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

Abstract

ABSTRACT 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 XCO factor (XCO,iso) in individual GMCs using correlation diagrams of the CO isotopologues. It is shown that XCO,iso is variable in each GMC within the range of XCO,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 XCO,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.