CO (J = 2-1) line observations of the galactic center molecular cloud complex. I. On-plane structure

Tomoharu Oka, Tetsuo Hasegawa, Toshihiro Handa, Masahiko Hayashi, Sehchi Sakamoto

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We present results of a mapping observation of the Galactic center region in the CO ( J = 2-1) line at 230 GHz, using the Tokyo-NRO 60 cm survey telescope (9′ beam). The CO (J = 2-1)/CO (J = 1-0) intensity ratio, R(2-1)/(1-0), exceeds unity for the Galactic center molecular gas with bright CO emission. The intermediate-density [n(H2) ≳ 103 cm-3] molecular gas traced by CO (J = 2-1) line shows a highly asymmetric pattern in the l-V plane. This asymmetry appears more prominently in CS and NH3 line emissions which probe denser molecular gas. Nevertheless, sites of recent massive star formation traced by hydrogen recombination lines (H II regions) and evolved intermediate-mass stars traced by OH maser emission (OH/IR stars) show a relatively symmetric pattern in the l-V plane. This simple kinematics indicates that star formation has been taking place, continuously or intermittently, in a region about 100 pc in radius ("Star-forming ring"). A closer comparison shows that the H II regions have systematically lower velocities (in absolute values) than the OH/IR stars. If the kinematics of OH/IR stars reflect the galactic gravitational potential, this difference can be understood as a result of bisymmetric distribution of the H II regions. A possible interpretation is that the outer region of the star-forming ring is near the first inner Lindbrad resonance (ILR), where molecular gas accumulates in two arms to be self-gravitating and intense star formation is triggered, which we see as "twin peaks" in the central regions of barred galaxies. This small ILR radius requires the existence of a small-scale (∼ 1 kpc), rapidly rotating bar in the central region of the Galaxy. We calculated the intensity ratio between our CO (J = 2-1) data and the CO (J = 1-0) data taken with the Columbia 1.2 m telescope. The average ratio for the gas at -3° ≤ / ≤ +3° within 4′.5 (10 pc) from the Galactic plane is 0.74 ± 0.02. Regions with R(2-1)/(1-0) ≥ 1-2 are found along the high-velocity side of the CO emission. The cloud complex located close to Sgr A* (Sgr A molecular cloud complex), which contains two well-known giant clouds (20 km s-1 and 40 km s-1) exhibit an extremely high intensity ratio K(2-1)/(1-0) ≅ 1-4. An anticorrelation is seen in the l-V diagram between the gas with high ratio [R(2-1)/(1-0) ≥ 1.2] and H II regions [R(2-1)/(1-0) ∼ 1]. The observed large R(2-1)/(1-0) exceeding unity can be interpreted as different sizes of " photospheres " for the two lines in clumps which compose the Galactic center clouds.

Original languageEnglish
Pages (from-to)334-342
Number of pages9
JournalAstrophysical Journal
Issue number1 PART I
Publication statusPublished - 1996
Externally publishedYes


  • Galaxy: Center
  • ISM: Clouds
  • ISM: Kinematics and dynamics
  • ISM: Molecules

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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