Published papers

The Mt. Fuji submillimeter-wave telescope has been operated since November 1998 to survey neutral atomic carbon (CI) toward the Milky Way. It has a 1.2 m main reflector with a surface accuracy of 10 μm in rms. A dual polarization superconductor-insulator-superconductor (SIS) mixer receiver mounted on the Nasmyth focus receives 810/492/345 GHz bands in DSB simultaneously. An acousto-optical spectrometer (AOS) has 1024 channels for 0.8 GHz bandwidth. The telescope was installed with a helicopter and bulldozers at the summit of Mt. Fuji (alt. 3725 m) in July 1998 after a test operation at Nobeyama for a year. It has been remotely operated via a satellite communication from Tokyo or Nobeyama. Atmospheric opacity at Mt. Fuji was 0.4 - 1.0 at 492 GHz in 30% of time and 0.07 - 0.5 at 345 GHz in 60% of time during winter five months. The system noise temperature was typically 1200 K (SSB) at 492 GHz and 500 K (DSB) at 345 GHz. The beam size was measured to be 2.′2 and 3.′1 at 492 and 345 GHz, respectively. We have conducted a large-scale survey of the CI (492 GHz) and CO (3-2:345 GHz) emission from nearby molecular clouds with a total area of 10 square degrees. We describe the telescope system and report the performance obtained in the 1998 winter.

We present first results from a submillimeter continuum survey of the Galactic center "central molecular zone" (CMZ), made with the Submillimeter Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope. SCUBA's scan-map mode has allowed us to make extremely wide field maps of thermal dust emission with unprecedented speed and sensitivity. We also discuss some issues related to the elimination of artifacts in scan-map data. Our simultaneous 850/450 μm maps have a total size of approximately 2°.8 × 0°5 (400 × 75 pc) elongated along the Galactic plane. They cover the Sagittarius A region, including Sgr A *, the circumnuclear disk, and the 20 and 50 km s-1 clouds; the area around the Pistol; Sgr B2, the brightest feature on the maps; and at their Galactic western and eastern edges the Sgr C and Sgr D regions. There are many striking features such as filaments and shell-like structures as well as point sources such as Sgr A * itself. The total mass in the CMZ is greater than that revealed in previous optically thin molecular line maps by a factor of ∼3, and new details are revealed on scales down to 0.33 pc across this 400 pc-wide region.

We present a coarsely sampled longitude-velocity (l-V) map of the region l = 300°-354°, b = 0° in the 492 GHz fine-structure transition of neutral atomic carbon (C0 3P1- 3P0; [C I]), observed with the Portable 18 cm Submillimeter-wave Telescope (POST18). The l- V distribution of the [C I] emission resembles closely that of the CO J=1-0 emission, snowing a widespread distribution of atomic carbon on the Galactic scale. The ratio of the antenna temperatures, RC I/CO, concentrates on the narrow range from 0.05 to 0.3. A large velocity gradient (LVG) analysis shows that the [C I] emission from the Galactic disk is dominated by a population of neutral gas with high C 0/CO abundance ratios and moderate column densities, which can be categorized as diffuse translucent clouds. The ratio of bulk emissivity, J C I/JCO, shows a systematic trend, suggesting the bulk C°/CO abundance ratio increasing with the Galactic radius. A mechanism related to kiloparsec-scale structure of the Galaxy may control the bulk C 0/CO abundance ratio in the Galactic disk. Two groups of high-ratio (RC I/CO >co 0.3) areas reside in the l-V loci several degrees inside of tangential points of the Galactic spiral arms. These could be gas condensations just accumulated in the potential well of spiral arms and be in the early stages of molecular cloud formation. © 2005. The American Astronomical Society. All rights reserved.

