Fermi-LAT gamma-ray study of the interstellar medium and cosmic rays in the Chamaeleon Molecular-Cloud Complex: A look at the dark gas as optically thick HI

We report a Fermi-LAT \(\gamma\)-ray analysis for the Chamaeleon molecular-cloud complex using a total column density (NH) model based on the dust optical depth at 353 GHz (\(\tau_{353}\)) with the Planck thermal dust emission model. Gamma rays with energy from 250 MeV to 100 GeV are fitted with the...

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Bibliographic Details
Published in:arXiv.org 2019-09
Main Authors: Hayashi, Katsuhiro, Mizuno, Tsunefumi, Fukui, Yasuo, Okamoto, Ryuji, Yamamoto, Hiroaki, Hidaka, Naoya, Okumura, Akira, Tajima, Hiroyasu, Sano, Hidetoshi
Format: Article
Language:English
Subjects:
Cosmic rays
Dust
Emissivity
Gamma rays
Interstellar chemistry
Interstellar matter
Molecular gases
Optical thickness
Uncertainty
Vapor phases
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Summary:We report a Fermi-LAT \(\gamma\)-ray analysis for the Chamaeleon molecular-cloud complex using a total column density (NH) model based on the dust optical depth at 353 GHz (\(\tau_{353}\)) with the Planck thermal dust emission model. Gamma rays with energy from 250 MeV to 100 GeV are fitted with the NH model as a function of \(\tau_{353}\), NH \(\propto\) \(\tau_{353}^{1/\alpha}\) (\(\alpha\) \(\geq\) 1.0), to explicitly take into account a possible nonlinear \(\tau_{353}\)/NH ratio. We found that a nonlinear relation, $\alpha$$\sim\(1.4, gives the best fit to the \)\gamma\(-ray data. This nonlinear relation may indicate dust evolution effects across the different gas phases. Using the best-fit NH model, we derived the CO-to-H2 conversion factor (XCO) and gas mass, taking into account uncertainties of the NH model. The value of XCO is found to be (0.63-0.76) \)\times\(10\)^{20}\( cm\)^{-2}\( K\)^{-1}\( km\)^{-1}\( s, which is consistent with that of a recent \)\gamma\(-ray study of the Chamaeleon region. The total gas mass is estimated to be (6.0-7.3) \)\times\( 10\)^{4}\( Msun, of which the mass of additional gas not traced by standard HI or CO line surveys is 20-40%. The additional gas amounts to 30-60% of the gas mass estimated in the case of optically thin HI and has 5-7 times greater mass than the molecular gas traced by CO. Possible origins of the additional gas are discussed based on scenarios of optically thick HI and CO-dark H2. We also derived the \)\gamma\(-ray emissivity spectrum, which is consistent with the local HI emissivity derived from LAT data within the systematic uncertainty of \)\sim$20%
ISSN:2331-8422
DOI:10.48550/arxiv.1909.03754