Dust-to-gas ratio, $X_{\mathrm{CO}}$ factor and CO-dark gas in the Galactic anticentre: an observational study

Abstract

We investigate the correlation between extinction and H~{\sc i} and CO emission at intermediate and high Galactic latitudes ($|b|>10\degr$) within the footprint of the Xuyi Schmidt Telescope Photometric Survey of the Galactic anticentre (XSTPS-GAC) on small and large scales. In Paper I (Chen et al. 2014), we present a three-dimensional dust extinction map within the footprint of XSTPS-GAC, covering a sky area of over 6,000\,deg${}^2$ at a spatial angular resolution of 6\,arcmin. In the current work, the map is combined with data from gas tracers, including H~{\sc i} data from the Galactic Arecibo L-band Feed Array H~{\sc i} survey and CO data from the Planck mission, to constrain the values of dust-to-gas ratio $DGR=A_V/N(\mathrm{H})$ and CO-to-${\mathrm{H}}_2$ conversion factor $X_{\mathrm{CO}}=N({\mathrm{H}}_2)/W_{\mathrm{CO}}$ for the entire GAC footprint excluding the Galactic plane, as well as for selected star-forming regions (such as the Orion, Taurus and Perseus clouds) and a region of diffuse gas in the northern Galactic hemisphere. For the whole GAC footprint, we find $DGR=(4.15\pm 0.01)\times 10^{-22}$\,$\mathrm{mag}\mathrm{c}{\mathrm{m}}^2$ and $X_{\mathrm{CO}}=(1.72\pm 0.03)\times 10^{20}$\,$\mathrm{c}{\mathrm{m}}^{-2}(\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1}{)}^{-1}$. We have also investigated the distribution of "CO-dark" gas (DG) within the footprint of GAC and found a linear correlation between the DG column density and the $V$-band extinction: $N(\mathrm{DG})\simeq 2.2\times 10^{21}(A_V-A_V^c)\mathrm{c}{\mathrm{m}}^{-2}$. The mass fraction of DG is found to be $f_{\mathrm{DG}}\sim 0.55$ toward the Galactic anticentre, which is respectively about 23 and 124 per cent of the atomic and CO-traced molecular gas in the same region. This result is consistent with the theoretical work of Papadopoulos et al. but much larger than that expected in the ${\mathrm{H}}_2$ cloud models by Wolfire et al.

11 pages, 7 figures, accepted for publication in MNRAS

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