We have constructed a 1.2-m submillimeter-wave telescope at the summit of Mt. Fuji to observe molecular clouds in two CI lines, 3P1-3P0 (492 GHz) and 3P2-3P1 (809 GHz). The telescope has been operated successfully for four observing seasons since the installation on 1998. We have obtained large-scale CI 492 GHz distributions of many molecular clouds, including Orion MC, Taurus MC, DR15, ρ-Oph, DR21, NGC2264, M17, W3, W44, W51, Rosette MC, covering more than 40 square degrees of the sky. The distribution of CI 492 GHz emission is found to be different from those of the 13CO or C18O emission in some clouds. We found C0-rich areas (C/CO ∼ 1) in several dark clouds without strong UV sources. Away from UV sources the spatial sequence appears to be C+/CO/C0. This seems to be inconsistent with the standard photodissociation region picture. These results are discussed in relation to formation processes of molecular clouds and dense cloud cores. © 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.

Molecular gas toward the supernova remnant (SNR) W44 (G34.7-0.4) was extensively mapped in CO J = 1-0 emission with the 17′ beam of the Nobeyama 45 m radio telescope. We detected high-velocity (>25 km s -1) CO line wings. They are confined to compact (≈1.5 pc) spots, and they are located adjacent to bright radio filaments or knots. The low 13CO/ 12CO intensity ratio of 0.03 and high HCO +/ 12CO intensity ratio of 0.3 suggest that the wing-emitting gas has a moderate 12CO opacity of ≈1 and a high density of n(H 2) > 10 5 cm -3. This gas might be shocked molecular gas that has been accelerated and compressed by the expanding blast waves of W44. In addition, the high spatial resolution CO maps reveal several other features that may reveal the interaction between the SNR and the surrounding interstellar gas. The giant molecular cloud CO G34.8-0.6 (v LSR = 48 km s -1) has a sharp edge coincident with the eastern radio continuum rim of W44, which may indicate that we observe the SNR/cloud interaction almost edge-on. The existence of the "edge" suggests that most of the molecular mass resides in smaller clumps that evaporate rapidly after the passage of the supernova blast wave. We also find spatially extended moderately broad emission (SEMBE) with a moderately large intensity of ≈30 K km s -1 in CO J = 1-0 and a typical line width of ≈10 km s -1 (FWHM). Its extent coincides with the brighter region of the radio synchrotron emission. We discuss the SEMBE in terms of molecular clumps shocked and disturbed by the compressed shell of the SNR.

We present large (30′ × 40′) maps in the C0 3P0-3P0 ([C I]; 492 GHz) and CO J = 3-2 (346 GHz) spectral lines of the NGC 1333 star-forming complex. We also report detections of the C0 3P2- 3P1 (809 GHz) spectral line from 3 × 3 positions centered on HH 12. The overall extents of [C I] and CO J = 3-2 emission are similar to those of 13CO and C18O J = 1-0 emission. The CO J = 3-2 intensity peak is adjacent to the illuminating star SVS 3, while the [C I] intensity peaks at 5′ southwest of the reflection nebula, filling a hollow surrounded by a C-shaped dense molecular shell. We suggest that the observed [C I] distributions might be the result of nonequilibrium chemistry on the basis of the time-dependent chemical simulation. A huge expanding H I shell associated with the NGC 1333 cloud lends further support to this notion.

