Galactic HCO+ absorption toward compact extragalactic radio sources

• Published in: Astrophysics and space science Vol. 363; no. 7; pp. 1 - 9
• Main Authors: Park, Geumsook, Koo, Bon-Chul, Kim, Kee-Tae, Byun, Do-Young, Heiles, Carl E.
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Netherlands 01.07.2018; Springer Nature B.V
• Subjects: Absorption; Astrobiology; Astronomy; Astrophysics; Astrophysics and Astroparticles; Carbon monoxide emissions; Cosmology; Emission; Emissions; Extinction; Extragalactic radio sources; Far infrared radiation; Hydrogen; Hydrogen storage; Millimeter waves; Molecular chains; Molecular gases; Observations and Techniques; Original Article; Physics; Physics and Astronomy; Space Exploration and Astronautics; Space Sciences (including Extraterrestrial Physics; ISM: clouds; Radio lines: ISM
• DOI: 10.1007/s10509-018-3372-4
• ISSN: 0004-640X

As part of the search for the “dark molecular gas (DMG),” we report on the results of HCO + J = 1 – 0 absorption observations toward nine bright extragalactic millimeter wave continuum sources. The extragalactic sources are at high Galactic latitudes ( | b | > 10 ∘ ) and seen at small extinction ( E ( B − V ) ≲ 0.1  mag). We have detected the HCO + absorption lines toward two sources, B0838+133 and B2251+158. The absorption toward B2251+158 was previously reported, while the absorption toward B0838+133 is a new detection. We derive hydrogen column densities or their upper limits toward the nine sources from our observations and compare them to those expected from CO line emission and far-infrared dust continuum emission. Toward the seven sources with no HCO + detection, CO emission has not been detected, either. Thus the sight lines are likely to be filled with almost pure atomic gas. Toward the two sources with HCO + detection, CO emission has been also detected. Comparison of the H 2 column densities from HCO + absorption and CO emission suggests a non-negligible amount of DMG toward B0838+133.