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In order to investigate the global physical condition of the Milky Way Galaxy, we carried out simultaneous observations of 12CO and 13CO J =2-1 with . We covered the Galactic plane at b=0 deg over l=10 deg-245 deg with a 3.075 arcmin grid using the waveguide-type sideband-separating receiver (2SB receiver) on the Tokyo-NRO 60-cm telescope (renamed the AMANOGAWA telescope; Nakajima et al. (2007)). Using the 12CO (J=1-0) dataset of Dame et al. (2001), which has almost the same spatial resolution, we showed the distribution of the 12CO (J=2-1)/12CO (J =1-0) and the 13CO (J=2-1)/12CO (J=2-1) ratios on the l-v plane and the intensity correlations between their intensities.

We found that for the majority of data points, the intensity correlation between 12CO (J=2-1) and 13CO (J=2-1) is aligned along a curve, and that between 12CO (J=2-1) and 12CO (J=1-0) lies along a straight line with an intensity ratio of R21=10 = 0.640. From these results, we conclude that the optical depth in the 12CO (J=2-1) line is much greater than that in 13CO (J=2-1), and most of the data points follow the two simple relationships shown in equations 7 and 8. Using the best fit values of parameters α and β in these equations, we estimated typical values for the physical condition of the molecular gas in the Galactic disk as follows: the excitation temperatures in 12CO (J=1-0), 12CO (J=2-1), and 13CO (J=2-1) are higher than 19 K, 13 K, and 9 K, respectively. The gas kinetic temperature is higher than 19 K. The H2 volume density in a cloud is more than 100 cm-3. The beam filling factor is much less than 0.7.

We found a number of data points that deviate from the global trend. This means that the gas temperature and density in these locations are different from the typical values we obtained. Some of these are located close to HII regions or an X-ray source, while the others are located in the interarm region.