Published papers

This Letter presents a Nyquist-sampled, high-resolution [C I] 3 P 1-3 P 0 map of the -02 < l < 12 × -01 < b < 0° region in the Central Molecular Zone (CMZ) taken with the Atacama Submillimeter Telescope Experiment 10 m telescope. We have found that molecular clouds in the CMZ can be classified into two groups according to their [C I]/13CO intensity ratios: a bulk component consisting of clouds with a low, uniform [C I]/13CO ratio (0.45) and another component consisting of clouds with high [C I]/13CO ratios (>0.8). The [C I]-enhanced regions appear in M-0.02-0.07, the circumnuclear disk, the 180 pc ring, and the high-velocity compact cloud CO+0.02-0.02. We have carried out a large velocity gradient analysis and have derived the C0/CO column density ratio for M-0.02-0.07 as 0.47, which is approximately twice that of the bulk component of the CMZ (0.26). We propose several hypotheses on the origin of high C0 abundance in M-0.02-0.07, including cosmic-ray/X-ray dissociation and mechanical dissociation of CO in the pre-existing molecular clouds. We also suggest the possibility that M-0.02-0.07 is a cloud at an early stage of chemical evolution from diffuse gas, which was possibly formed by the bar-induced mass inflow in the Galactic center region. © 2011. The American Astronomical Society. All rights reserved.

The Tornado nebula (G357.7-0.1) is a mysterious radio source with a bright "head" and faint "tail" located in the direction of the Galactic center (GC) region. We here report on the discovery of two diffuse X-ray sources at the head and tail of Tornado with the Suzaku satellite. We found emission lines from highly ionized atoms in the two sources. The spectra have been reproduced by an optically thin thermal plasma with a common temperature of 0.6-0.7 keV. The interstellar absorption (NH) of these sources are the same, and are slightly larger than that of the GC distance. Since the estimated distance using the NH value is consistent with radio observations of Tornado, these X-ray sources are likely to be associated with the Tornado nebula. The twin-plasma morphology at both ends of Tornado suggests that the system is a bipolar/outflow source. © 2011. Astronomical Society of Japan.

We consider the high-energy astrophysics of the inner ∼200pc of the Galaxy. Our modelling of this region shows that the supernovae exploding here every few thousand years inject enough power to (i) sustain the steady-state, in situ population of cosmic rays (CRs) required to generate the region's non-thermal radio and TeV γ-ray emission; (ii) drive a powerful wind that advects non-thermal particles out of the inner Galactic Centre; (iii) supply the low-energy CRs whose Coulombic collisions sustain the temperature and ionization rate of the anomalously warm envelope detected throughout the Central Molecular Zone; (iv) accelerate the primary electrons which provide the extended, non-thermal radio emission seen over ∼150pc scales above and below the plane (the Galactic Centre lobe); and (v) accelerate the primary protons and heavier ions which, advected to very large scales (up to ∼10kpc), generate the recently identified Wilkinson Microwave Anisotropy Probe (WMAP) haze and corresponding Fermi haze/bubbles. Our modelling bounds the average magnetic field amplitude in the inner few degrees of the Galaxy to the range 60 < B/μ G < 40 0 (at 2σ confidence) and shows that even TeV CRs likely do not have time to penetrate into the cores of the region's dense molecular clouds before the wind removes them from the region. This latter finding apparently disfavours scenarios in which CRs - in this starburst-like environment - act to substantially modify the conditions of star formation. We speculate that the wind we identify plays a crucial role in advecting low-energy positrons from the Galactic nucleus into the bulge, thereby explaining the extended morphology of the 511keV line emission. We present extensive appendices reviewing the environmental conditions in the Galactic Centre, deriving the star formation and supernova rates there, and setting out the extensive prior evidence that exists, supporting the notion of a fast outflow from the region. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

