Atacama Submillimeter Telescope Experiment Research Articles

CO J = 3–2 Emission from the “Water Fountain” Sources IRAS 16342-3814 and IRAS 18286-0959

Abstract We observed CO $J$$=$ 3–2 emission from “water-fountain” sources, which exhibit high-velocity collimated stellar jets traced by H$_{2}$O maser emission, with the Atacama Submillimeter Telescope Experiment (ASTE) 10 m telescope. We detected CO emission from two sources: IRAS 16342$-$3814 and IRAS 18286$-$0959. The IRAS 16342$-$3814 CO emission exhibits a spectrum that could be well fit to a Gaussian profile, rather than to a parabolic profile, with a velocity width (FWHM) of 158$ \pm $6 km s$^{-1}$ and an intensity peak at $V_{\rm LSR}$$=$ 50$ \pm $2 km s$^{-1}$. The mass-loss rate of the star is estimated to be $\sim $2.9 $\times$ 10$^{-5} M_{ \odot} $yr$^{-1}$. Our morpho-kinematic models suggest that the CO emission is optically thin, and associated with a bipolar outflow rather than with a (cold and relatively small) torus. The IRAS 18286$-$0959 CO emission has a velocity width (FWHM) of 3.0$ \pm $0.2 kms$^{-1}$, smaller than typically seen in AGB envelopes. The narrow velocity width of the CO emission suggests that it originates from either an interstellar molecular cloud or a slowly-rotating circumstellar envelope that harbors the H$_{2}$O maser source.

Discovery of large-scale gravitational infall in a massive protostellar cluster

We report Mopra (ATNF), Anglo-Australian Telescope, and Atacama Submillimeter Telescope Experiment observations of a molecular clump in Carina, BYF73 = G286.21+0.17, which give evidence of large-scale gravitational infall in the dense gas. From the millimetre and far-infrared data, the clump has mass ~ 2 x 10^4 Msun, luminosity ~ 2-3 x 10^4 Lsun, and diameter ~ 0.9 pc. From radiative transfer modelling, we derive a mass infall rate ~ 3.4 x 10^-2 Msun yr-1. If confirmed, this rate for gravitational infall in a molecular core or clump may be the highest yet seen. The near-infrared K-band imaging shows an adjacent compact HII region and IR cluster surrounded by a shell-like photodissociation region showing H2 emission. At the molecular infall peak, the K imaging also reveals a deeply embedded group of stars with associated H2 emission. The combination of these features is very unusual and we suggest they indicate the ongoing formation of a massive star cluster. We discuss the implications of these data for competing theories of massive star formation.

ASTE CO (3-2) MAPPING TOWARD THE WHOLE OPTICAL DISK OF M 83: PROPERTIES OF INTER-ARM GIANT MOLECULAR-CLOUD ASSOCIATIONS

We present a new on-the-fly (OTF) mapping of CO(J=3-2) line emission with the Atacama Submillimeter Telescope Experiment (ASTE) toward the 8' x 8' (or 10.5 x 10.5 kpc at the distance of 4.5 Mpc) region of the nearby barred spiral galaxy M 83 at an effective resolution of 25''. Due to its very high sensitivity, our CO(J=3-2) map can depict not only spiral arm structures but also spur-like substructures extended in inter-arm regions. This spur-like substructures in CO(J=3-2) emission are well coincident with the distribution of massive star forming regions traced by Halpha luminosity and Spitzer/IRAC 8 um emission. We have identified 54 CO(J=3-2) clumps as Giant Molecular-cloud Associations (GMAs) employing the CLUMPFIND algorithm, and have obtained their sizes, velocity dispersions, virial masses, and CO luminosity masses. We found that the virial parameter alpha, which is defined as the ratio of the virial mass to the CO luminosity mass, is almost unity for GMAs in spiral arms, whereas there exist some GMAs whose alpha are 3 -- 10 in the inter-arm region. We found that GMAs with higher $\alpha$ tend not to be associated with massive star forming regions, while other virialized GMAs are. Since alpha mainly depends on velocity dispersion of the GMA, we suppose the onset of star formation in these unvirialized GMAs with higher alpha are suppressed by an increase in internal velocity dispersions of Giant Molecular Clouds within these GMAs due to shear motion.

