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We use a neural network model and ALMA observations of HCN and HNC to constrain the physical conditions, most notably the cosmic-ray ionization rate (CRIR, zeta), in the Central Molecular Zone (CMZ) of the starburst galaxy NGC 253. Using output from the chemical code UCLCHEM, we train a neural network model to emulate UCLCHEM and derive HCN and HNC molecular abundances from a given set of physical conditions. We combine the neural network with radiative transfer modeling to generate modeled integrated intensities, which we compare to measurements of HCN and HNC from the ALMA Large Program ALCHEMI. Using a Bayesian nested sampling framework, we constrain the CRIR, molecular gas volume and column densities, kinetic temperature, and beam-filling factor across NGC 253's CMZ. The neural network model successfully recovers UCLCHEM molecular abundances with about 3 percent error and, when used with our Bayesian inference algorithm, increases the parameter inference speed tenfold. We create images of these physical parameters across NGC 253's CMZ at 50 pc resolution and find that the CRIR, in addition to the other gas parameters, is spatially variable with zeta a few times 10^{14} s^{-1} at greater than 100 pc from the nucleus, increasing to zeta greater than 10^{-13} s^{-1} at its center. These inferred CRIRs are consistent within 1 dex with theoretical predictions based on non-thermal emission. Additionally, the high CRIRs estimated in NGC 253's CMZ can be explained by the large number of cosmic-ray-producing sources as well as a potential suppression of cosmic-ray diffusion near their injection sites.

We investigate the dynamical and physical structures of bright-rimmed clouds (BRCs) in a nearby HII region. We focused on carbon- and oxygen-bearing species that trace photon-dominated regions (PDRs) and warm molecular cloud surfaces in order to understand the effect of UV radiation from the exciting stars on the cloud structure. We mapped four regions around the most prominent BRCs at scales of 4--10 arcmin in the HII region IC 1396 in [CII] 158 micron with (up)GREAT on board SOFIA. IC 1396 is predominantly excited by an O6.5V star. Toward IC 1396A, we also observed [OI] 63 micron and 145 micron. We combined these observations with JCMT archive data, which provide the low-J transitions of CO, $^{13}$CO, and C$^{18}$O. All spectra are velocity-resolved. The line profiles show a variety of velocity structures, which we investigated in detail for all observed emission lines. We find no clear sign of photoevaporating flows in the [CII] spectra, although the uncertainty in the location of the BRCs along the line of sight makes this interpretation inconclusive. Our analysis of the [$^{13}$CII] emission in IC 1396A suggests that the [CII] is likely mostly optically thin. The heating efficiency, measured by the ([CII]+[OI] 63 micron)/far-infrared intensity ratio, is higher in the northern part of IC 1396A than in the southern part, which may indicate a difference in the dust properties of the two areas. The complex velocity structures identified in the BRCs of IC 1396, which is apparently a relatively simple HII region, highlight the importance of velocity-resolved data for disentangling different components along the line of sight and thus facilitating a detailed study of the dynamics of the cloud. We also demonstrate that the optically thin [$^{13}$CII] and [OI] 145 micron emission lines are essential for a conclusive interpretation of the [CII] 158 micron and [OI] 63 micron line profiles., Comment: Accepted for publication in A&A, 26 pages including Appendices

For efficient spectroscopic redshift identification of early galaxies in the northern hemisphere, we aim to combine the Large Millimeter Telescope (LMT) with a wide-band heterodyne receiver, FINER, which will cover radio frequencies of 120--360 GHz and offer a 3--21 GHz intermediate frequency (IF) per sideband and polarization. To take full advantage of such wide IFs, we present a novel 10.24-GHz-wide digital spectrometer, DRS4 (Elecs Industry Co., Ltd.). It incorporates 20.48 Gsps samplers with an FPGA-based digital signal processing module. To mitigate the noise contamination from the image sideband, it is equipped with a digital sideband separation function to improve the sideband rejection up to 25 dB. Laboratory performance evaluations show that it exhibits an Allan time of at least ~100 s and a total power dynamic range of at least 7 dB. These results demonstrate its capability of instantaneously wide-band spectroscopy toward high-redshift galaxies with position-switching observations., Comment: 10 pages, 7 figures, Proceedings paper presented in SPIE Astronomical Telescope and Instrumentation 2024

Unveiling the emergence and prevalence of massive/bright galaxies during the epoch of reionization and beyond, within the first 600 million years of the Universe, stands as a pivotal pursuit in astronomy. Remarkable progress has been made by JWST in identifying an immense population of bright galaxies, which hints at exceptionally efficient galaxy assembly processes. However, the underlying physical mechanisms propelling their rapid growth remain unclear. With this in mind, millimeter and submillimeter-wave spectroscopic observations of redshifted far-infrared spectral lines, particularly the [O III] 88 micron and [C II] 158 micron lines, offers a crucial pathway to address this fundamental query. To this end, we develop a dual-polarization sideband-separating superconductor-insulator-superconductor (SIS) mixer receiver, FINER, for the Large Millimeter Telescope (LMT) situated in Mexico. Harnessing advancements from ALMA's wideband sensitivity upgrade (WSU) technology, FINER covers radio frequencies spanning 120-360 GHz, delivering an instantaneous intermediate frequency (IF) of 3-21 GHz per sideband per polarization, which is followed by a set of 10.24 GHz-wide digital spectrometers. At 40% of ALMA's light-collecting area, the LMT's similar atmospheric transmittance and FINER's 5 times wider bandwidth compared to ALMA culminate in an unparalleled spectral scanning capability in the northern hemisphere, paving the way for finer spectral-resolution detection of distant galaxies., Comment: 12 pages, 8 figures, and 3 tables. Proceedings paper presented in SPIE Astronomical Telescope and Instrumentation 2024

