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. (arXiv:1909.03754v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Hayashi_K/0/1/0/all/0/1">Katsuhiro Hayashi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Mizuno_T/0/1/0/all/0/1">Tsunefumi Mizuno</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fukui_Y/0/1/0/all/0/1">Yasuo Fukui</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Okamoto_R/0/1/0/all/0/1">Ryuji Okamoto</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Yamamoto_H/0/1/0/all/0/1">Hiroaki Yamamoto</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hidaka_N/0/1/0/all/0/1">Naoya Hidaka</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Okumura_A/0/1/0/all/0/1">Akira Okumura</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tajima_H/0/1/0/all/0/1">Hiroyasu Tajima</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sano_H/0/1/0/all/0/1">Hidetoshi Sano</a>
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$$sim$1.4, gives the best fit to the $gamma$-ray data. This nonlinear
relation may indicate dust evolution effects across the different gas phases.
Using the best-fit NH model, we derived the CO-to-H2 conversion factor (XCO)
and gas mass, taking into account uncertainties of the NH model. The value of
XCO is found to be (0.63-0.76) $times$10$^{20}$ cm$^{-2}$ K$^{-1}$ km$^{-1}$
s, which is consistent with that of a recent $gamma$-ray study of the
Chamaeleon region. The total gas mass is estimated to be (6.0-7.3) $times$
10$^{4}$ Msun, of which the mass of additional gas not traced by standard HI or
CO line surveys is 20-40%. The additional gas amounts to 30-60% of the gas mass
estimated in the case of optically thin HI and has 5-7 times greater mass than
the molecular gas traced by CO. Possible origins of the additional gas are
discussed based on scenarios of optically thick HI and CO-dark H2. We also
derived the $gamma$-ray emissivity spectrum, which is consistent with the
local HI emissivity derived from LAT data within the systematic uncertainty of
$sim$20%
We report a Fermi-LAT $gamma$-ray analysis for the Chamaeleon
molecular-cloud complex using a total column density (NH) model based on the
dust optical depth at 353 GHz ($tau_{353}$) with the Planck thermal dust
emission model. Gamma rays with energy from 250 MeV to 100 GeV are fitted with
the NH model as a function of $tau_{353}$, NH $propto$
$tau_{353}^{1/alpha}$ ($alpha$ $geq$ 1.0), to explicitly take into account
a possible nonlinear $tau_{353}$/NH ratio. We found that a nonlinear relation,
$alpha$$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%
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