The CO-to-H2 conversion factor of Galactic giant molecular clouds using CO isotopologues: high-resolution XCO maps

We investigated the correlation between intensities of the ¹²CO and ¹³CO (J = 1-0) lines towards the Galactic giant molecular clouds (GMCs) W51A, W33, N35-N36 complex, W49A, M17SW, G12.02-00.03, W43, and M16 using the FUGIN (FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope) CO line data. All the GMCs show intensity saturation in the ¹²CO line when the brightness temperature of ¹³CO is higher than a threshold temperature of about ~5 K. We obtained high-resolution (~20 arcsec) distribution maps of the X_CO factor (X_CO,iso) in individual GMCs using correlation diagrams of the CO isotopologues. It is shown that X_CO,iso is variable in each GMC within the range of X_CO,iso ~ (0.9-5) × 10²⁰ cm^-2 (K km s^-1)^-1. Despite the variability in the GMCs, the average value among the GMCs is found to be nearly constant at X_CO,iso = (2.17 ± 0.27) × 10²⁰ cm^-2 (K km s^-1)^-1, which is consistent with that from previous studies in the Milky Way.