The physical properties of galactic molecular outflows are important as they could constrain outflow formation mechanisms. We study the properties of the southwest (SW) outflow streamer including gas kinematics, optical depth, dense gas fraction, and shock strength in the central molecular zone of the starburst galaxy NGC 253. We image the molecular emission at a spatial resolution of $\sim$27 pc based on data from the ALCHEMI program. We trace the kinematics of molecular gas with CO(1-0) line. We constrain the optical depth of CO emission with CO/$^{13}$CO(1-0) ratio, the dense gas fraction with HCN/CO(1-0) ratio, as well as the shock strength with SiO(2-1)/$^{13}$CO(1-0) ratio. The CO/$^{13}$CO(1-0) integrated intensity ratio is $\sim$21 in the SW streamer region, which approximates the C/$^{13}$C isotopic abundance ratio. The higher integrated intensity ratio compared to the disk can be attributed to the optically thinner environment for CO(1-0) emission inside the SW streamer. The HCN/CO(1-0) and SiO(2-1)/$^{13}$CO(1-0) integrated intensity ratios both approach $\sim$0.2 in three giant molecular clouds (GMCs) at the base of the outflow streamers, which implies the higher dense gas fraction and enhanced strength of fast shocks in those GMCs than in the disk. The contours of those two integrated intensity ratios are extended towards the directions of outflow streamers, which connects the enhanced dense gas fraction and shock strength with molecular outflow. Moreover, the molecular gas with enhanced dense gas fraction and shock strength located at the base of the SW streamer shares the same velocity with the outflow. These phenomena suggest that the star formation inside the GMCs can trigger the shocks and further drive the molecular outflow., Comment: Accepted for publication in A&A, 14 pages, 11 figures

Molecular lines are powerful diagnostics of the physical and chemical properties of the interstellar medium (ISM). These ISM properties, which affect future star formation, are expected to differ in starburst galaxies from those of more quiescent galaxies. We investigate the ISM properties in the central molecular zone of the nearby starburst galaxy NGC 253 using the ultra-wide millimeter spectral scan survey from the ALMA Large Program ALCHEMI. We present an atlas of velocity-integrated images at a 1".6 resolution of 148 unblended transitions from 44 species, including the first extragalactic detection of HCNH$^+$ and the first interferometric images of C$_3$H$^+$, NO, HCS$^+$. We conduct a principal component analysis (PCA) on these images to extract correlated chemical species and to identify key groups of diagnostic transitions. To the best of our knowledge, our dataset is currently the largest astronomical set of molecular lines to which PCA has been applied. The PCA can categorize transitions coming from different physical components in NGC 253 such as i) young starburst tracers characterized by high-excitation transitions of HC$_3$N and complex organic molecules (COMs) versus tracers of on-going star formation (radio recombination lines) and high-excitation transitions of CCH and CN tracing PDRs, ii) tracers of cloud-collision-induced shocks (low-excitation transitions of CH$_3$OH, HNCO, HOCO$^+$, and OCS) versus shocks from star-formation-induced outflows (high-excitation transitions of SiO), as well as iii) outflows showing emission from HOC$^+$, CCH, H$_3$O$^+$, CO isotopologues, HCN, HCO$^+$, CS, and CN. Our findings show these intensities vary with galactic dynamics, star formation activities, and stellar feedback., Comment: 65 pages, 39 figures. Accepted for publication in ApJS

We present a spatially-resolved excitation analysis for the central molecular zone (CMZ) of the starburst galaxy NGC 253 using the data from the ALMA Large program ALCHEMI, whereby we explore parameters distinguishing NGC 253 from the quiescent Milky Way's Galactic Center (GC). Non-LTE analyses employing a hierarchical Bayesian framework are applied to Band 3-7 transitions from nine molecular species to delineate the position-position-velocity distributions of column density ($N_\mathrm{H_2}$), volume density ($n_\mathrm{H_2}$), and temperature ($T_\mathrm{kin}$) at 27 pc resolution. Two distinct components are detected: a low-density component with $(n_\mathrm{H_2},\ T_\mathrm{kin})\sim(10^{3.3}\ \mathrm{cm}^{-3}, 85 K)$ and a high-density component with $(n_\mathrm{H_2},\ T_\mathrm{kin})\sim (10^{4.4}\ \mathrm{cm}^{-3}, 110\ \mathrm{K})$, separated at $n_\mathrm{H_2}\sim10^{3.8}\ \mathrm{cm}^{-3}$. NGC 253 has $\sim10$ times the high-density gas mass and $\sim3$ times the dense-gas mass fraction of the GC. These properties are consistent with their HCN/CO ratio but cannot alone explain the factor of $\sim30$ difference in their star formation efficiencies (SFEs), contradicting the dense-gas mass to star formation rate scaling law. The $n_\mathrm{H_2}$ histogram toward NGC 253 exhibits a shallow declining slope up to $n_\mathrm{H_2}\sim10^6\ \mathrm{cm}^{-3}$, while that of the GC steeply drops in $n_\mathrm{H_2}\gtrsim10^{4.5}\ \mathrm{cm}^{-3}$ and vanishes at $10^5\ \mathrm{cm}^{-3}$. Their dense-gas mass fraction ratio becomes consistent with their SFEs when the threshold $n_\mathrm{H_2}$ for the dense gas is taken at $\sim 10^{4.2\mbox{-}4.6}\ \mathrm{cm}^{-3}$. The rich abundance of gas above this density range in the NGC 253 CMZ, or its scarcity in the GC, is likely to be the critical difference characterizing the contrasting star formation in the centers of the two galaxies., Comment: 49 pages, 27 figures, 7 tables, accepted for publication in the Astrophysical Journal