The data from the Nobeyama Radio Observatory 45 m telescope Galactic Center CO survey have been analyzed to generate a compilation of molecular clouds with intense CO emission in this region. The measured parameters of identified clouds are analyzed and cross-correlated to compare with those of clouds in the Galactic disk. We diagnosed gravitational stability of identified clouds using their viral theorem mass and CO luminosity. If we assume that the disk clouds are nearly at the onset of gravitational instability, all the clouds and cloud complexes in the Galactic center must be gravitationally stable. This indicates that the GC clouds could be in equilibrium with high pressure in the Galactic center environment. The velocity dispersion of a cloud correlates inversely with the degree of gravitational instability. Gravitationally less stable clouds follow the main ridge of intense CO emission, part of which define two rigidly-rotating molecular arms. It is concluded that mechanisms such as orbit crowding at a resonant radius could promote dynamical evolution of clouds and thereby trigger subsequent star formation. © 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The 3P1-3P0 fine-structure line of the neutral carbon atom ([C I]) has been mapped over the 1°.8 × 1°.3 area of the L1688 cloud in the ρ Ophiuchi region with the Mount Fuji submillimeter-wave telescope. The 3P2- 3P1 line of [C I] has also been observed toward two representative positions to evaluate the excitation temperature of the [C I] lines. The overall extent of the [C I] distribution generally resembles that of the 13CO distribution. The [C I] distribution has two major peaks; one (peak I) is at ρ Oph A, and the other (peak II) is toward the east side of the C18O core in the southern part of L1688. Peak II is located beyond the C18O core with respect to the exciting star HD 147889. The C0 column density is 5.0 × 1017 cm-2 toward peak II. The spatial distribution of the [C I] emission is compared with plane-parallel photodissociation region (PDR) models, which suggest that peak II is associated with a lower density PDR front, adjacent to the dense cloud cores observed in the C18O line emission. Alternatively, peak II is in the early stage of chemical evolution, where C0 has not been completely converted to CO. In this case, the difference in the [C I] and C 18O distributions represents an evolutionary sequence. This is consistent with a picture of a shock-compressed formation of the dense cores in this region due to influences from the Sco OB2 association.

The large-scale distribution of the C I (3P1- 3P0, 492 GHz) emission line from Orion A and B giant molecular clouds has been imaged with the Mount Fuji submillimeter-wave telescope. The total area observed is about 15 deg2. The overall spatial and velocity structure of the C I line is found to be similar to that of the 13CO (J = 1-0) line. The derived column density ratio N(C I)/N(CO) shows a large variation ranging from 0.2 to 2.9 toward the cloud edges, whereas it is relatively constant between 0.1 and 0.2 toward the interior region of the entire Orion clouds in both massive star-forming and dark cloud regions. This almost constant ratio suggests that C I can coexist with CO even in the deep inside of the molecular cloud.

The data from the Nobeyama Radio Observatory 45 m telescope Galactic Center CO survey have been analyzed to generate a compilation of molecular clouds with intense CO emission in this region. Clouds are identified in an automated manner through the main part of the survey data for all CO emission peaks exceeding 10 K (T*R). The measured parameters of identified clouds are analyzed and cross-correlated to compare with those of clouds in the Galactic disk. For the clouds in the Galactic center (GC), we find the scaling laws of the type σV~S0.40 and MVT~(LCO)0.88, which are similar to those of clouds in the Galactic disk. All the GC clouds identified have larger velocity widths and virial theorem masses each above the σV-S and LCO-MVT lines of the disk clouds. We diagnosed gravitational stabilities of identified clouds assuming that the disk clouds are nearly at the onset of gravitational instability. All the clouds and cloud complexes in the GC are gravitationally stable, indicating they are in equilibrium with high pressure in the GC environment. Gravitationally less stable clouds follow the main ridge of intense CO emission, part of which define two rigidly rotating molecular arms. The velocity dispersion of a cloud correlates inversely with the degree of gravitational instability. It is concluded that mechanisms such as orbit crowding at the inner Lindblad resonance may promote gravitational instability and subsequent star formation.

Large-scale CO images taken with the 2 × 2 focal-plane array receiver on the 45 m telescope of Nobeyama Radio Observatory reveal parsec-scale structure of molecular gas around the Galactic center radio arc. The LVG analysis shows that the gas kinetic temperature is considerably enhanced (Tk ≥ 70K) in CO 0.13-0.13, a cloud which seems to be in contact with the nonthermal filaments of the Galactic center radio arc. This lends support for an interaction between CO 0.13-0.13 and the nonthermal filaments. The interaction with the strong magnetic field in and around the nonthermal filaments may heat up the molecular gas in CO 0.13-0.13 through shock dissipation and/or magnetic viscosity. We also find an expanding cavity adjacent to the nonthermal filaments. The shell has a radius of ∼ 5 pc and an expansion velocity of 25kms-1. This expanding cavity might have been formed by several hundred supernovae or a single hypernova within 3 × 105 yr. Magnetic tubes in this region could have been swept up by the expanding cavity, and compressed by interactions with CO 0.13-0.13. An interaction between the cloud and magnetic field may accelerate electrons to relativistic energies through shock acceleration or magnetic field reconnection.