Search of SO 2 in the Martian atmosphere by ground-based submillimeter observation

We searched sulfur oxide (SO 2 and SO) in the Martian atmosphere by the Atacama Submillimeter Telescope Experiment (ASTE). Sulfur oxide is one of the most evident species in terrestrial volcanic gases. Although it has not yet been detected at Mars, this detection can constraint the Martian crustal and volcanic activities. We observed northern winter of Mars on 26 December 2007 ( Ls=8.1°) in 346 GHz range with ∼1 h integration. We get the upper limit of the SO 2 mixing ratio, 2 ppb. This is mostly equivalent to the value observed in northern summer ( Ls=205°) ( Krasnopolsky, 2005). We concluded that the crustal or volcanic gas produced into the atmosphere is tenuous in northern winter. Even at Mars, it can be expected that the crustal or volcanic gas includes non-negligible amount of SO 2 produced as degasification from the magma. Our result and recent detections of CH 4 suggest a possibility of some kind of carbon hydride sources under the ground reservoir, independent of usual crustal and volcanic activities.

A molecular outflow revealing star formation activity in the vicinity of the HII region G034.8-0.7 and the SNR W44

This work aims at investigating the molecular gas component in the vicinity of two young stellar object (YSO) candidates identified at the border of the HII region G034.8-0.7 that is evolving within a molecular cloud shocked by the SNR W44. The purpose is to explore signatures of star forming activity in this complex region. We performed a near and mid infrared study towards the border of the HII region G034.8-0.7 and observed a 90" X 90" region near 18h 56m 48s, +01d 18' 45" (J2000) using the Atacama Submillimeter Telescope Experiment (ASTE) in the 12CO J=3--2, 13CO J=3--2, HCO+ J=4--3 and CS J=7--6 lines with an angular resolution of 22". Based on the infrared study we propose that the source 2MASS 18564827+0118471 (IR1 in this work) is a YSO candidate. We discovered a bipolar 12CO outflow in the direction of the line of sight and a HCO+ clump towards IR1, confirming that it is a YSO. From the detection of the CS J=7--6 line we infer the presence of high density (>10^7 cm^-3) and warm (>60 K) gas towards IR1, probably belonging to the protostellar envelope where the YSO is forming. We investigated the possible genetic connection of IR1 with the SNR and the HII region. By comparing the dynamical time of the outflows and the age of the SNR W44, we conclude that the possibility of the SNR has triggered the formation of IR1 is unlikely. On the other hand, we suggest that the expansion of the HII region G034.8-0.7 is responsible for the formation of IR1 through the "collect and collapse" process.

A bright, dust-obscured, millimetre-selected galaxy beyond the Bullet Cluster (1E0657−56)

Deep 1.1 mm continuum observations of 1E0657−56 (the ‘Bullet Cluster’) taken with the millimeter-wavelength camera AzTEC on the 10-m Atacama Submillimeter Telescope Experiment (ASTE), have revealed an extremely bright (S1.1 mm= 15.9 mJy) unresolved source. This source, MMJ065837−5557.0, lies close to a maximum in the density of underlying mass distribution, towards the larger of the two interacting clusters as traced by the weak-lensing analysis of Clowe et al. Using optical–infrared (IR) colours, we argue that MMJ065837−5557.0 lies at a redshift of z= 2.7 ± 0.2. A lensing-derived mass model for the Bullet Cluster shows a critical line (caustic) of magnification within a few arcsec of the AzTEC source, sufficient to amplify the intrinsic millimetre-wavelength flux of the AzTEC galaxy by a factor of ≫20. After subtraction of the foreground cluster emission at 1.1 mm due to the Sunyaev-Zel'dovich effect, and correcting for the magnification, the rest-frame far-IR luminosity of MMJ065837−5557.0 is ≤1012 L⊙, characteristic of a luminous infrared galaxy (LIRG). We explore various scenarios to explain the colours, morphologies and positional offsets between the potential optical and IR counterparts, and their relationship with MMJ065837−5557.0. Until higher resolution and more sensitive (sub)millimetre observations are available, the detection of background galaxies close to the caustics of massive lensing clusters offers the only opportunity to study this intrinsically faint millimetre-galaxy population.