We present position-position-velocity (PPV) cubes of the physical and chemical properties of the molecular medium in the central 1.2 kpc region of the active galaxy NGC 613 at a PPV resolution of 0.$^{\prime\prime}$8$\times$0.$^{\prime\prime}$8$\times$10 km s$^{-1}$ (0.$^{\prime\prime}$8 = $\sim$68 pc). We used eight molecular lines obtained with ALMA. Non-LTE calculation with hierarchical Bayesian inference was used to construct PPV cubes of the gas kinetic temperature ($T_\mathrm{kin}$), molecular hydrogen volume density ($n_\mathrm{H_2}$), column densities ($N_\mathrm{H_2}$), and fractional abundances of four molecules ($^{12}$C$^{18}$O, HCN, HCO$^+$, and CS). The derived $n_\mathrm{H_2}$, $N_\mathrm{H_2}$, and $T_\mathrm{kin}$ ranged 10$^{3.21-3.85}$ cm$^{-3}$, 10$^{20.8-22.1}$ cm$^{-2}$, and 10$^{2.33-2.64}$ K, respectively. Our first application of the non-LTE method with the hierarchical Bayesian inference to external galaxies yielded compatible results compared with the previous studies of this galaxy, demonstrating the efficacy of this method for application to other galaxies. We examined the correlation between gas surface density $\Sigma_\mathrm{H_2}$ (converted from $N_\mathrm{H_2}$) and the star formation rate $\Sigma_\mathrm{SFR}$ obtained from the 110 GHz continuum flux map and found two distinct sequences in the $\Sigma_\mathrm{H_2}$-$\Sigma_\mathrm{SFR}$ diagram; the southwestern subregion of the star-forming ring exhibited a $\sim$0.5 dex higher star formation efficiency (SFE; $\Sigma_\mathrm{SFR}/\Sigma_\mathrm{H_2}$) than the eastern subregion. However, they exhibited no systematic difference in $n_\mathrm{H_2}$, which is often argued as a driver of SFE variation. We suggest that the deficiency of molecular gas in the southwestern subregion, where no significant gas supply is evident along the offset ridges in the bar, is responsible for the elevated SFE., Comment: 18 pages, 14 figures, accepted to ApJ

We analyze HCN and HNC emission in the nearby starburst galaxy NGC 253 to investigate its effectiveness in tracing heating processes associated with star formation. This study uses multiple HCN and HNC rotational transitions observed using ALMA via the ALCHEMI Large Program. To understand the conditions and associated heating mechanisms within NGC 253's dense gas, we employ Bayesian nested sampling techniques applied to chemical and radiative transfer models which are constrained using our HCN and HNC measurements. We find that the volume density $n_{\text{H}_{2}}$ and cosmic ray ionization rate (CRIR) $\zeta$ are enhanced by about an order of magnitude in the galaxy's central regions as compared to those further from the nucleus. In NGC 253's central GMCs, where observed HCN/HNC abundance ratios are lowest, $n \sim 10^{5.5}$ cm$^{-3}$ and $\zeta \sim 10^{-12}$ s$^{-1}$ (greater than $10^4$ times the average Galactic rate). We find a positive correlation in the association of both density and CRIR with the number of star formation-related heating sources (supernova remnants, HII regions, and super hot cores) located in each GMC, as well as a correlation between CRIRs and supernova rates. Additionally, we see an anticorrelation between the HCN/HNC ratio and CRIR, indicating that this ratio will be lower in regions where $\zeta$ is higher. Though previous studies suggested HCN and HNC may reveal strong mechanical heating processes in NGC 253's CMZ, we find cosmic ray heating dominates the heating budget, and mechanical heating does not play a significant role in the HCN and HNC chemistry., Comment: 33 pages, 23 figures, accepted for publication by the Astrophysical Journal

Centers of starburst galaxies may be characterized by a specific gas and ice chemistry due to their gas dynamics and the presence of various ice desorption mechanisms. This may result in a peculiar observable composition. We analyze abundances of $CO_2$, a reliable tracer of ice chemistry, from data collected as part of the ALMA large program ALCHEMI, a wide-frequency spectral scan toward the starburst galaxy NGC~253 with an angular resolution of 1.6$''$. We constrain the $CO_2$ abundances in the gas phase using its protonated form $HOCO^+$. The distribution of $HOCO^+$ is similar to that of methanol, which suggests that $HOCO^+$ is indeed produced from the protonation of $CO_2$ sublimated from ice. The $HOCO^+$ fractional abundances are found to be $(1-2)\times10^{-9}$ at the outer part of the central molecular zone (CMZ), while they are lower ($\sim10^{-10}$) near the kinematic center. This peak fractional abundance at the outer CMZ is comparable to that in the Milky Way CMZ, and orders of magnitude higher than that in Galactic disk star-forming regions. From the range of $HOCO^+/CO_2$ ratios suggested from chemical models, the gas-phase $CO_2$ fractional abundance is estimated to be $(1-20)\times10^{-7}$ at the outer CMZ, and orders of magnitude lower near the center. We estimate the $CO_2$ ice fractional abundances at the outer CMZ to be $(2-5)\times10^{-6}$ from the literature. A comparison between the ice and gas $CO_2$ abundances suggests an efficient sublimation mechanism. This sublimation is attributed to large-scale shocks at the orbital intersections of the bar and CMZ., Comment: 22 pages, 9 figures. Accepted for publication in the Astrophysical Journal