Molecular line, millimetre/submillimetre continuum, and mid-IR observations are reported of the opaque fingers which cross the Eagle Nebula. The fingers are surprisingly warm when viewed in the CO J= 3-2 lines, with kinetic temperatures approaching 60 K, although the lines are relatively narrow. Most of the mass in the fingers is concentrated in cores which lie at the tips of the fingers, and contain from ∼ 10 to 60 M⊙, representing 55-80% of the mass of the individual fingers. The integrated mass contained in the three fingers and the nearby extended material is ∼ 200 M⊙. The velocity fields of the gas are complex and the material is very clumpy. The best evidence for coherent velocity structure is seen running along the central finger, which has a velocity gradient ∼ 1.7 kms-1 pc-1. The fingers contain several embedded submm continuum cores, with the most intense located at the tips of the fingers. The continuum spectra of these cores shows that they are much cooler, Tdust ∼ 20 K, than Tgas ∼ 60 K of their respective fingers. A simple thermal and chemical model of a finger was developed to study the physical environment, which takes into account the external UV illumination (∼ 1700 G0), and the chemical and thermal structure of a finger. The model predictions are consistent with all of the available observations. The fingers appear to have been formed after primordial dense clumps in the original cloud were irradiated by the light of its OB stars. These clumps then shielded material lying behind from the photoevaporative dispersal of the cloud, and facilitated the formation of the finger structures. The cores in the tips of the fingers appear to be at a very early stage of pre-protostellar development: there are no embedded infrared sources or molecular outflows present. The pressure inside the cores is just less than that of the surrounding gas, allowing them to be compressed by the external pressure. The cores are probably just starting the final stages of collapse, which will lead to the formation of a condensed, warm object. It is well known that such characteristics are expected from the earliest stages of objects popularly known as 'protostars'. The cores in the tips of the Eagle Nebula 's fingers have characteristics similar to those expected to occur in the earliest stages of proto stellar formation.

We have observed the Galactic center in 12CO(J = 2-1) and 13CO(J = 2-1) lines using the Tokyo-Onsala-ESO-Calán 60-cm telescope and compared the data with the 12CO(J = 1-0) Columbia survey (Bitran et al. 1997). The 12CO J = 2-1/J = 1-0 intensity ratio is very high (0.96) in the central 900 pc of the Galaxy, although it is about 0.6-0.7 for typical molecular clouds in the Galactic disk. The observed intensity ratios, 12CO J = 2-1/J = 1-0 and 13CO/12CO (J = 2-1), indicate that even the 12CO line is not very optically thick: τ12CO(J=1-0) ∼ 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. © 1999 COSPAR. Published by Elsevier Science Ltd.

The data from the Nobeyama Radio Observatory 45 m telescope Galactic Center CO survey have been analyzed to generate a compilation of molecular clouds with intense CO emission in this region. Clouds are identified in an automated manner throughout the main part of the survey data for all CO emission peaks exceeding 10 K (T*R). Correlations between the size, velocity dispersion, virial mass, and the CO luminosity, for the molecular clouds in the Galactic center were shown. We diagnosed gravitational stabilities of identified clouds assuming that the disk clouds are nearly at the onset of gravitational instability. Most of the clouds and cloud complexes in the Galactic center are gravitationally stable, while some clouds with intense CO emission are gravitationally unstable. @1999 COSPAR. Published by Elsevier Science Ltd.

CO J = 1-0 mapping observations of the dark nebula Lynds 227, which is possibly associated with the unidentified EGRET source 2EG J1811-2339, are presented. We detected a large amount of molecular gas along Lynds 227 with a total mass of (1-2) × 104 M⊙ surrounding an X-ray synchrotron nebula, which was detected by the ASCA within the error circle of 2EG J1811-2339. Molecular gas along Lynds 227 shows spatial anticorrelation with the X-ray synchrotron nebula, suggesting that the synchrotron nebula is interacting with the Lynds 227 cloud. We propose a γ-ray emission mechanism for 2EG J1811-2339: high-energy electrons are injected from a rotation-powered pulsar and further accelerated in shock waves generated by the interaction with ambient matter. The high-energy electrons move into the molecular cloud at the Lynds 227 and collide with dense interstellar matter to yield high-energy γ-ray photons, mainly through relativistic bremsstrahlung.

