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

Abstract

We investigate the correlation between extinction and H I 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 H I data from the Galactic Arecibo L-band Feed Array H I survey and CO data from the Planck mission, to constrain the values of dust-to-gas ratio DGR = A_V/N(H) and CO-to-H₂ conversion factor X_CO = N(H₂)/W_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 ± 0.01) × 10^-22 mag cm² and X_CO = (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) ∼eq 2.2 × 10^{21} (A_V - A^c_V) cm^{-2}. The mass fraction of DG is found to be f_DG ∼ 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.