Kazuo Sorai

An Isotope Study of Carbon Monoxide in the Edge-on Galaxy NGC 891

We present high-resolution simultaneous observations of the edge-on galaxy NGC 891 in 12CO and 13CO emissions. The molecular thin-disk component within 10.7 kpc from the galactic center was completely covered. This data set with accurate relative calibrations of intensity scale and pointing is analyzed to examine radial variation in the physical properties of the molecular gas. The total 13CO/12CO luminosity ratio is 1/6.6. A low 13CO/12CO intensity ratio of 1/(15.4 ± 6.0) is observed in the nuclear disk of about 550 pc in radius. There exists a systematic gradient of the 13CO/12CO intensity ratio in the main galactic disk as a function of galactocentric distance: the 13CO/12CO intensity ratio exhibits a notable peak of 1/4.5 near 4 kpc, and decreases systematically outward down to 1/10 at 10 kpc. The observational results are analyzed on the basis of a CO excitation analysis. The low 13CO/12CO intensity ratio in the nuclear disk may be attributed to a predominance of warm molecular gas (40 K) of moderate gas density (~103 cm-3), while the systematic gradient of the 13CO/12CO intensity ratio in the main disk can be interpreted in terms of radial decrease in the dense molecular gas fraction. Since interstellar gas in the inner part of the galaxy is mostly molecular, this variation will be ascribed to compression of molecular gas with its strength dependent on galactocentric distance rather than dissociation of low-density molecular gas by UV photons from young stars in the inner part of the galaxy.

Distribution and Kinematics of Molecular Gas in Barred Spiral Galaxies. I. NGC 3504

We present the results of the CO mapping observations of the barred spiral galaxy NGC3504 with the Nobeyama 45-m telescope. The distribution of the molecular gas shows offset ridges which correspond to the distribution of HII regions along the bar. The velocity perpendicular to the bar decreases abruptly at the ridge. The velocity change implies that the molecular gas changes the direction of its motion to parallel to the bar at the ridge. Since the position angle of the major axis of the bar and the line of nodes are almost the same in NGC 3504, an upper limit to the pattern speed of the bar can be derived directly from the radial velocity. The resultant upper limit is 41 km/s/kpc which is much smaller than that derived with an assumption that the corotation radius is located at the end of the bar (77 km/s/kpc). The corotaion radius derived from our upper limit is more than two times larger than the length of the semi-major axis of the bar in NGC 3504.

A CO(J=2-1) line survey of the Galactic center

We have observed the Galactic center in 12CO(J= 2 - 1) and 13CO( J = 2 - 1) lines using the TokyoOnsala-ESO-Cal&n 60-cm telescope and compared the data with the 12CO(J= 1-O) Columbia survey (Bitran et al. 1997). The 12C0 J = 2- l/J = 1-O intensity ratio is very high (0.96) in the central 9OOpc of the Galaxy, although it is about 0.6-0.7 for typical molecular clouds in the Galactic disk. The observed intensity ratios, 12C0 J=2-l/J=l-0 and ‘3CO/‘2C0 (J=2-l), indicate that even the 12C0 line is not very optically thick: 71zco(k14) - 1 or smaller in the Galactic center. This may be due to high temperature and large velocity dispersion. The large velocity dispersion is probably caused by large external pressure and tidal forces. 01999 COSPAR. Published by Elsevier Science Ltd.

4.7. The CO 2–1/1–0 ratio in the disk and center of the Milky Way Galaxy

We are conducting a survey of the CO J =2-1 line emission in the southern Milky Way (the Tokyo-Onsala-ESO-Calan Galactic CO survey), using the 60-cm radio telescope ( VST2 ) in La Silla, Chile. It is combined with the northern survey made with the identical telescope ( VST1 ) at Nobeyama ( e.g. , Sakamoto et al. 1995), and is directly comparable with the Columbia survey of the CO J =1-0 emission ( e.g. , Dame et al. 1987).