The Tokyo-Onsala-Eso-Calán Galactic CO J = 2-1 survey. I. The Galactic center region

As a part of the Tokyo-Onsala-ESO-Calán Galactic CO survey, we have made large-scale mapping observations of the Galactic center in the CO J = 2-1 line using the 60 cm survey telescope in Chile. The data were taken with the same beam size (9′) and sampling grid (7′.5) as the CO J = 1-0 Columbia survey so that the two data sets can be compared directly. Velocity channel maps and longitude-velocity diagrams of the ¹²CO J = 2-1 line, covering -6° ≤ l ≤ 6°, -2° ≤ b ≤ 2°, -300 ≤ υ_LSR ≤ 300 km s^-1, are presented, along with the corresponding maps of the ¹²CO J = 2-1/J = 1-0 intensity ratio [R_2-1/1-0(¹²CO)]. A longitude-velocity diagram of the ¹³CO J = 2-1 line intensity and corresponding ¹³CO J = 2-1/¹²CO J = 2-1 intensity ratio [R_13/12(J = 2-1)] is also presented at b = 0°. The overall R_2-1/1-0(¹²CO) 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 [R_2-1/1-0(¹²CO) and R_13/12(J = 2-1)] indicate that the optical depth of the ¹²CO J = 1-0 line is ∼1 or smaller in the Galactic center molecular clouds, much smaller than those of the giant molecular clouds (GMCs) in the Galactic disk. Longitude-velocity distributions of physical properties of molecular gas are derived using a large velocity gradient analysis. Molecular gas in the Galactic center generally shows a high pressure. In particular, there is a high-pressure region in the central ∼100 pc where the pressure is an order of magnitude higher than that in GMCs in the Galactic disk. This region is dominated by high-density gas and contains star-forming regions.