The cosmic ray ionization rate (CRIR) is a key parameter in understanding the physical and chemical processes in the interstellar medium. Cosmic rays are a significant source of energy in star formation regions, which impacts the physical and chemical processes which drive the formation of stars. Previous studies of the circum-molecular zone (CMZ) of the starburst galaxy NGC 253 have found evidence for a high CRIR value; $10^3-10^6$ times the average cosmic ray ionization rate within the Milky Way. This is a broad constraint and one goal of this study is to determine this value with much higher precision. We exploit ALMA observations towards the central molecular zone of NGC 253 to measure the CRIR. We first demonstrate that the abundance ratio of H$_3$O$^+$ and SO is strongly sensitive to the CRIR. We then combine chemical and radiative transfer models with nested sampling to infer the gas properties and CRIR of several star-forming regions in NGC 253 due to emission from their transitions. We find that each of the four regions modelled has a CRIR in the range $(1-80)\times10^{-14}$ s$^{-1}$ and that this result adequately fits the abundances of other species that are believed to be sensitive to cosmic rays including C$_2$H, HCO$^+$, HOC$^+$, and CO. From shock and PDR/XDR models, we further find that neither UV/X-ray driven nor shock dominated chemistry are a viable single alternative as none of these processes can adequately fit the abundances of all of these species., Comment: 24 pages, 15 figures, accepted for publication in ApJ, updated to include changes made for proofs

Molecular abundances are sensitive to UV-photon flux and cosmic-ray ionization rate. In starburst environments, the effects of high-energy photons and particles are expected to be stronger. We examine these astrochemical signatures through multiple transitions of HCO$^+$ and its metastable isomer HOC$^+$ in the center of the starburst galaxy NGC 253 using data from the ALMA large program ALCHEMI. The distribution of the HOC$^+$(1-0) integrated intensity shows its association with "superbubbles", cavities created either by supernovae or expanding HII regions. The observed HCO$^+$/HOC$^+$ abundance ratios are $\sim 10-150$, and the fractional abundance of HOC$^+$ relative to H$_2$ is $\sim 1.5\times 10^{-11} - 6\times 10^{-10}$, which implies that the HOC$^+$ abundance in the center of NGC 253 is significantly higher than in quiescent spiral-arm dark clouds in the Galaxy and the Galactic center clouds. Comparison with chemical models implies either an interstellar radiation field of $G_0\gtrsim 10^3$ if the maximum visual extinction is $\gtrsim 5$, or a cosmic-ray ionization rate of $\zeta \gtrsim 10^{-14}$ s$^{-1}$ (3-4 orders of magnitude higher than that within clouds in the Galactic spiral-arms) to reproduce the observed results. From the difference in formation routes of HOC$^+$, we propose that a low-excitation line of HOC$^+$ traces cosmic-ray dominated regions, while high-excitation lines trace photodissociation regions. Our results suggest that the interstellar medium in the center of NGC 253 is significantly affected by energy input from UV-photons and cosmic rays, sources of energy feedback., Comment: 57 pages, 32 figures, accepted for publication in ApJ

Being one of the most abundant atomic/molecular species observed in dense molecular gas, atomic carbon ($\mathrm{C}^0$) is a potential good tracer of molecular gas mass in many chemical/physical environments, though the $\mathrm{C^0}$ abundance variation outside the Galactic disk region is yet to be fully known. This paper presents a wide-field 500 GHz [CI] map of the Galactic central molecular zone (CMZ) obtained with the ASTE 10-m telescope. Principal component analysis and non-LTE multi-transition analysis have shown that the [CI] emission predominantly originates from the low-excitation gas component with a 20-50 K temperature and $\sim 10^3\ \mathrm{cm}^{-3}$ density, whereas $\mathrm{C^0}$ abundance is likely suppressed in the high-excitation gas component. The average $N(\mathrm{C}^0)$/$N(\mathrm{CO})$ abundance ratio in the CMZ is 0.3-0.4, which is 2-3 times that in the Galactic disk. The $N(\mathrm{C}^0)$/$N(\mathrm{CO})$ ratio increases to 0.7 in the innermost 10 pc region and to $\sim2$ in the circumnuclear disk. We discovered $\mathrm{C^0}$-rich regions distributed in a ring-shape encircling the supernova remnant (SNR) Sgr A east, indicative that the $\mathrm{C}^0$-enrichment in the central 10 pc region is a consequence of a molecular cloud-SNR interaction. In the 15 atom/molecules included in principal component analysis (PCA), CN is the only other species that increases in the [CI]-bright ring. The origin of the [CI]-bright ring is likely a cosmic-ray dominated region created by low-energy cosmic-ray particles accelerated by Sgr A east or primitive molecular gas collected by the SNR in which the $\mathrm{C}^0$-to-CO conversion has not reached the equilibrium., Comment: 23 pages, 16 figures, accepted for publication in Astrophysical Journal

