The Tokyo-Onsala-ESO-Calan Galactic CO J=2-1 survey. I. The Galactic center region

As a part of the Tokyo-Onsala-ESO-Calan Galactic CO survey, we have made la rge-scale mapping observations of the Galactic center in the CO J = 2-1 lin e using the 60 cm survey telescope in Chile. The data were taken with the s ame beam size (9 ') and sampling grid (7.'5) as the CO J = 1-0 Columbia sur vey, so that the two data sets can be compared directly. Velocity channel m aps and longitude-velocity diagrams of the (CO)-C-12 J = 2-1 line, covering - 6 degrees less than or equal to I less than or equal to 6 degrees, - 2 d egrees less than or equal to b less than or equal to 2 degrees, -300 less t han or equal to nu (LSR) less than or equal to 300 km s(-1), are presented, along with the corresponding maps of the (CO)-C-12 J = 2-1/1J = 1-0 intens ity ratio [R2-1/1-0((CO)-C-12)]. A longitude-velocity diagram of the (CO)-C -13 J = 2-1 line intensity and corresponding (CO)-C-13 J = 2-1/(CO)-C-12 J = 2-1 intensity ratio [R-13/12(J = 2-1)] is also presented at b = 0 degrees . The overall R2-1/1-0((CO)-C-12) in the central 900 pc of the Galaxy is 0. 96 +/- 0.01, which is higher than the typical value in the Galactic disk, 0 .6-0.7. The isotopic intensity ratio R-13/12(J = 2-1) is 0.10 +/- 0.01. The two observed intensity ratios [R2-1/1-0((CO)-C-12) and R-13/12(j = 2-1)] i ndicate that the optical depth of the (CO)-C-12 J = 1-0 line is similar to1 or smaller in the Galactic center molecular clouds, much smaller than thos e of the giant molecular clouds (GMCs) in the Galactic disk. Longitude-velo city distributions of physical properties of molecular gas are derived usin g a large velocity gradient analysis. Molecular gas in the Galactic center generally shows a high pressure. In particular, there is a high-pressure re gion in the central similar to 100 pc where the pressure is an order of mag nitude higher than that in GMCs in the Galactic disk. This region is domina ted by high-density gas and contains star-forming regions.