Dust-to-gas ratio, XCO factor and CO-dark gas in the Galactic anticentre: An observational study

Abstract

We investigate the correlation between extinction and HI and CO emission at intermediate and high Galactic latitudes (|b| > 10°) within the footprint of the Xuyi Schmidt Telescope Photometric Survey of the Galactic anticentre (XSTPS-GAC) on small and large scales. In Paper I, we present a three-dimensional (3D) dust extinction map within the footprint of XSTPS-GAC, covering a sky area of over 6000 deg2 at a spatial angular resolution of 6 arcmin. In the current work, the map is combined with data from gas tracers, including HI data from the Galactic Arecibo L-band Feed Array HI survey and CO data from the Planck mission, to constrain the values of dust-to-gas ratio DGR = AV/N(H) and CO-to-H2 conversion factor XCO = N(H2)/WCO 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 ± 0.01) × 10-22 mag cm2 and XCO = (1.72 ± 0.03) × 1020 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(DG) ≃ 2.2 × 1021(AV - AVc) cm-2. The mass fraction of DG is found to be fDG ~ 0.55 towards 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 H2 cloud models by Wolfire et al.