We report a statistical analysis exploring the origin of the overall low star formation efficiency (SFE) of the Galactic central molecular zone (CMZ) and the SFE diversity among the CMZ clouds using a wide-field HCN $J$=4-3 map, whose optically thin critical density ($\sim10^7\,\mathrm{cm}^{-3}$) is the highest among the tracers ever used in CMZ surveys. Logistic regression is performed to empirically formulate star formation probability of 195 HCN clumps, 13 of which contain star formation signatures. The explanatory parameters in the best-fit model are reduced into the virial parameter $\alpha_{\mathrm{vir}}$ without significant contribution from other parameters, whereas the performance of the model without $\alpha_{\mathrm{vir}}$ is no better than that using randomly generated data. The threshold $\alpha_{\mathrm{vir}}$ is 6, which translates into a volume density ($n_{\mathrm{H_2}}$) of $10^{4.6}\,\mathrm{cm}^{-3}$ with the $n_{\mathrm{H_2}}$-$\alpha_{\mathrm{vir}}$ correlation. The scarcity of the low-$\alpha_{\mathrm{vir}}$ clumps, whose fraction to all HCN clumps is 0.1, can be considered as one of the immediate causes of the suppressed SFE. No correlation between the clump size or mass and star formation probability is found, implying that HCN $J$=4-3 does not immediately trace the mass of star-forming gas above a threshold density. Meanwhile, star-forming and non-star-forming clouds are degenerate in the physical parameters of the CS $\mathit{J}$=1-0 clouds, highlighting the efficacy of the HCN $\mathit{J}$=4-3 line to probe star-forming regions in the CMZ. The time scale of the high-$\alpha_{\mathrm{vir}}$ to low-$\alpha_{\mathrm{vir}}$ transition is $\lesssim2$ Myr, which is consistent with the tidal compression and X1/X2 orbit transition models but possibly does not fit the cloud-cloud collision picture., Comment: 25 pages, 7 figures, 5 tables, accepted for publication in the Astrophysical Journal

This paper reports a re-analysis of archival ALMA data of the high velocity (-width) compact cloud (HVCC) CO-0.40-0.22, which has recently been hypothesized to host an intermediate-mass black Hole (IMBH). If beam-smearing effects, difference in beam sizes among frequency bands, and Doppler shift due to the motion of the Earth are considered accurately, none of the features reported as evidence for an IMBH in previous studies are confirmed in the re-analyzed ALMA images. Instead, through analysis of the position--velocity structure of the HCN J=3-2 data cube, we have found kinematics typical of a cloud-cloud collision (CCC), namely, two distinct velocity components bridged by broad emission features with elevated temperatures and/or densities. One velocity component has a straight filamentary shape with approximately constant centroid velocities along its length but with a steep, V-shaped velocity gradient across its width. This contradicts the IMBH scenario but is consistent with a collision between two dissimilar-sized clouds. From a non-LTE analysis of the multi-transition methanol lines, the volume density of the post-shock gas has been measured to be $\gtrsim 10^6\ \mathrm{cm}^{-3}$, indicating that the CCC shock can compress gas in a short timescale to densities typical of star-forming regions. Evidence for star formation has not been found, possibly because the cloud is in an early phase of CCC-triggered star formation or because the collision is non-productive., Comment: 14 pages, 14 figures, accepted for publication in the Astrophysical Journal

This {\it supplement} paper presents the maps of HCN $J$=4-3, HNC $J$=1-0, $\mathrm{H^{13}CN}$ $J$=1-0, and HC$_3$N $J$=10-9 for the Galactic central molecular zone (CMZ), which have been obtained using the Atacama Submillimeter Telescope Experiment and Nobeyama Radio Observatory 45-m telescope. Three-dimensional maps (2-D in space and 1-D in velocity) of the gas kinetic temperature ($T_\mathrm{kin}$), hydrogen volume density ($n_\mathrm{H_2}$), and fractional abundances of eight molecules (HCN, HNC, $\mathrm{HC_3N}$, HCO$^+$, $\mathrm{H_2CO}$, SiO, CS, and $\mathrm{N_2H^+}$) have been constructed from our and archival data. We have developed a method with hierarchical Bayesian inference for this analysis, which has successfully suppressed the artificial correlations among the parameters created by systematic errors due to the deficiency in the simple one-zone excitation analysis and the calibration uncertainty. The typical values of $T_\mathrm{kin}$ and $n_\mathrm{H_2}$ are $10^{1.8}\ $K and $10^{4.2}\ \mathrm{cm}^{-3}$, respectively, and the presence of an additional cold/low-density component is also indicated. The distribution of high-temperature regions is poorly correlated with known active star-forming regions, while a few of them coincide with shocked clouds. Principal component analysis has identified two distinct groups in the eight analyzed molecules: one group with large PC1 and PC2 scores and the other with a large $T_\mathrm{kin}$ dependence, which could be explained using two regimes of shock chemistry with fast ($\gtrsim 20\ \mathrm{km\,s}^{-1}$) and slow ($\lesssim 20\ \mathrm{km\,s}^{-1}$) velocity shocks, respectively. This supports the idea that the mechanical sputtering of dust grains and the mechanical heating play primary roles in the chemical and thermal processes in CMZ clouds., Comment: 51 pages, 58 figures, Accepted for publication in Astrophysical Journal Supplement Series

