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Physical conditions of molecular gas are key parameters to the formation rate and initial mass function of stars formed in molecular clouds. The ongoing Tokyo-NRO survey has been observing the Galactic CO (J=2–1) emission with a beamsize matched to the Columbia CO (J=1–0) survey. Intensities of the two lines should reflect physical conditions of the CO-emitting gas. An out-of-plane survey of the inner Galaxy which covers from 20° to 60° in galactic longitude and from −1° to +1° in galactic latitude with grid spacings of 0.25° has already been made (Sakamoto et al. 1994). Its coverage is large enough to draw conclusions on global properties of molecular gas in the inner Galaxy.

Recent high resolution CO images of the Galactic center (GC) molecular clouds reveal many arc and/or shell structures. A considerable fraction of them may most probably be formed by an interaction between supernova remnants (SNRs) and molecular clouds. Studies of such cases in less confused areas are needed to clarify this picture. Our study of the SNR W44 supports this SNR-induced shell formation scenario in the GC. By using the Tokyo-NRO 60-cm survey telescope, the 45-m telescope at Nobeyama,and the 15-m JCMT on Mauna Kea, we established that W44 is the first unambiguous example of interaction between a SNR and giant molecular clouds. Our millimeter and submillimeter observations revealed that the SNR changed the surface of the giant molecular clouds into filaments of shocked molecular gas. They run perpendicular to the direction of the SNR expansion. Sequential interactions between SNRs and the molecular cloud which are expected in the GC may further form these molecular filaments into networks, which are observed as shell/arc structures often made of high density molecular gas.

Molecular clouds in the Galactic center region are characterised by their large velocity widths and physical conditions which differ from clouds in the Galactic disk (e.g., Morris 1996). These clouds may not be gravitationally bound, but in equilibrium with the high external pressure in the Galactic bulge (Spergel & Blitz 1992, Oka et al. 1997a).

Sgr B2 is one of the most active star-forming complexes in our Galaxy located ~100 pc from the Galactic center. Whiteoak et al. (1987) found that groups of the OH and H2CO masers and the compact HII regions are aligned in a north-south line, and suggested that star formation there is being triggered by a single large-scale event such as an interaction between molecular clouds. In order to investigate the total molecular cloud distribution and the triggering mechanism of the active massive-star formation in the Sgr B2 region, we mapped it in the 13CO and C18O (J = 1–0) lines with the Nobeyama 45 m telescope in 1988 March and May. In the 13CO line, an area was mapped of 345″ in right ascension and 495″ in declination covering the whole Sgr B2 molecular cloud at a grid spacing of 15″. In the C18O line, more restricted area was observed at 7.5″ or 15″ spacing. The HPBW of the 45 m telescope was 16″ at 110 GHz. Full description of the observations will be given elsewhere (Whiteoak et al. 1997).

Surely the most striking radio feature in the Galactic center may be a sheaf of straight vertical filaments (VFs, e.g., Yusef-Zadeh, Morris, & Chance 1984) of the radio arc. The VFs are believed to be the manifestations of strong magnetic field lines (≥1mG) which have been illuminated by some local relativistic particle source.

Sgr B2, located at a distance of ∼100 pc from the Galactic center, is one of the most active, recent massive-star formation regions in the Galaxy. Based on the 13CO (J = 1–0) line data taken with the Nobeyama 45 m telescope, we presented a cloud collision scenario as the triggering mechanism of the burst of massive-star formation there (Hasegawa et al. 1994). In order to obtain further evidence supporting our model, we observed the Sgr B2 molecular cloud in various molecular lines with the 45 m telescope in 1992 February. Twelve points each with 20″ spacings were observed along several strips 3.67′ long at constant galactic longitudes through the major HII region complexes. Here we report the results of the two lines in the 110 GHz band, C18O (J = 1–0) and HNCO (50,5–40,4). Full description of the observations will be given elsewhere (Sato et al. 1997).

We are conducting a survey of the CO J=2-1 line emission in the southern Milky Way (the Tokyo-Onsala-ESO-Calán 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).

As a key program of the 45-m telescope at Nobeyama Radio Observatory, we have made high-resolution CO images of the Galactic center region with the 2×2 focal plane array receiver. The data consist of 44,000 12CO and 13,000 13CO spectra taken with 16″ beams spaced by 34″. The 12CO image covers roughly −1.5° ≤ l ≤ +3.4° and −0.6° ≤ b ≤ +0.6° (Oka et al. 1997b)

Analyses of the distribution of far infrared point sources in the Galactic bulge have suggested that from a face-on perspective the bulge has a bar like shape. Here, we investigate how a rotating bar-like bulge affects the global gas dynamics in a disk and compare the longitude-velocity (LV) maps from selfgravitating hydrodynamical simulations with observed maps of neutral hydrogen and carbon monoxide in the Galaxy. We found that the features on the numerical LV maps depend strongly on four factors: the pattern speed of the bar, the position angle of the Sun, the strength of the bar potential and the ratio of the gas mass to total dynamical mass. We conclude that our Galaxy has a rotating, weak, bar-like bulge (a/b ∼ 0.8) observed from nearly end on (θp < 20°). The allowed range of pattern speed of the bar is surprisingly narrow (∼ 20 km s–1 kpc–1) and is consistent with recent observations of bulge stars. Selfgravity of the interstellar matter is needed to account for some of the observations.

