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

While molecular gas mass is usually derived from ¹²CO(J = 1-0)—the most fundamental line for exploring molecular gas—it is often derived from ¹²CO(J = 2-1) assuming a constant ¹²CO(J = 2-1) $/$ ¹²CO(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 ¹²CO data obtained with the Nobeyama 45 m radio telescope and IRAM 30 m telescope. The median of R_2/1 for all galaxies is 0.61, and the weighted mean of R_2/1 by ¹²CO(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 (∼0.8) in the inner ∼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 ¹²CO(J = 2-1) is underestimated/overestimated by ∼20%, and at most by 35%. The scatter of molecular gas surface density within each galaxy becomes larger by ∼30%, and at most by 120%. Indices of the spatially resolved Kennicutt-Schmidt relation by ¹²CO(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 ¹²CO(J = 2-1) instead of ¹²CO(J = 1-0).