論文

We present high-resolution CO images of the Galactic center region taken with the 2 × 2 focal-plane array receiver mounted on the 45 m telescope of Nobeyama Radio Observatory. We have collected about 44,000 12C16O (J = 1-0) spectra and over 13,000 13C16O (J = 1-0) spectra with a 34″ (1.4 pc) grid spacing. The 12CO mapping area is roughly - 1°.5 ≤ l ≤ + 3°.4 and -0°.6 ≤ b ≤ +0°.6, which covers almost the full extent of the molecular gas concentration in the Galactic center. These CO images demonstrate extremely complex distribution and kinematics of molecular gas in the Galactic center. While its large-scale behavior can be attributed to the well-known coherent features, bright CO emission arises from a number of compact (d ≤ 10 pc) clouds with large velocity widths (ΔV ≥ 30 km s-1). The small-scale structure of molecular gas is characterized by filaments, arcs, and shells. The boisterous molecular gas kinematics there may be a result of violent release of kinetic energy by a number of supernova explosions and/or Wolf-Rayet stellar winds.

Physical conditions of molecular gas in the first quadrant of the Galaxy are examined through comparison of the CO J = 2-1 data of the Tokyo-Nobeyama Radio Observatory survey with the CO J = 1-0 data of the Columbia survey. A gradient of the CO J = 2-1/J = 1-0 intensity ratio (≡R2-1/1-0) with Galactocentric distance is reported. The ratio varies from ≃0.75 at 4 kpc to ≃0.6 at 8 kpc in Galactocentric distance. This confirms the early in-plane results reported by Handa et al. We classify molecular gas into three categories in terms of R2-1/1-0 on the basis of a large velocity gradient model calculation. Very high ratio gas (VHRG; R2-1/1-0 > 1.0) is either dense, warm, and optically thin gas or externally heated, dense gas. High ratio gas (HRG; R2-1/1-0 = 0.7-1.0) is warm and dense gas with high-excitation temperature of the J = 2-1 transition (Tex ≳ 10 K), and it is often observed in central regions of giant molecular clouds. Low ratio gas (LRG; R2-1/1-0 < 0.7) has low-excitation temperature of the J = 2-1 transition (Tex ≲ 10 K) because of low density or low kinetic temperature, or both, and is often observed in dark clouds and outer envelopes of giant molecular clouds. It is shown that the CO J = 2-1 emission is better characterized as a tracer of dense gas rather than a tracer of warm gas for molecular gas with kinetic temperature higher than 10 K. The observed large-scale decrease in R2-1/1-0 as a function of Galactocentric distance is ascribed to the fractional decrease of HRG and VHRG from ≃40% near 5 kpc to ≃20% near the solar circle. The HRG and VHRG are found predominantly along the Sagittarius and Scutum arms, probably in their downstream. This fact and the deficiency of atomic gas compared with molecular gas in the inner Galaxy indicate that physical conditions of interstellar gas are affected by grand-design, nonlinear processes, such as compression by spiral density waves followed by gravitational collapse, and not by dissociation of low-density molecular gas by young stars. © 1997. The American Astronomical Society. All rights reserved.

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. © 1996. The American Astronomical Society. All rights reserved.