The FFX Correlator

Abstract We established a new algorithm for a correlation process in radio astronomy. This scheme consists of a 1st-stage Fourier transform as a filter and a 2nd-stage Fourier transform for spectroscopy. The “FFX” correlator stands for Filter and FX architecture, since the 1st-stage Fourier transform is performed as a digital filter, and the 2nd-stage Fourier transform is performed as a conventional FX scheme. We developed FFX correlator hardware not only for verifying the FFX scheme algorithm, but also for applying to the Atacama Submillimeter Telescope Experiment (ASTE) telescope toward high-dispersion and wideband radio observations at submillimeter wavelengths. In this paper, we present the principle of the FFX correlator and its properties, as well as evaluation results with the production version.

Millimeter‐ and Submillimeter‐Wave Observations of the OMC‐2/3 Region. II. Observational Evidence for Outflow‐triggered Star Formation in the OMC‐2 FIR 3/4 Region

We have observed the Orion Molecular Cloud-2 FIR 3/4 region in the H13CO+ ( -->J = 1?0),12CO ( -->J = 1?0), SiO ( -->v = 0, -->J = 2?1), and CS ( -->J = 2?1) lines and the 3.3 mm continuum emission with the Nobeyama Millimeter Array (NMA) and in the CO ( -->J = 3?2) and CH3OH ( -->JK = 7K?6K) lines with Atacama Submillimeter Telescope Experiment (ASTE). Our NMA observations in the H13CO+ emission have revealed 0.07 pc scale dense gas associated with FIR 4 (FIR 4 clump). The 12CO ( -->J = 3?2,1-0) emission shows high-velocity blue- and redshifted components to both the northeast and southwest of FIR 3, suggesting an outflow from FIR 3 along the plane of the sky. The SiO and CH3OH emission, known as shock tracers, are detected around the interface between the outflow and FIR 4 clump. Furthermore, the 12CO ( -->J = 1?0) emission shows an L-shaped structure in the PV diagram. These results suggest the presence of an interaction between the outflow and FIR 4 clump. Moreover, our interferometric 3.3 mm continuum observations have first found that FIR 4 consists of 11 dusty cores at a scale of ~2000 AU. The separation among these cores (~ -->5 ? 103 AU) is on the same order of the Jeans length (~ -->13 ? 103 AU), and the estimated time scale of the fragmentation (~ -->3.8 ? 104 yr) is similar to the time scale of the outflow interaction (~ -->1.4 ? 104 yr). We suggest that the interaction triggered the fragmentation into these dusty cores, and hence the next generation of the cluster formation in FIR 4.

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

Abstract We present CO(3-2) emission observations toward the 3′$\times$3′ (or 20kpc$\times$20kpc at a distance of 23Mpc) region of the southern barred spiral galaxy NGC 986 using the Atacama Submillimeter Telescope Experiment (ASTE). This effort is a part of our on-going extragalactic CO(3-2) imaging project, ADIoS (ASTE Dense gas Imaging of Spiral galaxies). Our CO(3-2) image revealed the presence of a large (the major axis is 14kpc in total length) gaseous bar filled with a dense molecular medium along the dark lanes observed in optical images. This is the largest “dense-gas rich bar” known to date. The dense gas bar, discovered in NGC 986, could be a huge reservoir of possible “fuel” for future starbursts in the central region, and we suggest that star formation in the central region of NGC 986 could still be in a growing phase. We found a good spatial coincidence between the overall distributions of dense molecular gas traced by CO(3-2) and massive star formation depicted by H$\alpha $. The global CO(3-2) luminosity, $L’_{\rm CO(3-2)}$, of NGC 986 was determined to be (5.4$\pm$1.1)$\times$10$^8$Kkms$^{-1}$pc$^2$. The CO(3-2)$/$CO(1-0) integrated intensity ratio was found to be 0.60$\pm$0.13 at a spatial resolution of 44$”$ or 5kpc, and the CO(3-2)$/$CO(2-1) ratio was 0.67$\pm$0.14 at a beam size of $\sim $25$”$ or $\sim $2.8kpc. These line ratios suggest moderate excitation conditions of CO lines ($n_{\rm H_2}$$\sim $10$^{3-4}$cm$^{-3}$) in a few kiloparsec region of central NGC 986.