We report the results of millimeter and submillimeter molecular line mapping observations of the Galactic circumnuclear disk (CND). The CND appears as a large, asymmetric disk of warm molecular gas with a high CO J = 3-2/CO J = 1-0 intensity ratio exceeding 1.5. It has a mass of (2-5) × 105 M⊙ and a diameter of about 10pc, including a well-known 2-pc radius ring of dense molecular gas around the minispiral. The CND can be clearly traced by the J = 1-0 lines of HCN, H13CN, HCO+, and HNC, but it is barely traced by N2H+, SiO, CCS, and HC 3N lines. These data confirm the entity of the CND, and the 2-pc ring is just a part of it. Line ratios suggest that the CND is chemically immature, having higher density and higher temperature than the ambient gas. A one-zone large-velocity-gradient analysis finds that molecular gas in the CND has a typical kinetic temperature of T k ≃ 63K and H2 density of n(H2) ≃ 104.1cm-3. The bulk of the CND seems to have an overall, systematic infall motion, with a velocity of V infall ≃ 50kms-1. Our results are consistent with the scenario that the CND has been formed by tidal capture and disruption of a giant molecular cloud (GMC). The progenitor GMC may have been formed recently inside the 120-pc ring, possibly by the encounter with the small inner bar of the Galaxy. Toomre's Q parameter indicates that the CND is gravitationally stable now, but it will become unstable and fragment as gas accumulates at r ≃ 2pc. It would trigger a burst of star formation, and subsequent processes could enhance the mass accretion rate to SgrA*. © 2011. The American Astronomical Society. All rights reserved.

Results of molecular line observations toward the Sagittarius B1 complex are reported. Maps of the HCN, HCO + and SiO J = 1-0 emissions were taken with the NRO 45-m telescope. With these data, combined with the ASTE CO J = 3-2 survey data, we have investigated the spatial structure, kinematics and physical conditions of two peculiar molecular features: CO0.55+0.07 and SiO0.56-0.01. The CO arc, CO0.55+0.07, shows clear expanding motion with sizes of 8.5 × 6.8 pc 2 and an expansion velocity of 40 km s -1. The SiO shell, SiO0.56-0.01, has a size of 3.0×3.4 pc 2, and surrounds an X-ray Fe line source, GO.570-0.018. The mass and the kinetic energy of CO0.55+0.07 are estimated to be 10 5.5M⊙ and 10 51.5erg, respectively. The kinetic energy of SiO0.56-0.01 is ∼ 10 50.4erg. An LVG analysis shows that the typical density and kinetic temperature are 10 3.8 cm -3 and 28 K, respectively. High-density clumps with a density of 10 4.0-4.5 cm -3 associated with CO0.55+0.07 and SiO0.56-0.01 have been found, supporting the idea that they consist of swept-up material. The huge kinetic energy of CO0.55+0.07 is considered to be injected by a series of supernova explosions that took place within ∼ 2× 10 5yr, which would suggest that it is a 'bubble' created by a massive stellar cluster, whose mass is estimated to be 10 3.5-4.5 M⊙. The origin of SiO0.56-0.01 is rather unclear, but we suggest that it could be related to the X-ray source G0.570-0.018. © 2009. Astronomocal Society of Japan.

In a J, H, and Ks band survey of the Galactic center region over an area of 2° x 5°, we have found many dark clouds, among which a distinguished chain of dark clouds can be identified with a quiescent CO cloud. The distances of the clouds are estimated to be 3.2-4.2kpc, corresponding to the Norma arm, by our new method for determining the distances to dark clouds using the cumulative number of stars against the J - Ks color. By adopting these estimated distances, the size and total mass of the cloud are estimated to be ̃ 70 pc in length and 6 x 10 4 M G, respectively. Three compact H II regions exist in the cloud, indicating that star-forming activities are going on at the cores of the quiescent CO cloud on the spiral arm. © 2009 Astronomical Society of Japan.

We observed CO J = 1-0 and HCN J = 1-0 line emission toward a high-velocity, compact molecular cloud, CO 0.02-0.02, near the center of our Galaxy, using the Nobeyama Millimeter Array (NMA). A CO velocity-integrated map with a resolution of 4.″3 × 2.″4 shows two oval clumps with sizes of 0.8 pc. These clumps are seperated by 1.2 pc, being located at the eastern and southwestern peripheries of the CO 0.02-0.02 cloud. The overall distribution of HCN emission coincides with that taken with the Nobeyama 45 m telescope. The HCN velocity-integrated map with a resolution of 6.″ × 3.″4 shows two prominent peaks in the cloud center. Both NMA maps at VLSR ∼ 110km s-1 show an arc-shaped edge in the southeast, which may correspond to the edge of the "emission cavity" found in the CO J = 3-2 integrated-intesity map. We also noticed a faint radio continuum "arc" that encircles the bulk of the CO 0.02-0.02 cloud. These results support the notion that CO 0.02-0.02 has been accelerated, heated, and compressed in a series of supernova shocks that occurred within the last (3-5) × 104 yr. We suggest that a massive compact cluster with an age of 10-30 Myr is responsible for the formation of the CO 0.02-0.02 cloud. © 2008. Astronomical Society of Japan.

