Tomoharu Oka

A Deep Submillimeter Survey of the Galactic Center

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. SCUBAs 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 28 × 05 (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.

CO (J = 2-1) Line Observations of the Galactic Center Molecular Cloud Complex. II Dynamical Structure and Physical Conditions

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 70012C16O (J = 2-1) spectra in the region -25 ? l ? 25 and |b| ? 1? with 0125 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| ? 025 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?, -25 ? l ? 25, and |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 [R(2-1)/(1-0) ? 1.0] and associated UV sources.

Statistical Properties of Molecular Clouds in the Galactic Center

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). 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.

An Out-of-Plane CO (J = 2-1) Survey of the Milky Way. II. Physical Conditions of Molecular Gas

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.

Detection of Shocked Molecular Gas by Full-Extent Mapping of the Supernova Remnant W44

Molecular gas toward the supernova remnant (SNR) W44 (G34.7� 0.4) was extensively mapped in CO J ¼ 1 0 emission with the 17 0 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 13 CO/ 12 CO intensity ratio of 0.03 and high HCO + / 12 CO intensity ratio of 0.3 suggest that the wing-emitting gas has a moderate 12 CO opacity of � 1 and a high density of n(H2 )>1 0 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 (vLSR ¼ 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 ¼ 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.

Atomic carbon and CO isotope emission in the vicinity of DR 15

We present observations of the P-3(1)-P-3(o) fine-structure transition of atomic carbon [C I], the J = 3-2 transition of CO, and the J = 1-0 transitions of (CO)-C-13 and (CO)-O-18 toward DR 15, an H II region associated with two mid-infrared dark clouds (IRDCs). The (CO)-C-13 and (CO)-O-18 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-circle dot, respectively, at the positions of the cataloged IRDCs. Our data show a rough spatial correlation between [C I] and (CO)-C-13 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.

Distribution of the [C I] Emission in the ρ Ophiuchi Dark Cloud

The 3P1-3P0 fine-structure line of the neutral carbon atom ([C I]) has been mapped over the 18 × 13 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 C18O 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.

Atomic Carbon in the AFGL 333 Cloud

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 AFGL 333 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.

ATOMIC CARBON IN THE W 3 GIANT MOLECULAR CLOUD

We have mapped the W 3 giant molecular cloud in the ([CI]) line with the Mount Fuji Submillimeter-wave Telescope. The [CI] emission is extended over the molecular cloud, having peaks at three star forming clouds; W 3(Main), W 3(OH), and AFGL 333. The [CI] emission is found to be strong in the AFGL 333 cloud. We have also observed the , and lines by using the Nobeyama Radio Observatory 45 m telescope. In the AFGL 333 cloud, we find two massive cores, which are highly gravitationally bound and have no sign of active star formation. The high []/[CO] and [CCS]/[] abundance ratios suggest that the AFGL 333 cloud is younger than the W 3(Main) and W 3(OH) clouds.

The Mt. Fuji submillimeter-wave telescope

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 ...