CO Multi-line Imaging of Nearby Galaxies (COMING). IX. (CO)-C-12(J=2-1)/(CO)-C-12(J=1-0) line ratio on kiloparsec scales

While molecular gas mass is usually derived from (CO)-C-12(J = 1-0)-the most fundamental line for exploring molecular gas-it is often derived from (CO)-C-12(J = 2-1) assuming a constant (CO)-C-12(J = 2-1)/(CO)-C-12(J = 1-0) line ratio (R-2/1). We present variations of R-2/ 1 and effects of the assumption that R-2/1 is a constant in 24 nearby galaxies using (CO)-C-12 data obtained with the Nobeyama 45mradio telescope and IRAM 30mtelescope. The median of R-2/1 for all galaxies is 0.61, and the weighted mean of R-2/1 by (CO)-C-12(J = 1-0) integrated intensity is 0.66 with a standard deviation of 0.19. The radial variation of R-2/1 shows that it is high (similar to 0.8) in the inner similar to 1 kpc while its median in disks is nearly constant at 0.60 when all galaxies are compiled. In the case that the constant R-2/1 of 0.7 is adopted, we found that the total molecular gas mass derived from (CO)-C-12(J = 2-1) is underestimated/ overestimated by similar to 20%, and at most by 35%. The scatter of molecular gas surface density within each galaxy becomes larger by similar to 30%, and at most by 120%. Indices of the spatially resolved Kennicutt-Schmidt relation by (CO)-C-12(J = 2-1) are underestimated by 10%-20%, at most 39%, in 17 out of 24 galaxies. R-2/1 has good positive correlations with star-formation rate and infrared color, and a negative correlation with molecular gas depletion time. There is a clear tendency of increasing R-2/1 with increasing kinetic temperature (T-kin). Further, we found that not only T-kin but also pressure of molecular gas is important in understanding variations of R-2/1. Special considerations should be made when discussing molecular gas mass and molecular gas properties inferred from (CO)-C-12(J = 2-1) instead of (CO)-C-12(J = 1-0).