We have surveyed the central molecular zone (CMZ) of our Galaxy in the CO J = 3-2 line with the Atacama Submillimeter-wave Telescope Experiment (ASTE). Molecular gas in the Galactic center shows a high J=3-2/J=1-0 intensity ratio (∼ 0.9), while gas in the spiral arms of the Galactic disk shows a lower ratio (∼ 0.5). The high-velocity compact cloud CO 0.02-0.02 and the hyperenergetic shell CO 1.27+0.01, as well as gas in the Sgr A region, exhibit high J=3-2/J=1-0 intensity ratios exceeding 1.5. We also found a number of small spots of high-ratio gas. Some of these high-ratio spots have large velocity widths and some seem to be associated with nonthermal threads or filaments. These could be spots of hot molecular gas shocked by unidentified supernovae, which may be abundant in the CMZ. © 2007. Astronomical Society of Japan.

We report on the results of molecular line observations toward the l = 1,3 complex, an anomalous cloud complex in the central molecular zone of the Galaxy, The CO J=3-2 survey recently performed with the Atacama Submillimeter Telescope Experiment (ASTE) 10 m telescope has found that the complex has an enhanced CO J=3-2/J=1-0 intensity ratio. We have made high-resolution maps of the CO J=1-0, HCN J=1-0, HCO+ J=1-0, SiO J=1-0, and J=2-1 lines with the Nobeyama 45 m telescope. The complex is found to be rich in shells and arcs of dense molecular gas. We identified 9 expanding shells in HCN maps and compact SiO features associated with the shells. The intensity ratios of HCN/CO, HCO +/CO, and CO J=3-2/J=1-0 are coherently enhanced by a factor of a few in gas with an LSR velocity higher than 110 km s-1. The high-velocity gas has a high density (nH ∼ 104.5 cm-3) and high SiO/13CO intensity ratio, indicating that the gas was shocked. The typical HCN/HCO+ intensity ratio is found to be 2.3, being higher by a factor of a few than those in the Galactic disk clouds. The typical kinetic energy and expansion time of the shells are estimated to be 1050.9-52.5 erg and 104.6-5.3 yr, respectively. The kinetic energy could be furnished by multiple supernova and/or hypernova explosions at a rate of 10-3-4 yr-1. These estimates suggest that the expanding shells as a whole may be in the early stage of superbubble formation. This proto-superbubble may have originated in a massive cluster formation that took place 106.8-7.6 yr ago. © 2007. Astronomical Society of Japan.

We estimated the physical conditions of molecular gas in the central molecular zone (CMZ) of the Galaxy, using our CO J = 3-2 data obtained with the Atacama Submillimeter Telescope Experiment (ASTE) in conjunction with J = 1-012CO and 13CO data previously observed with the Nobeyama Radio Observatory (NRO) 45 m telescope. The large velocity gradient (LVG) approximation was employed. The distributions of gas density, kinetic temperature, and CO column density were derived as functions of the position and the velocity for the entire coverage of the CO J = 3-2 data. We fairly determined the physical conditions for 69% of data points in the CMZ with ≥ 1 σ CO detections. The kinetic temperature was found to be roughly uniform in the CMZ, while the gas density was higher in the 120 pc star-forming ring than in the outer dust lanes. The physical conditions of high J = 3-2/J = 1-0 features are also discussed. © 2007. Astronomical Society of Japan.

We have mapped two massive clumps, clump A and B, of the AFGL 333 cloud in the N2H+ J = 1-0, CCS JN = 43-3 2, HC3N J = 5-4, and NH3 (J, K) = (1,1) and (2,2) lines with the Nobeyama Radio Observatory 45 m telescope. Intense N 2H+ emission comes from the two clumps, and its overall distribution is similar to that of C18O J = 1-0. On the other hand, the CCS and HC3N emissions are more intense toward clump B than toward clump A. Thus, the column density ratios of [CCS]/[N2 H +] and [HC3N]/[N2H+] tend to be higher toward clump B than toward clump A, indicating that clump B is younger than clump A. This is supported by a fact that the 2MASS sources with H -K > 2 and the Spitzer 24 μm sources are mostly associated with clump A, whereas only a few such sources exist in clump B. Clump B involves a few dense cores that are dark in the mid-infrared maps. Such cores are thought to be good candidates of starless cores for high- or intermediate-mass stars. © 2007. The American Astronomical Society. All rights reserved.

