Seiichi Sakamoto

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.

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.

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.

Carbon Isotope Ratio in 12CO/13CO toward Local Molecular Clouds with Near-Infrared High-Resolution Spectroscopy of Vibrational Transition Bands

We report the carbon monoxide isotope ratio in local molecular clouds toward LkHα 101, AFGL 490, and Mon R2 IRS 3. The vibrational transition bands of 12CO ν = 2 ← 0 and 13CO ν = 1 ← 0 were observed with high-resolution near-infrared spectroscopy (R = 23,000) to measure the 12CO/13CO ratio. The isotopic ratios are 12CO/13CO = 137 ± 9 (LkHα 101), 86 ± 49 (AFGL 490), and 158 (Mon R2 IRS 3), which are 1.5-2.8 times higher than the local interstellar medium value of 12CO/13CO = 57 ± 5 from millimeter C18O emission observations. This is not easily explained by saturation of the 13CO absorption. It is also questionable whether the selective photodestruction of 13CO can account for the difference between the Galactic trend and the present observation, because the molecular clouds are with high visible extinction (AV = 10-70 mag), well shielded from destructive FUV radiation. The molecular gas associated with AFGL 490 and Mon R2 IRS 3 consists of multiple temperature components lying in the lines of sight. In the cool component (Tex < 100 K), the excitation temperature of 12CO is twice that of 13CO. We attribute the temperature discrepancy to the photon-trapping effect, which makes the radiative cooling of the main isotopomer less effective.

A Search for CO (J = 3–2) Emission from the Host Galaxy of GRB 980425 with the Atacama Submillimeter Telescope Experiment

We report on a deep search for 12 CO (J = 3–2) line emission from the host galaxy of GRB 980425 with the Atacama Submillimeter Telescope Experiment (ASTE). We observed five points of the galaxy, covering the entire region. After combining all of the spectra, we obtained a global spectrum with a rms noise level of 3.3 mK in the Tmb scale at a velocity resolution of 10 km s � 1 .N os ignificant emission was detected, though we found am arginal emission feature in the velocity range corresponding to the redshift of the galaxy. We derived 3 � upper limits on the global properties: the velocity-integrated CO (3–2) intensity of ICO(3–2) <0 :26 Kk m s � 1 , by adopting a velocity width of 67 km s � 1 ;a n H 2 column density of N.H2 /<3 � 10 20 cm � 2 ;am olecular gas mass of M.H2 /<3 � 10 8 Mˇ ,b y assuming a CO line luminosity to H2 molecular gas mass conversion factor of

Exploration of Southern Sky with Atacama Submillimeter Telescope Experiment (ASTE)

The Atacama Submillimeter Telescope Experiment (ASTE) is a joint project between Japan and Chile to install and operate a 10 m high precision telescope for exploration of the Southern sky at submillimeter wavelengths. Due to the excellent atmospheric conditions at the site, Pampa la Bola (4800 m) in the Atacama desert in northern Chile, ASTE offers an unique opportunity to make extensive submillimeter observations. We successfully started scientific observations in August 2003 at 800, 500, and 350 GHz bands.

ASTE CO(3-2) Observations of the Southern Barred Spiral Galaxy NGC 986: a Large Gaseous Bar Filled with a Dense Molecular Medium

We present CO (3–2) emission observations toward the 3 0 � 3 0 (or 20 kpc � 20 kpc at a distance of 23 Mpc) region of the southern barred spiral galaxy NGC 986 using the Atacama Submillimeter Telescope Experiment (ASTE). This effort is a part of our on-going extragalactic CO (3–2) imaging project, ADIoS (ASTE Dense gas Imaging of Spiral galaxies). Our CO (3–2) image revealed the presence of a large (the major axis is 14 kpc in total length) gaseous bar filled with a dense molecular medium along the dark lanes observed in optical images. This is the largest “densegas rich bar” known to date. The dense gas bar, discovered in NGC 986, could be a huge reservoir of possible “fuel” for future starbursts in the central region, and we suggest that star formation in the central region of NGC 986 could still be in a growing phase. We found a good spatial coincidence between the overall distributions of dense molecular gas traced by CO (3–2) and massive star formation depicted by H˛ .T heglobal CO (3–2) luminosity, L 0 .3� 2/ ,o fNGC 986 was determined t ob e (5.4˙ 1.1) � 10 8 Kk m s � 1 pc 2 .T he CO (3–2)=CO (1–0) integrated intensity ratio was found to be 0.60 ˙ 0.13 at a spatial resolution of 44 00 or 5 kpc, and the CO (3–2)=CO (2–1) ratio was 0.67 ˙ 0.14 at a beam size of � 25 00 or � 2.8 kpc. These line ratios suggest moderate excitation conditions of CO lines (nH2 � 10 3� 4 cm � 3 )i n a few kiloparsec region of central NGC 986.

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.

A Possible Detection of CO (J = 3–2) Emission from the Host Galaxy of GRB980425 with the Atacama Submillimeter Telescope Experiment

We observed CO (J = 3–2) line emission from the host galaxy of GRB980425 using the Atacama Submillimeter Telescope Experiment (ASTE). Five points were observed covering the entire region of the galaxy, and we find possible emission features (S/N∼ 3 σ) at the velocity range corresponding to the redshift of the galaxy. After combining all spectra of five points, we obtain a global spectrum with a ∼4 σ emission feature. If the features are real, this is the first detection of CO among GRB host galaxies. We derive the total gas mass of M(H2) = 7 ± 1 × 10 M assuming a CO-to-H2 conversion factor of αCO = 8.0 M (K km s −1 pc2)−1. The dynamical mass is calculated to be Mdyn = 2× 10 M , and M(H2)/Mdyn ∼ 3% is consistent with those of nearby dwarfs and normal spirals. The derived star formation rate is 0.4 ± 0.1 M yr−1 based on the Schmidt law. This SFR agrees with the results of previous Hα observations (SFR = 0.35 M yr −1; Sollerman et al. 2005), suggesting that there is no significant obscured star formation in this host galaxy. This result implies that there is a variety of GRB hosts in terms of the presence of obscured star formation.