This paper reports the discovery of evidence for physical contact between the Galactic circumnuclear disk (CND) and an exterior giant molecular cloud. The central 10 pc of our Galaxy has been imaged in the HCN J=1-0, HCO+ J=1-0, CS J=2-1, H13CN J=1-0, SiO J=2-1, SO N_J=2_3-1_2, and HC3N J=11-10 lines using the Nobeyama Radio Observatory 45 m radio telescope. Based on our examination of the position-velocity maps of several high-density probe lines, we have found that an emission "bridge" may be connecting the +20 km/s cloud (M-0.13-0.08) and the negative-longitude extension of the CND. Analyses of line intensity ratios imply that the chemical property of the bridge is located between the +20 km/s cloud and the CND. We introduce a new interpretation that a part of the CND may be colliding with the 20 km/s cloud and the collision may be responsible for the formation of the bridge. Such collisional events could promote mass accretion onto the CND or into the inner ionized cavity, which may be further tested by proper motion studies., Comment: Accepted for publication in The Astrophysical Journal

We present $1''.4$ (98 pc) resolution ALMA observations of $^{13}$CO($J$=1-0), C$^{18}$O($J$=1-0), CS($J$=2-1) and CH$_3$OH($J_K$=$2_K$-$1_K$) molecular lines in the central $1'$ (4.2 kpc) region of NGC 1068 to study the physical and chemical properties of giant molecular clouds (GMCs) and to test whether these GMC-scale properties are linked to the larger-scale galactic environment. Using the derived $^{13}$CO cube, we have identified 187 high-significance ($> 8\sigma$) GMCs by employing the CLUMPFIND algorithm. The molecular gas masses of GMCs $M_{\rm ^{13}CO}$, derived from the $^{13}$CO data, range from $1.8 \times 10^4 ~ M_\odot$ to $4.2 \times 10^7 ~M_\odot$. A mass function of GMCs in NGC 1068 has been obtained for the first time at $\sim$100 pc resolution. We find the slope of the mass function $\gamma = -1.25 \pm 0.07$ for a mass range of $M_{\rm ^{13}CO} \geq 10^5M_\odot$. This is shallower than the GMCs in the disk regions of the Milky Way, M51 and NGC 300. Further, we find that the high mass cut-off of the GMC mass function occurs at $M_{\rm ^{13}CO} \sim 6 \times 10^7 M_\odot$, which is an order of magnitude larger than that in the nuclear bar region of M51, indicating that the more massive clouds dominate the mass budget in NGC 1068. The observed C$^{18}$O)/$^{13}$CO intensity ratios are found to be fairly uniform (0.27 $\pm$ 0.05) among the identified GMCs. In contrast, the CH$_3$OH/$^{13}$CO ratios exhibit striking spatial variation across the disk, with the smallest values around the bar-end ($<$ 0.03), and larger ratios along the spiral arms ($\sim$ 0.1-0.2). We find that GMCs with detectable methanol emission tend to have systematically larger velocity widths than those without methanol emission, suggesting that (relatively weak) shocks are responsible for the enhancement of the CH$_3$OH/$^{13}$CO ratios of GMCs in the disk of NGC 1068., Comment: 35 pages, 13 figures, in press at Publ. Astron. Soc. Japan

The high velocity dispersion compact cloud CO-0.30-0.07 is a peculiar molecular clump discovered in the central moleculr zone of the Milky Way, which is characterized by its extremely broad velocity emissions ($\sim 145\ \rm{km s^{-1}}$) despite the absence of internal energy sources. We present new interferometric maps of the cloud in multiple molecular lines in frequency ranges of 265--269 GHz and 276--280 GHz obtained using the Sumbmillimeter Array, along with the single-dish images previously obtained with the ASTE 10-m telescope. The data show that the characteristic broad velocity emissions are predominantly confined in two parallel ridges running through the cloud center. The central ridges are tightly anti-correlated with each other in both space and velocity, thereby sharply dividing the entire cloud into two distinct velocity components (+15 km s$^{-1}$ and +55 km s$^{-1}$). This morphology is consistent with a model in which the two velocity components collide with a relative velocity of 40 $\mathrm{km s^{-1}}$ at the interface defined by the central ridges, although an alternative explanation with a highly inclined expanding-ring model is yet to be fully invalidated. We have also unexpectedly detected several compact clumps ($\lesssim 0.1\ $pc in radius) likely formed by shock compression. The clumps have several features in common with typical star-forming clouds: high densities ($10^{6.5-7.5}\ \mathrm{cm^{-3}}$), rich abundances of hot-core-type molecular species, and relatively narrow velocity widths apparently decoupled from the furious turbulence dominating the cloud. The cloud CO-0.30-0.07 is possibly at an early phase of star formation activity triggered by the shock impact., Comment: 29 pages, 10 figures, accepted for publication in ApJ

We investigate the effects of Supergiant Shells (SGSs) and their interaction on dense molecular clumps by observing the Large Magellanic Cloud (LMC) star forming regions N48 and N49, which are located between two SGSs, LMC 4 and LMC 5. $^{12}$CO ($J$=3-2, 1-0) and $^{13}$CO ($J$=1-0) observations with the ASTE and Mopra telescopes have been carried out towards these regions. A clumpy distribution of dense molecular clumps is revealed with 7 pc spatial resolution. Large velocity gradient analysis shows that the molecular hydrogen densities ($n({\rm H}_2)$) of the clumps are distributed from low to high density ($10^3$-$10^5$ cm$^{-3}$) and their kinetic temperatures ($T_{\rm kin}$) are typically high (greater than $50$ K). These clumps seem to be in the early stages of star formation, as also indicated from the distribution of H$\alpha$, young stellar object candidates, and IR emission. We found that the N48 region is located in the high column density HI envelope at the interface of the two SGSs and the star formation is relatively evolved, whereas the N49 region is associated with LMC 5 alone and the star formation is quiet. The clumps in the N48 region typically show high $n({\rm H}_2)$ and $T_{\rm kin}$, which are as dense and warm as the clumps in LMC massive cluster-forming areas (30 Dor, N159). These results suggest that the large-scale structure of the SGSs, especially the interaction of two SGSs, works efficiently on the formation of dense molecular clumps and stars., Comment: 26 pages, 7 tables, 16 figures