We have surveyed the central molecular zone (CMZ) of our Galaxy in the CO J3-2 line with the Atacama Submillimeter-wave Telescope Experiment (ASTE). Molecular gas in the Galactic center shows high J3-2/J1-0 intensity ratio (∼ 0.9) while gas in the Galactic disk shows the lower ratio (∼ 0.5). The high-velocity compact cloud CO 0.02-0.02 and the hyperenergetic shell CO 1.27+0.01, as well as gas in the Sgr A region exhibit high J3-2/J1-0 intensity ratio exceeding 1.5. We also found a number of small spots of high ratio gas. Some of these high ratio spots have large velocity widths and some seem to associate with nonthermal threads or filaments. These could be spots of hot molecular gas shocked by unidentified supernovae which may be abundant in the CMZ. © 2006 IOP Publishing Ltd.

We have mapped the W3 giant molecular cloud in the C0 3P1 - 3P0 ([C I] 492 GHz) and 12CO J = 3-2 emission lines with the Mount Fuji Submillimeter-wave Telescope. The [C I] distribution is extended over the molecular cloud, having peaks at three star forming clouds, W3 Main, W3(OH), and AFGL 333. The [C I] emission is found to be strong in the AFGL 333 cloud, where the 12CO J = 3-2 emission is relatively weak. In order to characterize the physical and chemical states of the AFGL 333 cloud, we have also observed the CO J = 1-0 isotopomer lines and the CCS and N2H+ lines with the Nobeyama Radio Observatory 45 m Telescope. The [C0]/[CO] and [CCS]/[N2H+] abundance ratios are found to be higher in the AFGL 333 cloud than in the W3(OH) cloud, suggesting that the AFGL 333 cloud is younger than the W3(OH) cloud. In the AFGL333 cloud we have found two massive cores without any sign of active star formation. They are highly gravitationally bound and are regarded as good candidates for a massive prestellar core.

We have found massive clumps without any sign of active star formation in the AFGL 333 cloud. We present a study of the physical and chemical properties of the AFGL 333 cloud. © 2007 International Astronomical Union.

We report on the detection of a high-velocity feature, CO 1.27+0.01, in the Galactic-center molecular-cloud complex. In our 12CO images taken with the NRO 45m telescope, CO 1.27+0.01 appears as the second prominent high-velocity feature, next to CO 0.02-0.02, having a velocity width of Δ V ≃ 100kms-1 and a size of 9pc × 12pc. It seems to consist of two expanding shells with sizes of ∼ 10 pc. The major shell has a kinetic energy of about an order of magnitude larger than the gravitational energy, indicating that the shell is apparently gravitationally unbound. We propose that CO 1.27+0.01 may have been accelerated by a series of supernova explosions which occurred within the last ∼ 6 × 104 yr. No association of a nonthermal radio continuum source suggests a strong magnetic field (B ≳ 0.4mG) in the vicinity of CO 1.27+0.01.

As a part of the Tokyo-Onsala-ESO-Calán Galactic CO survey, we have made large-scale mapping observations of the Galactic center in the CO J = 2-1 line using the 60 cm survey telescope in Chile. The data were taken with the same beam size (9′) and sampling grid (7′.5) as the CO J = 1-0 Columbia survey so that the two data sets can be compared directly. Velocity channel maps and longitude-velocity diagrams of the 12CO J = 2-1 line, covering -6° ≤ l ≤ 6°, -2° ≤ b ≤ 2°, -300 ≤ υLSR ≤ 300 km s-1, are presented, along with the corresponding maps of the 12CO J = 2-1/J = 1-0 intensity ratio [R2-1/1-0(12CO)]. A longitude-velocity diagram of the 13CO J = 2-1 line intensity and corresponding 13CO J = 2-1/12CO J = 2-1 intensity ratio [R13/12(J = 2-1)] is also presented at b = 0°. The overall R2-1/1-0(12CO) in the central 900 pc of the Galaxy is 0.96 ± 0.01, which is higher than the typical value in the Galactic disk, 0.6-0.7. The isotopic intensity ratio R13/12(J = 2-1) is 0.10 ± 0.01. The two observed intensity ratios [R2-1/1-0(12CO) and R13/12(J = 2-1)] indicate that the optical depth of the 12CO J = 1-0 line is ∼1 or smaller in the Galactic center molecular clouds, much smaller than those of the giant molecular clouds (GMCs) in the Galactic disk. Longitude-velocity distributions of physical properties of molecular gas are derived using a large velocity gradient analysis. Molecular gas in the Galactic center generally shows a high pressure. In particular, there is a high-pressure region in the central ∼100 pc where the pressure is an order of magnitude higher than that in GMCs in the Galactic disk. This region is dominated by high-density gas and contains star-forming regions.