On-The-Fly Observing System of the Nobeyama 45-m and ASTE 10-m Telescopes

Abstract We have developed a spectral line On-The-Fly (OTF) observing mode for the Nobeyama Radio Observatory 45-m and the Atacama Submillimeter Telescope Experiment 10-m telescopes. Sets of digital autocorrelation spectrometers are available for OTF with heterodyne receivers mounted on the telescopes, including the focal-plane 5$\times$5 array receiver, BEARS, on the 45-m. During OTF observations, the antenna is continuously driven to cover the mapped region rapidly, resulting in a high observing efficiency and accuracy. Pointing of the antenna and readouts from the spectrometer are recorded as fast as 0.1s. In this paper we report on improvements made to the software and instruments, requirements and optimization of observing parameters, the data-reduction process, and verification of the system. It is confirmed that, using optimal parameters, the OTF is about twice as efficient as the conventional position-switch observing method.

A Molecular Line Observation toward Massive Clumps Associated with Infrared Dark Clouds

We have surveyed the N2H+ -->J = 1?0, HC3N -->J = 5?4, CCS -->JN = 43?32, NH3 (J, K) = (1, 1), (2, 2), (3, 3), and CH3OH -->J = 7?6 lines toward the 55 massive clumps associated with infrared dark clouds by using the Nobeyama Radio Observatory 45 m telescope and the Atacama Submillimeter Telescope Experiment 10 m telescope. The N2H+, HC3N, and NH3 lines are detected toward most of the objects. On the other hand, the CCS emission is detected toward none of the objects. The [CCS]/[N2H+] ratios are found to be mostly lower than unity even in the Spitzer 24 ?m dark objects. This suggests that most of the massive clumps are chemically more evolved than the low-mass starless cores. The CH3OH emission is detected toward 18 out of 55 objects. All the CH3OH-detected objects are associated with the Spitzer 24 ?m sources, suggesting that star formation has already started in all the CH3OH-detected objects. The velocity widths of the CH3OH -->JK = 70?60 -->A+ and -->7?1?6?1 E lines are broader than those of N2H+ -->J = 1?0. The CH3OH -->JK = 70?60 -->A+ and -->7?1?6?1 E lines tend to have broader line width in the MSX dark objects than in the others, the former being younger or less luminous than the latter. The origin of the broad emission is discussed in terms of the interaction between an outflow and an ambient cloud.

Tracing star formation in galaxies with molecular line and continuum observations

We report our recent progress on extragalactic spectroscopic and continuum observations, including HCN(J=1–0), HCO+(J=1–0), and CN(N=1–0) imaging surveys of local Seyfert and starburst galaxies using the Nobeyama Millimeter Array, high-J CO observations (J=3–2 observations using the Atacama Submillimeter Telescope Experiment (ASTE) and J=2–1 observations with the Submillimeter Array) of galaxies, and λ 1.1 mm continuum observations of high-z violent starburst galaxies using the bolometer camera AzTEC mounted on ASTE.

NRO 45 M and ASTE 10 M observations of dense gas in giant HIIregions of M 33

Wepresent 12 CO( J = 3–2) and 12 CO( J = 1–0) observations of the supergiant Hii region NGC 604 in the nearest face-on spiral galaxy M 33 using the Atacama Submillimeter Telescope Experiment (ASTE) 10-m and the Nobeyama Radio Observatory (NRO) 45-m telescopes. We found high 12 CO( J = 3–2)/ 12 CO( J = 1–0) ratio gas with an arc-like distribution (“high-ratio gas arc”) surrounding the central star cluster of NGC 604. Our results suggest that dense gas formation and second-generation star formation occur in the surrounding gas compressed by the stellar wind and/or supernova of the first-generation stars of NGC 604, i.e. , the central star cluster of NGC 604. Thus, NGC 604 is an example of large-scale sequential star formation.