We have performed unbiased spectral line surveys at 3 mm band toward the Galactic circumnuclear disk (CND) and Sgr A* using the Nobeyama Radio Observatory (NRO) 45 m radio telescope. The target positions are two tangential points of the CND and the direction of Sgr A*. We have obtained three wide-band spectra which cover the frequency range from 81.3 GHz to 115.8 GHz, detecting 46 molecular lines from 30 species including 10 rare isotopomers and four hydrogen recombination lines. Each line profile consists of multiple velocity components which arise from the CND, +50 km/s and +20 km/s clouds (GMCs), and the foreground spiral arms. We define the specific velocity ranges which represent the CND and the GMCs toward each direction, and classify the detected lines into three categories: the CND-/GMC-/HBD-types, based on the line intensities integrated over the defined velocity ranges. The CND- and GMC-types are the lines which mainly trace the CND and the GMCs, respectively. The HBD-type possesses the both characteristics of the CND-/GMC-types. We also present the lists of line intensities and other parameters, as well as intensity ratios, which must be useful to investigate the difference between nuclear environments of our Galaxy and of others., Comment: Accepted for publication in The Astrophysical Journal Supplement Series, 41 pages, 9 figures

We report the results of millimeter-wave molecular line observations of the Tornado Nebula (G357.7--0.1), which is a bright radio source behind the Galactic Center region. A 15'x15' area was mapped in the J=1--0 lines of CO, 13CO, and HCO+ with the Nobeyama Radio Observatory 45-m telescope. The VLA archival data of OH at 1720 MHz were also reanalyzed. We found two molecular clouds with separate velocities, V_LSR=-14 km/s and +5 km/s. These clouds show rough spatial anti-correlation. Both clouds are associated with OH 1720 MHz emissions in the area overlapping with the Tornado Nebula. The spatial and velocity coincidence indicates violent interaction between the clouds and the Tornado nebula. Modestly excited gas prefers the position of the Tornado "head" in the -14 km/s cloud, also suggesting the interaction. Virial analysis shows that the +5 km/s cloud is more tightly bound by self-gravity than the -14 km/s cloud. We propose a formation scenario for the Tornado Nebula; the +5 km/s cloud collided into the -14 km/s cloud, generating a high-density layer behind the shock front, which activates a putative compact object by Bondi-Hoyle-Lyttleton accretion to eject a pair of bipolar jets., Comment: Accepted for publication in The Astrophysical Journal, 8 pages, 7 figures

We report the detection of extremely broad emission toward two molecular clumps in the Galactic central molecular zone. We have mapped the Sagittarius C complex ($-0^\circ.61 < l < -0^\circ.27$, $-0^\circ.29 < b < 0^\circ.04$) in the HCN $J$ = 4--3, $\mathrm{^{13}CO}$ $J$ = 3--2, and $\mathrm{H^{13}CN}$ $J$ = 1--0 lines with the ASTE 10 m and NRO 45 m telescopes, detecting bright emission with $80\mbox{--}120$\ $\mathrm{km\,s^{-1}}$ velocity width (in full-width at zero intensity) toward CO$-0.30$$-0.07$ and CO$-0.40$$-0.22$, which are high velocity compact clouds (HVCCs) identified with our previous CO $J$ = 3--2 survey. Our data reveal an interesting internal structure of CO$-0.30$$-0.07$ comprising a pair of high velocity lobes. The spatial-velocity structure of CO$-0.40$$-0.22$ can be also understood as multiple velocity component, or a velocity gradient across the cloud. They are both located on the rims of two molecular shells of about 10 pc in radius. Kinetic energies of CO$-0.30$$-0.07$ and CO$-0.40$$-0.22$ are $\left(0.8\mbox{--}2\right)\times10^{49}$ erg and $\left(1\mbox{--}4\right)\times10^{49}$ erg, respectively. We propose several interpretations of their broad emission: collision between clouds associated with the shells, bipolar outflow, expansion driven by supernovae (SNe), and rotation around a dark massive object. There scenarios cannot be discriminated because of the insufficient angular resolution of our data, though the absence of visible energy sources associated with the HVCCs seems to favor the cloud--cloud collision scenario. Kinetic energies of the two molecular shells are $1\times10^{51}$ erg and $0.7\times10^{51}$ erg, which can be furnished by multiple SN or hypernova explosions in $2\times10^5$ yr. These shells are candidates of molecular superbubbles created after past active star formation., Comment: 23 pages, 8 figures, accepted for publication in Astrophysical Journal