We present observations of the 3P1-3P0 fine-structure transition of atomic carbon [C I], the J = 3-2 transition of CO, and the J = 1-0 transitions of 13CO and C18O toward DR 15, an H 11 region associated with two mid-infrared dark clouds (IRDCs). The 13CO and C18O J = 1-0 emissions closely follow the dark patches seen in optical wavelength, showing two self-gravitating molecular cores with masses of 2000 and 900 M⊙, respectively, at the positions of the cataloged IRDCs. Our data show a rough spatial correlation between [C I] and 13CO J = 1-0. Bright [C I] emission occurs in the relatively cold gas behind the molecular cores but does not occur in either highly excited gas traced by CO J = 3-2 emission or in the H II region/molecular cloud interface. These results are inconsistent with those predicted by standard photodissociation region models, suggesting an origin for interstellar atomic carbon unrelated to photodissociation processes.

We observed 34 positions throughout the disk of the Large Magellanic Cloud in the CO J = 2-1 emission line with the Tokyo-Onsala-ESO-Calá 60 cm radio telescope. Comparing the spectra with those of the J = 1-0 line at the same angular resolution (≃ 9′, or 130 pc at 50 kpc), we found that the CO J = 2-1/J = 1-0 intensity ratio (R2-1/1-0) scatters in a range of 0.5-1.3. The luminosity ratio averaged for all observed points is 0.92 ± 0.05. The ratio R2-1/1-0 is approximately unity (0.95 ± 0.06) in 30 Dor, consistent with optically thick and thermalized emission, even in the southern part where massive star formation does not occur yet. This suggests that the high R2-1/1-0 is not primarily due to the UV radiation from young stars but rather to the intrinsic nature of the molecular gas that is relatively dense (≳103 cm-3) and may be ready to form stars. In addition to a cloud-to-cloud difference R2-1/1-0, there exists a radial gradient of the ratio of 0.94 ± 0.11 in the inner region (≳2 kpc from the kinematic center) and 0.69 ± 0.11 in the outer region (≳2 kpc from the center, excluding the 30 Doradus complex). The higher R2-1/1-0 the inner galaxy might be due to relatively higher gas densities within CO clumps in molecular clouds and/or higher external heating in that region.

SS 433 is a bright X-ray binary with two sided jets moving at 0.26 c. It is located at the center of the supernova remnant W50, suggesting that SS 433 is associated with W50. A large molecular cloud also surrounds the SS 433/W50 system. Recent observations with ROSAT and ASCA have shown X-ray hot spots interpreted as SS 433 jets interacting with the ambient interstellar medium. To verify this hypothesis, we have performed observations of the molecular cloud around SS 433 in the J=1-0 and J=2-1 lines of CO transitions along and perpendicular to the jet axis and have found two clouds with velocities of 32 km/s and 50 km/s respectively. Due to a strong correlation between the 50 km/s map and the X-ray hot spots, we conclude that SS 433 is inside or at the edge of the 50 km/s molecular cloud. © 2000 COSPAR. Published by Elsevier Science Ltd.

We have developed a 1.2 m submillimeter-wave telescope at the summit of Mt. Fuji to survey emission lines of the neutral carbon atom (CI) toward the Milky Way. A superconductor-insulator-superconductor mixer receiver on the Nasmyth focus is used to observe the 492 GHz band in SSB and the 345 GHz band in DSB simultaneously. The receiver noise temperature is 300 K in SSB and 200 K in DSB for 492 and 345 GHz, respectively. The intermediate frequency frequency is 1.8-2.5 GHz. An acousto-optical spectrometer which has the total bandwidth of 0.9 GHz and 1024 channel outputs has also been developed. The telescope was installed at the summit of Mt. Fuji (alt. 3725 m) in July 1998. It has been remotely operated via a satellite communication system from Tokyo or Nobeyama. Atmospheric opacity at Mt. Fuji was 0.4-1.0 at 492 GHz during 30% of the time and 0.07-0.5 at 345 GHz during 60% of the time from November 1998 to February 1999. The system noise temperature was 1000-3000 K in SSB at 492 GHz and 500-2000 K in DSB at 345 GHz. We observed the CI (3P1-3P0: 492 GHz) and CO (J = 3 - 2: 345 GHz) emission lines from nearby molecular clouds with the beam size of 2′.2 and 3′.1, respectively. We describe the telescope system and report the performance obtained in the 1998 winter. © 2000 American Institute of Physics. [S0034-6748(00)04907-8].