We report the detection of a peculiar molecular cloud, CO 0.02-0.02, lying about 5′ Galactic east from the center of the Galaxy. 12CO images taken with Nobeyama Radio Observatory (NRO) 45 m telescope showed that it is relatively compact (∼3 × 4 pc2) as well as having a very large velocity width (ΔV ≥ 100 km s-1). The cloud has a virial mass about 1 order of magnitude larger than the LTE mass, 9 × 104 M⊙, indicating the cloud is apparently gravitationally unbound. New observations with the James Clerk Maxwell Telescope 15 m and the NRO 45 m telescopes show that CO 0.02-0.02 is very bright in the CO (J = 3-2) and in the HCN and HCO+ (J = 1-0) lines. It appears that the environment may have an unusually high density and temperature, which may be related to the very broad CO line width. We propose that CO 0.02 -0.02 may have been accelerated, heated, and compressed in a series of supernovae shocks that have occurred within the last (3-5) × 104 yr.

VLSR| ≤ 150 km s-1. This indicates that low-density gas ≲50 pc away from the plane dominates the total CO luminosity of the central 400 pc of the Galaxy. The fractional distribution of the molecular gas with R(2-1)/(1-0) for each cloud complex clearly demonstrates the close relationship between the gas with a very high ratio [K(2-1)/(1-0) ≥ 1.0] and associated UV sources. © 1998. The American Astronomical Society. All rights reserved.A large-scale 12C16O (J = 2-1) survey of the inner few hundred parsecs of the Galaxy has been conducted using the University of Tokyo-Nobeyama Radio Observatory 60 cm survey telescope. We have taken about 700 12C16O (J = 2-1) spectra in the region -2°.5 ≤ l ≤ 2°.5 and |b| ≤ 1° with 0°.125 grid spacing, covering the entire region of the huge molecular cloud complex in the Galactic center. We refer to the CO (J = 1-0) data taken with the Columbia 1.2 m telescope and calculate the J = 2-1 to J = 1-0 intensity ratio. Velocity channel maps and longitude-velocity maps of CO (J = 2-1) line are presented, with corresponding maps of J = 2-1/J = 1-0 intensity ratio. Large-scale CO maps enable us to identify several giant molecular cloud complexes and many characteristic features of molecular gas. We identify 15 molecular cloud complexes larger than ∼30 pc in our CO (J = 2-1) data. Their virial masses are at least 1 order of magnitude larger than the masses estimated from the CO luminosity. This discrepancy can be removed if we notice that they may not be gravitationally bound but are in pressure equilibrium with the hot gas and/or magnetic field in this region. Using the expressions of virial mass and CO mass for a cloud in the pressure equilibrium case, we get the X-factor for the Galactic center molecular clouds as X = 0.24 × 1020 cm-2 (K km s-1)-1, which is 1 order of magnitude lower than that in the Galactic disk (X = 3.0 × 1020 cm-2 [K km s-1]-1). We estimate the total molecular mass in the Galactic center as M/(H2) = 2 × 107 M⊙ as a lower limit; the actual total gas mass within the central 400 pc of the Galaxy must be M/(H2) = (2-6) × 107 M⊙. We diagnose the physical conditions of the molecular gas in the Galactic center using the intensity ratio between the J = 2-1 and J = 1-0 lines. Although the CO J = 2-1/J = 1-0 line intensity ratio is high (∼0.74) in the midplane, molecular gas at |b| ≥ 0°.25 exhibits low J = 2-1/J = 1-0 ratios (∼0.6). The overall J = 2-1/J = 1-0 luminosity ratio is R(2-1)/(1-0) = 0.64 ± 0.01 if we include all the emission within |b| ≤ 1°, -2°.5 ≤ / ≤ 2°.5, and