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

Tsuyoshi Sawada, Tetsuo Hasegawa, Toshihiro Handa, Jun Ichi Morino, Tomoharu Oka, Roy Booth, Leonardo Bronfman, Masahiko Hayashi, Abraham Luna Castellanos, Lars Åke Nyman, Seiichi Sakamoto, Masumichi Seta, Peter Shaver, Kazuo Sorai, Kumiko Sato Usuda

Research output: Contribution to journalReview articlepeer-review

71 Citations (Scopus)


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 ≤ υLSR ≤ 300 km s-1, are presented, along with the corresponding maps of the 12CO J = 2-1/J = 1-0 intensity ratio [R2-1/1-0(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 [R13/12(J = 2-1)] is also presented at b = 0°. The overall R2-1/1-0(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 R13/12(J = 2-1) is 0.10 ± 0.01. The two observed intensity ratios [R2-1/1-0(12CO) and R13/12(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.

Original languageEnglish
Pages (from-to)189-219
Number of pages31
JournalAstrophysical Journal, Supplement Series
Issue number1
Publication statusPublished - 2001 Sept
Externally publishedYes


  • Galaxy: center
  • ISM: clouds
  • ISM: molecules
  • Radio lines: ISM
  • Surveys

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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