D. Paradis, K. Dobashi, T. Shimoikura, A. Kawamura, T. Onishi, Y. Fukui, J. -P. Bernard
When modeling infrared or gamma-ray data as a linear combination of observed gas tracers, an excess of emission has been pointed out with respect to expectations from known neutral and atomic gas as traced by HI and CO measurements respectively. This excess could result from an additional gas component. This gas, called "dark-gas" (DG) has been observed in our Galaxy, as well as in the Magellanic Clouds. In this paper, we investigate for the first time the correlation between visible extinction (Av) data and the gas tracers on large scales in the solar neighborhood. Our work focuses on the solar neighborhood ($|b|>10\degr$), as well as the inner and outer Galaxy and on four individual regions: Taurus, Orion, Cepheus-Polaris and Aquila-Ophiuchus. Thanks to the recent production of an all-sky Av map, we first perform the correlation between Av and HI and CO emission over the most diffuse regions, to derive the optimal (Av/NH)^(ref) ratio. We then iterate the analysis over the entire regions to estimate the CO-to-H2 conversion factor as well as the DG mass fraction. The average extinction to gas column density ratio in the solar neighborhood is found to be (Av/NH)^(ref)=6.53 10^(-22) mag. cm^2, with significant differences between the inner and outer Galaxy. We derive an average XCO value of 1.67 10^(20) H2 cm^(-2)/(K km s^(-1)). In the solar neighborhood, the gas mass in the dark component is found to be 19% relative to that in the atomic component and 164$%$ relative to the one traced by CO. These results are compatible with the recent analysis using Planck data within the uncertainties of our measurements. We estimate the fraction of dark gas to the total molecular gas to be 0.62 in the solar neighborhood. The HI-to-H2 and H2-to-CO transitions appear for Av $\simeq$0.2 and Av$\simeq1.5$, respectively, in agreement with theoretical models of dark-H2 gas.
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http://arxiv.org/abs/1205.3384
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