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

<jats:title>Abstract</jats:title> <jats:p>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 (R2/1). We present variations of R2/1 and effects of the assumption that R2/1 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 R2/1 for all galaxies is 0.61, and the weighted mean of R2/1 by 12CO(J = 1–0) integrated intensity is 0.66 with a standard deviation of 0.19. The radial variation of R2/1 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 R2/1 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. R2/1 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 R2/1 with increasing kinetic temperature (Tkin). Further, we found that not only Tkin but also pressure of molecular gas is important in understanding variations of R2/1. 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).</jats:p>