Dense gas in normal and active galaxies

Dense molecular medium plays essential roles in galaxies. As demonstrated by the tight and linear correlation between HCN(1–0) and FIR luminosities among star-forming galaxies, from very nearby to high-z ones, the observation of a dense molecular component is indispensable to understand the star formation laws in galaxies. In order to obtain a general picture of the global distributions of dense molecular medium in normal star-forming galaxies, we have conducted an extragalactic CO(3–2) imaging survey of nearby spiral galaxies using the Atacama Submillimeter Telescope Experiment (ASTE). From the survey (ADIoS; ASTE Dense gas Imaging of Star-forming galaxies), CO(3–2) images of M 83 and NGC 986 are presented. Emphasis is placed on the correlation between the CO(3–2)/CO(1–0) ratio and the star formation efficiency in galaxies. In the central regions of some active galaxies, on the other hand, we often find enhanced or overluminous HCN(1–0) emission. The HCN(1–0)/CO(1–0) and HCN(1–0)/HCO+(1–0) intensities are often enhanced up to ∼0.2–0.3 and ∼2–3, respectively. Such elevated ratios have never been observed in the nuclear starburst regions. One possible explanation for these high HCN(1–0)/CO(1–0) and HCN(1–0)/HCO+(1–0) ratios is X-ray induced chemistry in X-ray dominated regions (XDRs), i.e., the overabundance of the HCN molecule in the X-ray irradiated dense molecular tori. If this view is true, the known tight correlation between HCN(1–0) and the star-formation rate breaks in the vicinity of active nuclei. Although the interpretation of these ratios is still an open question, these ratios have a great potential for a new diagnostic tool for the energy sources of dusty galaxies in the ALMA era because these molecular lines are free from dust extinction.

A new evolutionary scenario of intermediate-mass star-formation revealed by multi-wavelength observations of OMC-2/3

We have performed millimeter- and submilli- meter-wave survey observations using the Nobeyama millimeter array (NMA) and the Atacama Submillimeter Telescope Experiment (ASTE) in one of the nearest intermediate-mass (IM) star-forming regions: Orion Molecular Cloud-2/3 (OMC-2/3). Using the high-resolution capabilities offered by the NMA (∼several arcsec), we observed dust continuum and H13CO+(1–0) emission in 12 pre- and proto-stellar candidates identified previously in single-dish millimeter observations. We unveiled the evolutionary changes with variations of the morphology and velocity structure of the dense envelopes traced by the H13CO+(1–0) emission. Furthermore, using the high-sensitivity capabilities offered by the ASTE, we searched for large-scale molecular outflows associated with these pre- and proto-stellar candidates observed with the NMA. As a result of the CO(3–2) observations, we detected six molecular outflows associated with the dense gas envelopes traced by H13CO+(1–0) and 3.3 mm continuum emission. The estimated CO outflow momentum increases with the evolutionary sequence from early to late type of the protostellar cores. We also found that the 24 μm flux increases as the dense gas evolutionary sequence. We propose that the enhancement of the 24 μm flux is caused by the growth of the cavity (i.e. the CO outflow destroys the envelope) as the evolutionary sequence. Our results show that the dissipation of the dense gas envelope plays an essential role in the evolution of the IM protostars. The extremely high-sensitivity and high-angular resolution offered by ALMA will reveal unprecedented details of the inner ∼50 AU of these protostars, which will provide us a break through in the classic scenario of IM star/disk formation.

ASTE Submillimeter Observations of a Young Stellar Object Condensation in Cederblad 110

We present results of submillimeter observations of a low-mass young stellar objects (YSOs) condensation in the Cederblad 110 region of the Chamaeleon I dark cloud with Atacama Submillimeter Telescope Experiment. Our HCO+(J=4-3) map reveals a dense molecular gas with an extent of ~ 0.1 pc, which is a complex of two envelopes associated with class I sources Ced110 IRS4 and IRS11 and a very young object Cha-MMS1. The other two class I sources in this region, IRS6 and NIR89, are located outside the clump and have no extended HCO+ emission. HCO+ abundance is calculated to be 2.6 x 10^{-10} for MMS1 and 3.4 x 10^{-9} for IRS4, which are comparable to the reported value for other young sources. Bipolar outflows from IRS4 and IRS6 are detected in our 12CO(J=3-2) map. The outflow from IRS4 seems to collide with Cha-MMS1. The outflow has enough momentum to affect gas motion in MMS1, although no sign has been detected to indicate that a triggered star formation has occurred.

