2012A&A...543A.103P

When modeling infrared or γ-ray data as a linear combination of observed gas tracers, excess emission has been detected compared to expectations from known neutral and molecular gas traced by HI and CO measurements, respectively. This excess might correspond to additional gas component. This so-called ``dark gas'' (DG) has been observed in our Galaxy, as well as the Magellanic Clouds. For the first time, we investigate the correlation between visible extinction (A_V) data and gas tracers on large scales in the solar neighborhood, to detect DG and to verify our compatibility with previous studies. Our work focuses on both the solar neighborhood (|b|>10°), and the inner and outer Galaxy, as well as on four individual regions: Taurus, Orion, Cepheus-Polaris, and Aquila-Ophiuchus. Thanks to the recent production of an all-sky A_V map, we first perform the correlation between A_V and both HI and CO emission over the most diffuse regions (with low-to-intermediate gas column densities), to derive the optimal (A_V/N_H)^ref ratio. We then iterate the analysis over the entire regions (including low and high gas column densities) 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 (A_V/N_H )^ref=6.53x10^–22mag.cm², with significant differences between the inner and outer Galaxy, of about 60%. We derive an average value of the CO-to-H₂ conversion factor of X_CO=1.67x10²⁰H₂/cm2/(K.km/s), with significant variations between nearby clouds. 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 a recent analysis of Planck data within the uncertainties of our measurements. We estimate the ratio of dark gas to total molecular gas to be 0.62 in the solar neighborhood. The HI-to-H₂ and H₂-to-CO transitions appear for A_V≃0.2mag and A_V≃1.5mag, respectively, in agreement with theoretical models of dark-H₂ gas.