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).

中文翻译：

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邻近星系的CO多线成像（COMING）。九。 千帕斯卡刻度上的¹² CO（J  = 2–1）/ ¹² CO（J = 1–0）线宽比

虽然分子气体质量通常衍生自¹² CO（Ĵ  = 1-0）为探索分子-the最根本的在线气体-它通常衍生自¹² CO（Ĵ  = 2-1）假定恒定的¹² CO（Ĵ  = 2 –1）| $ / $ | ¹² CO（J  = 1–0）线宽比（R _2/1）。利用Nobeyama 45 m射电望远镜和IRAM 30 m望远镜获得的^12个CO数据，我们介绍了R _2/1的变化以及假设R _2/1在24个附近星系中是常数的影响。R _2/1的中位数所有星系的总 强度为0.61，R _2/1乘以¹² CO（J = 1-0）的积分强度的加权平均值为0.66，标准偏差为0.19。R _2/1的径向变化表明，当所有星系都被汇编时，R _2/1在内部的〜1 kpc处较高（〜0.8），而其在磁盘中的中值几乎恒定为0.60。在采用常数R _2/1为0.7的情况下，我们发现总分子气体质量源自¹² CO（J = 2-1）被低估/被高估了约20％，最多不超过35％。每个星系中分子气体表面密度的散布增加约30％，至多增加120％。 在24个星系中的17个星系中，空间分辨的Kennicutt-Schmidt关系的指数被¹² CO（J = 2-1）低估了10％–20％，最多为39％。R _2/1与恒星形成率和红外色具有良好的正相关关系，与分子气体耗尽时间呈负相关关系。有明显的趋势是随着动力学温度（T _kin）的增加而增加R _2/1。此外，我们发现不仅是T_亲但是分子气体的压力对于理解R _2/1的变化也很重要。在讨论从¹² CO（J  = 2–1）而不是¹² CO（J  = 1–0）推断出的分子气体质量和分子气体性质时，应特别考虑。