Simple hydrides (OH and CH) trace the dark molecular gas

Di Li, Ningyu Tang, Hiep Nguyen, J. R. Dawson, Carl Heiles, Pei Wang, PRIMO collaboration

    Research output: Chapter in Book/Report/Conference proceedingConference proceeding contributionpeer-review


    Emission lines from CO and Hi are the standard tracers of molecular and atomic interstellar medium, respectively. In the past two decades, a consensus has formed that a substantial fraction of Galactic molecular gas evades detection by these two tracers, thus giving rise to the empirical concept of dark molecular gas (DMG). Largely based on the experience and evidence garnered from the Arecibo Millennium survey, we have formed an international consortium, the Pacific Rim Interstellar Matter Observers (PRIMO), to pursue alternative tracers of DMG, particularly absorption against background radio sources (quasars). PRIMO have carried out observing programs at Arecibo, JVLA, Delingha 13.7m, ATCA and ALMA, among others. Our observations reveal abundant hydrides, namely OH and CH, in DMG clouds. The historical difficulty of mapping OH can be explained by the measured OH excitation temperature f(TexOH) ∝1√2πσexp[-(ln(TexOH)-ln(3.4\K))2/(2σ2)]], which is a modified log-normal function peaking close to the numerical value of the L-band Galactic continuum background (synchrotron + CMB). Both OH and CH are shown to be better tracers of molecular hydrogen than CO, particularly in the intermediate extinction regions (Av ∼ 0.05-2 magnitude), where DMG dominates.

    Original languageEnglish
    Title of host publicationOrigins : from the protosun to the first steps of life
    Subtitle of host publicationProceedings of the 345th Symposium of the International Astronomical Union Held In Vienna, Austria 20–23 August, 2018
    EditorsBruce G. Elmegreen, L. Viktor Tóth, Manuel Güdel
    Place of PublicationCambridge
    PublisherCambridge University Press (CUP)
    Number of pages4
    ISBN (Print)9781108471602
    Publication statusPublished - 2019
    Event345th Symposium of the International Astronomical Union - Vienna, Austria
    Duration: 20 Aug 201823 Aug 2018

    Publication series

    NameProceedings of the International Astronomical Union
    PublisherCambridge University Press
    ISSN (Print)1743-9213


    Conference345th Symposium of the International Astronomical Union


    • astrochemistry
    • Galaxy: abundances
    • ISM: atoms
    • ISM: molecules
    • radio lines: ISM


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