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 $DGR=A_V/N({ \rm H})$ and CO-to-$\rm H_2$ conversion factor $X_{ \rm CO}=N({ \rm H_2})/W_{ \rm CO}$ 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 $DGR=(4.15\pm0.01) \times 10^{ -22}$\,$\rm mag\,cm^{ 2}$ and $X_{ \rm CO}=(1.72 \pm 0.03) \times 10^{ 20}$\,$\rm cm^{ -2}\,(K\,km\,s^{ -1})^{ -1}$. 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: $N({ \rm DG}) \simeq 2.2 \times 10^{ 21} (A_V - A^{ c}_{ V})\,\rm cm^{ -2}$. The mass fraction of DG is found to be $f_{ \rm DG}\sim 0.55$ 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 $\rm H_2$ cloud models by Wolfire et al.
Categories: astro-ph.GA