High-Resolution Mappings of the $l=1{\rlap{.}{^\circ}}3$ Complex in Molecular Lines: Discovery of a Proto-Superbubble

Abstract We report on the results of molecular line observations toward the $l=1{\rlap{\hskip.75pt{^\circ}}\hskip.75pt.\hskip1pt}3$ 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 $J$=$1$–$0, {\rm HCO^{+}} J$=$1$–$0$, 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, ${\rm HCO^{+}}$/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 $110\ {\rm kms^{-1}}$. The high-velocity gas has a high density (${n_{\rm H}}\sim10^{4.5}\ {\rm cm^{-3}}$) and high SiO/$^{13}{\rm CO}$ intensity ratio, indicating that the gas was shocked. The typical HCN/${\rm HCO^{+}}$ 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 $10^{50.9\mbox{-}52.5}$ erg and $10^{4.6\mbox{-}5.3}$ yr, respectively. The kinetic energy could be furnished by multiple supernova and/or hypernova explosions at a rate of $10^{-3\mbox{-}-4}$ 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 $10^{6.8\mbox{-}7.6}\ {\rm yr}$ ago.

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

Abstract 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 $T_{\mathrm{mb}}$ scale at a velocity resolution of 10 $\mathrm{km\ s^{-1}}$. No significant emission was detected, though we found a marginal emission feature in the velocity range corresponding to the redshift of the galaxy. We derived 3 $\sigma$ upper limits on the global properties: the velocity-integrated CO (3–2) intensity of $I_{\mathrm{CO}}$(3–2) $< 0.26\ \mathrm{K\ km\ s^{-1}}$, by adopting a velocity width of $67\ \mathrm{km\ s^{-1}}$; an H$_2$ column density of $N(\mathrm{H_2}) < 3 \times 10^{20} \mathrm{cm^{-2}}$; a molecular gas mass of $M(\mathrm{H_2}) < 3 \times 10^8\ M_{\odot}$, by assuming a CO line luminosity to H$_2$ molecular gas mass conversion factor of $X_{\mathrm{CO}}=5.0\times10^{20}\ \mathrm{cm^{-2}\ (K\ km\ s^{-1})^{-1}}$; and a star-formation rate of SFR $< 0.1\ M_{\odot}\ \mathrm{yr}^{-1}$, based on the Schmidt law. The SFR is consistent with the previous results of H$\alpha$ and mid-IR observations, thereby suggesting that there is no significant obscured star formation in the host galaxy of GRB 980425. This result implies that there is a variety of GRB hosts with regard to the presence of obscured star formation.

Physical Conditions of Molecular Gas in the Galactic Center

Abstract 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 = 1–0 $^{12}$CO and $^{13}$CO 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 $\geq 1 \hskip2pt\sigma$ 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.

ASTE CO(3-2) observations of M 83: Correlation between CO(3-2)/CO(1-0) ratios and star formation efficiencies

AbstractWe have performed CO(J=3−2) emission observations with the Atacama Submillimeter Telescope Experiment (ASTE) toward the 5′ × 5′ (or 6.6 × 6.6 kpc at the distance D = 4.5 Mpc) region of the nearby barred spiral galaxy M 83. We successfully resolved the major structures, i.e., the nuclear starburst region, bar, and inner spiral arms in CO(J=3−2) emission at a resolution of 22'' (or 480 pc), showing a good spatial coincidence between CO(J=3−2) and 6 cm continuum emissions.From a comparison of CO(J=3−2) data with CO(J=1−0) intensities measured with Nobeyama 45-m telescope, we found that the radial profile of CO(J=3−2)/CO(J=1−0) integrated intensity ratio R3−2/1−0 is almost unity in the central region (r<0.25 kpc), whereas it drops to a constant value, 0.6–0.7, in the disk region. The radial profile of star formation efficiencies (SFEs), determined from 6 cm radio continuum and CO(J=1−0) emission, shows the same trend as that of R3−2/1−0. At the bar-end (r ~ 2.4 kpc), the amounts of molecular gas and the massive stars are enhanced when compared with other disk regions, whereas there is no excess of R3−2/1−0 and SFE in that region. This means that a simple summation of the star forming regions at the bar-end and the disk cannot reproduce the nuclear starburst of M 83, implying that the spatial variation of the dense gas fraction traced by R3−2/1−0 governs the spatial variation of SFE in M 83.