2023PASJ...75..743Y

We simultaneously measured the spatially resolved CO-to-H₂ conversion factor (α_CO) and dust-to-gas ratio (DGR) in nearby galaxies on a kiloparsec scale. In this study, we used ¹²CO(J = 1-0) data obtained by the Nobeyama 45 m radio telescope with H I and dust mass surface densities. We obtained the values of global α_CO and DGR in 22 nearby spiral galaxies, with averages of 2.66 ± 1.36 M_⊙ pc^–2 (K km s^–1)^–1 and 0.0052 ± 0.0026, respectively. Furthermore, the radial variations of α_CO and DGR in four barred spiral galaxies (IC 342, NGC 3627, NGC 5236, and NGC 6946) were obtained by dividing them into inner and outer regions with a boundary of 0.2 R₂₅, where R₂₅ is the isophotal radius at 25 mag arcsec^–2 in the B band. The averages of α_CO and DGR in the inner region (≤0.2 R₂₅) are 0.36 ± 0.08 M_⊙ pc^–2 (K km s^–1)^–1 and 0.0199 ± 0.0058, while those in the outer region (>0.2 R₂₅) are 1.49 ± 0.76 M_⊙ pc^–2 (K km s^–1)^–1 and 0.0084 ± 0.0037, respectively. The value of α_CO in the outer region is 2.3 to 5.3 times larger than that of the inner region. When separated into the inner and outer regions, we find that α_CO and DGR correlate with the metallicity and the star formation rate surface density. The value of α_CO derived in this study tends to be smaller than those obtained in previous studies for the Milky Way and nearby star-forming galaxies. This fact can be attributed to our measurements being biased toward the inner region; we measured α_CO at 0.85 and 0.76 times smaller in radius than the previous works for nearby star-forming galaxies and the Milky Way, respectively.