PY - 2026
DA - 2026/09/17
JO - Astronomy & Astrophysics - A&A
DO - 10.1051/0004-6361/202658863
UR - https://hal.science/hal-05753723v1
AB - 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 N H2 ∼ 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.
SN - 0004-6361
SN - 1432-0746
ER -