The core region of the W3 molecular cloud has high molecular line luminosities (Dickel et al. 1980; Brackman and Scoville 1980). This region contains luminous infrared sources and ultra compact H II regions. A young star cluster may be forming in this region. Recent interferometric high angular-resolution observation revealed a bipolar outflow toward IRS 5 in the CO emission (Claussen et al. 1984) and a mass condensation in the HCN emission (Wright et al. 1984) in this region.

The Orion bright bar, a bar shaped optical feature, is seen about 2 arcmin southeast of the Trapezium stars in the Orion nebula. Since the bright bar is probably an ionization front seen edge on, this region has an ideal configuration to investigate a shocked region generated by an expanding H II region.

High resolution images of the Orion Nebula in the millimeter wave emission lines of CS and CO taken with the 45-m telescope at Nobeyama are presented. They cover a field approximately 400″ square with a 15″ – 34″ resolution and reveal a wealth of information on kinematic and density structures. The images of the J=1-0 (49 GHz) and J=2-1 (98 GHz) lines of CS show a long (>1 pc) and narrow (∼0.1 pc) N-S ridge of dense molecular gas. On the ridge, two major clumps are recognized; one is associated with the KL object and the other is 100″ south of it. The images of the J=1-0 (115 GHz) CO line indicate interaction between the molecular cloud and the H II region formed by the Trapezium stars. Bright CO emission is found towards the edges of the denser part of the H II region delineated by radio continuum emission. The CO emission coincides with the emission of vibrationally excited H2 and the 3.3 μm dust emission feature. The CO images reveal filamentary structures (“streamers”) stretching radially from the KL region. On the streamers there are Herbig-Haro objects moving away from the KL region. They may be tracers of weak interaction between the ambient molecular gas and mostly unseen, highly collimated, high-velocity (>200 km/s) jets.

We have made 15″ resolution observations of CO J = 1-0 emission toward L723 and S140 using the Nobeyama 45-m radio telescope. The maps resolved the molecular flow structures clearly. The outflow in the S140 molecular cloud was resolved to be a bipolar structure with its axis being nearly perpendicular to the elongation of the dense core observed in CS emission and to the direction of the infrared polarization. The blueshifted and redshifted components in L723 were resolved into two pairs of bipolar outflows with a point-symmetric structure.

Cold HI gas appears as self-absorption dips in the 21-cm line profiles in and around the giant molecular cloud near W3 and W4. The cold HI cloud is ∼150 pc long and extends along the galactic plane. It consists of several fragments, each of which is typically ∼25 pc in diameter and (1 − 4) × 104 M⊙ in mass. The [H2]/[HI] ratio is estimated to be 15 – 50. The mass of the entire HI cloud amounts to ∼105 M⊙, which is comparable to that observed in CO emission.

We have observed the CS (J = 1-0), C34S (J = 1-0) and H51α emission toward the W49A molecular cloud complex in an area of 3'x 2′ (α x δ) with an angular resolution of 33″. The CS emitting region is 100″ x 80″ or 6.7 pc x 5.4 pc (α x δ) at the half maximum level. Although the CO emission is self-absorbed due to the foreground cold gas, the CS optical depth of the foreground gas is found to be small. Therefore, the two CS peaks at VLSR = 4 km s−1 and 12 km s−1 imply the presence of two dense molecular clouds toward W49A. The brighter 12 km s−1 cloud peaks 35″ southeast of W49A IRS, the infrared and H2O/OH maser sources associated with the compact H II region, while the 4 km s−1 cloud has a peak at W49A IRS. The hydrogen column density through the c34S emitting region is (0.3-1.7) x 1024 cm−2. The estimated core mass of the W49A molecular cloud is (0.5-2.5) x 104 M⊙. This mass is closely packed in a small region of 3.4 pc in diameter, and is about an order of magnitude larger than the virial mass of the system. The massive core will collapse within 10 years unless there is some special supporting mechanism. There was a sudden increase in the star formation rate 104– 105 years ago, suggesting a triggered burst of star formation in the core of W49A. The collision of two velocity clouds might have triggered the formation of this massive core and the burst of star formation.

We have mapped the nuclear region of an Scd galaxy IC 342 in the CO (J = 1-0) emission with an angular resolution of 15″. This galaxy has a bright optical nucleus with a dark lane elongated in the north-south direction. A prominent molecular bar which has been discovered by Lo et al. (1984) using the Owens Valley Interferometer lies just on the dark lane. Their CO map was spatially limited by the primary beam pattern and the velocity coverage of their spectrometer was not sufficient. It is not known, therefore, whether the molecular bar is connected to the outer spiral arms or not. We made the highest resolution CO map accessible by a single-dish telescope with a sufficient velocity coverage.

Sato and Fukui (1978, hereafter SF) studied the HI 21-cm line in the giant molecular cloud near M17 and showed that the self-absorption of the 21-cm line is a useful tool for probing star-forming clouds. The giant molecular cloud at V(LSR) ∼ 20 km s-1 has been found to extend 4° (∼170 pc) southwest of M17 (Elmegreen et al. 1979). To gain a better knowledge of its physical conditions, we used the data from the Maryland-Green Bank Survey (Westerhout 1973) to examine the self-absorption features of the 21-cm line in and around the whole molecular cloud except the two fragments adjoining M17. The HI profiles at every in galactic latitude and longitude in the region were analyzed in almost the same manner as in SF by assuming that the spin temperature of the cold HI gas is equal to 20 K. The distance was taken to be 2.5 kpc.