J/A+A/710/A301 HyGAL. III. Absorption lines (Kim+, 2026)
HyGAL: Characterizing the Galactic ISM with observations of hydrides and other
small molecules. III. The absorption lines of [O I], CH, and OH.
Kim W.-J., Jacob A.M., Neufeld D.A., Schilke P., Wiesemeyer H., Gerin M.,
Wolfire M.G., Ossenkopf-Okada V., Valdivia V., Falgarone E., Lis D.C.,
Bialy S., Rugel M.R., Sanchez-Monge A., Busch M., Moeller T., Wyrowski F.,
Seifried D., Menten K.M., Saintonge A.
<Astron. Astrophys. 710, A301 (2026)>
=2026A&A...710A.301K 2026A&A...710A.301K (SIMBAD/NED BibCode)
ADC_Keywords: Interstellar medium ; Molecular clouds ; Spectroscopy ; Infrared
Keywords: astrochemistry - techniques: spectroscopic - ISM: abundances
Abstract:
The HyGAL Stratospheric Observatory for Infrared Astronomy (SOFIA)
legacy program aims at characterizing the interstellar medium in the
Milky Way using hydrides, [CII], and [OI] absorption lines with the
2.7m SOFIA telescope toward twenty-five submillimeter-bright Galactic
star-forming regions. As part of HyGAL, we investigated correlations
among the known H2 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, [OI], 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 H2 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(Htotal), is (3.09±0.64)x10-4, slightly below the
elemental solar value but consistent with the previous observations
measuring gas-phase abundances. We also find that N(HI) decreases
toward the regions where the molecular fraction exceeds fNH2∼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 HI and hydride ions, highlighting density variations in the
diffuse-to-translucent ISM along different lines of sight.
Description:
Tables for derived column densities of molecular and atomic species
studied in this work. One table is for column densities over VI(A)
velocity intervals. The other table is for column densities integrated
over VI(M) velocity intervals.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea1.dat 167 25 The summary of HyGAL source information
tablec1.dat 237 74 Column densities over VI(A)
tablec2.dat 237 101 Column densities over VI(M)
sp/* . 94 Individual spectra
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Byte-by-byte Description of file: tablea1.dat
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Bytes Format Units Label Explanations
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1- 17 A17 --- Name Source name
19- 20 I2 h RAh Right Ascension (J2000)
22- 23 I2 min RAm Right Ascension (J2000)
25- 28 F4.1 s RAs Right Ascension (J2000)
30 A1 --- DE- Declination sign (J2000)
31- 32 I2 deg DEd Declination (J2000)
34- 35 I2 arcmin DEm Declination (J2000)
37- 40 F4.1 arcsec DEs Declination (J2000)
42- 48 F7.3 deg GLON Galactic longitude
50- 55 F6.3 deg GLAT Galactic latitude
57- 61 F5.1 km/s vLSR LSR velocity
63- 66 F4.1 kpc d Distance or lower value of distance interval
67 A1 --- --- [-]
68- 71 F4.1 kpc d2 ? Upper value of distance interval
73- 75 F3.1 kpc Rgal Galactodentric distance or lower value of
Galactodentric radius interval
76 A1 --- --- [-]
77- 79 F3.1 kpc Rgal2 ? Upper value of Galactodentric distance
interval
81- 99 A19 --- spCCH Name of the CCH spectrum file
in subdirectory sp
101-115 A15 --- spCH Name of the CH spectrum file
in subdirectory sp
117-136 A20 --- spHCO+ Name of the HCO+ spectrum file
in subdirectory sp
138-151 A14 --- spOH Name of the OH spectrum file
in subdirectory sp
153-167 A15 --- spOI Name of the OI spectrum file
in subdirectory sp
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Byte-by-byte Description of file: tablec1.dat tablec2.dat
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Bytes Format Units Label Explanations
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1- 17 A17 --- Name Source name
19- 23 F5.1 km/s Vmin Minimum velocity
25- 29 F5.1 km/s Vmax Maximum velocity
32- 36 F5.2 kpc Rgalmin ?=- Minimum Galactocentric distance
38- 42 F5.2 kpc Rgalmax ?=- Maximum Galactocentric distance
44- 51 E8.3 cm-2 N(CH) ?=- CH column density
53- 60 E8.3 cm-2 e_N(CH) ?=- N(CH) lower limit sigma
62- 69 E8.3 cm-2 E_N(CH) ?=- N(CH) upper limit sigma
71- 78 E8.3 cm-2 N(OH) ?=- OH column density
80- 87 E8.3 cm-2 e_N(OH) ?=- N(OH) lower limit sigma
89- 96 E8.3 cm-2 E_N(OH) ?=- N(OH) upper limit sigma
98-105 E8.3 cm-2 N(HCO+) ?=- HCO+ column density
107-114 E8.3 cm-2 e_N(HCO+) ?=- N(HCO+) lower limit sigma
116-123 E8.3 cm-2 E_N(HCO+) ?=- N(HCO+) upper limit sigma
125-132 E8.3 cm-2 N(CCH) ?=- CCH column density
134-141 E8.3 cm-2 e_N(CCH) ?=- N(CCH) lower limit sigma
143-150 E8.3 cm-2 E_N(CCH) ?=- N(CCH) upper limit sigma
152-159 E8.3 cm-2 N(O) ?=- Oxygen column density
161-168 E8.3 cm-2 e_N(O) ?=- N(O) lower limit sigma
170-177 E8.3 cm-2 E_N(O) ?=- N(O) upper limit sigma
179-186 E8.3 cm-2 N(H2) ?=- H2 column density
188-195 E8.3 cm-2 e_N(H2) ?=- N(H2) lower limit sigma
197-204 E8.3 cm-2 E_N(H2) ?=- N(H2) upper limit sigma
206-213 E8.3 cm-2 N(HI) ?=- HI column density
215-222 E8.3 cm-2 e_N(HI) ?=- N(HI) lower limit sigma
224-231 E8.3 cm-2 E_N(HI) ?=- N(HI) upper limit sigma
233-237 F5.3 --- fH2 ?=- Molecular gas fraction (fH2)
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Byte-by-byte Description of file(#): sp/*
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Bytes Format Units Label Explanations
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1- 21 E21.18 km/s vLSR LSR velocity
22- 44 E23.30 K Ta* Antenna temperature
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Acknowledgements:
Won-Ju Kim,
License: CC-BY-4.0
References:
Jacob et al., Paper I 2022ApJ...930..141J 2022ApJ...930..141J
Kim et al., Paper II 2023A&A...670A.111K 2023A&A...670A.111K
(End) Won-Ju Kim [MPIfR, Germany], Patricia Vannier [CDS] 01-Jun-2026