CO Multi-line Imaging of Nearby Galaxies (COMING). XII. CO-to-H${}_2$ Conversion Factor and Dust-to-Gas Ratio

Abstract

We simultaneously measured the spatially-resolved CO-to-H${}_2$ conversion factor ($\hat{I}{\pm }_{\mathrm{CO}}$) and dust-to-gas ratio (DGR) in nearby galaxies on a kiloparsec scale. In this study, we used ${}^{12}$CO($J=1-0$) data obtained by the Nobeyama 45-m radio telescope with HI and dust mass surface densities. We obtained the values of global $\hat{I}{\pm }_{\mathrm{CO}}$ and DGR in 22 nearby spiral galaxies, with averages of $2.66\pm 1.36M_{\odot }{\mathrm{pc}}^{-2}(\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1}{)}^{-1}$ and $0.0052\pm 0.0026$, respectively. Furthermore, the radial variations of $\hat{I}{\pm }_{\mathrm{CO}}$ and DGR in four barred spiral galaxies (IC 342, NGC 3627, NGC 5236, and NGC 6946) were obtained by dividing them into the inner and outer regions with a boundary of $0.2R_{25}$, where $R_{25}$ is the isophotal radius at 25 mag arcsec${}^{-2}$ in the $B$ band. The averages of $\hat{I}{\pm }_{\mathrm{CO}}$ and DGR in the inner region ($\le 0.2R_{25}$) are $0.36\pm 0.08M_{\odot }{\mathrm{pc}}^{-2}(\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1}{)}^{-1}$ and $0.0199\pm 0.0058$, while those in the outer region ($>0.2R_{25}$) are $1.49\pm 0.76M_{\odot }{\mathrm{pc}}^{-2}(\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1}{)}^{-1}$ and $0.0084\pm 0.0037$, respectively. The value of $\hat{I}{\pm }_{\mathrm{CO}}$ 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 $\hat{I}{\pm }_{\mathrm{CO}}$ and DGR correlate with the metallicity and the star formation rate surface density. The value of $\hat{I}{\pm }_{\mathrm{CO}}$ 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 $\hat{I}{\pm }_{\mathrm{CO}}$ at 0.85 and 0.76 times smaller in radius than the previous works for nearby star-forming galaxies and the Milky Way, respectively.

64 pages, 32 figures, accepted to PASJ