We mapped molecular gas toward the supernova remnant W44 in the HCO+ J=1-0 line with the Nobeyama Radio Observatory 45 m telescope and in the CO J=3-2 line with the Atacama Submillimeter Telescope Experiment 10 m telescope. High-velocity emission wings were detected in both lines over the area where the radio shell of W44 overlaps the molecular cloud in the plane of the sky. We found that the average velocity distributions of the wing emission can be fitted by a uniform expansion model. The best-fit expansion velocities are 12.2+-0.3 km/s and 13.2+-0.2 km/s in HCO+ and CO, respectively. The non-wing CO J=3-2 component is also fitted by the same model with an expansion velocity of 4.7+-0.1 km/s . This component might be dominated by a post shock higher-density region where the shock velocity had slowed down. The kinetic energy of shocked molecular gas is estimated to be (3.5+-1.3)x10^{49} erg. Adding this and the energy of the previously identified HI shell, we concluded that (1.2+-0.2)x10^{50} erg has been converted into gas kinetic energy from the initial baryonic energy of the W44 supernova. We also found ultra-high-velocity CO J=3-2 wing emission with a velocity width of ~100 km/s at (l, b)=(+34.73d, -0.47d). The origin of this extremely high-velocity wing is a mystery., Comment: 11 pages, 8 figures

This paper reports the discovery of a helical molecular cloud in the central molecular zone (CMZ) of our Galaxy. This "pigtail" molecular cloud appears at (l, b, V_LSR) ~ (-0.7deg, +0.0deg, -70 to -30 km/s), with a spatial size of ~ (20 pc)^2 and a mass of (2-6) 10^5 solar masses. This is the third helical gaseous nebula found in the Galactic center region to date. Line intensity ratios indicate that the pigtail molecular cloud has slightly higher temperature and/or density than the other normal clouds in the CMZ. We also found a high-velocity wing emission near the footpoint of this cloud. We propose a formation model of the pigtail molecular cloud. It might be associated with a magnetic tube that is twisted and coiled because of the interaction between clouds in the innermost x_1 orbit and ones in the outermost x_2 orbit., Comment: 15 pages, 6 figures, accepted for publication in Astrophysical Journal

This letter presents a Nyquist-sampled, high-resolution [CI] 3P1-3P0 map of the -0.2 deg < l < 1.2 deg x -0.1 deg < b < 0 deg region in the Central Molecular Zone (CMZ) taken with the Atacama Submillimeter Telescope Experiment (ASTE) 10 m telescope. We have found that molecular clouds in the CMZ can be classified into two groups according to their [CI]/13CO intensity ratios: a bulk component consisting with clouds with a low, uniform [CI]/13CO ratio (0.45) and another component consisting of clouds with high [CI]/13CO ratios (> 0.8). The [CI]-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 (LVG) analysis and have derived the C^0/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 C^0 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., Comment: 16 pages, 5 figures, accepted for publication in Astrophysical Journal Letters

In order to precisely determine temperature and density of molecular gas in the Large Magellanic Cloud, we made observations of optically thin $^{13}$CO($J=3-2$) transition by using the ASTE 10m telescope toward 9 peaks where $^{12}$CO($J=3-2$) clumps were previously detected with the same telescope. The molecular clumps include those in giant molecular cloud (GMC) Types I (with no signs of massive star formation), II (with HII regions only), and III (with HII regions and young star clusters). We detected $^{13}$CO($J=3-2$) emission toward all the peaks and found that their intensities are 3 -- 12 times lower than those of $^{12}$CO($J=3-2$). We determined the intensity ratios of $^{12}$CO($J=3-2$) to $^{13}$CO($J=3-2$), $R^{12/13}_{3-2}$, and $^{13}$CO($J=3-2$) to $^{13}$CO($J=1-0$), $R^{13}_{3-2/1-0}$, at 45$\arcsec$ resolution. These ratios were used for radiative transfer calculations in order to estimate temperature and density of the clumps. The parameters of these clumps range kinetic temperature $T\mathrm{_{kin}}$ = 15 -- 200 K, and molecular hydrogen gas density $n(\mathrm{H_2})$ = 8$\times 10^2$ -- 7$\times 10^3$ cm$^{-3}$. We confirmed that the higher density clumps show higher kinetic temperature and that the lower density clumps lower kinetic temperature at a better accuracy than in the previous work. The kinetic temperature and density increase generally from a Type I GMC to a Type III GMC. We interpret that this difference reflects an evolutionary trend of star formation in molecular clumps. The $R^{13}_{3-2/1-0}$ and kinetic temperature of the clumps are well correlated with H$\alpha$ flux, suggesting that the heating of molecular gas $n(\mathrm{H_2})$ = $10^3$ -- $10^4$ cm$^{-3}$ can be explained by stellar FUV photons., Comment: 39 pages, 7 figures, 4 tables. Accepted for publication in The Astronomical Journal

We have carried out 12CO(J =2-1) and 12CO(J =3-2) observations at spatial resolutions of 1.0-3.8 pc toward the entirety of loops 1 and 2 and part of loop 3 in the Galactic center with NANTEN2 and ASTE. These new results revealed detailed distributions of the molecular gas and the line intensity ratio of the two transitions, R3-2/2-1. In the three loops, R3-2/2-1 is in a range from 0.1 to 2.5 with a peak at ~ 0.7 while that in the disk molecular gas is in a range from 0.1 to 1.2 with a peak at 0.4. This supports that the loops are more highly excited than the disk molecular gas. An LVG analysis of three transitions, 12CO J =3-2 and 2-1 and 13CO J =2-1, toward six positions in loops 1 and 2 shows density and temperature are in a range 102.2 - 104.7 cm-3 and 15-100 K or higher, respectively. Three regions extended by 50-100 pc in the loops tend to have higher excitation conditions as characterized by R3-2/2-1 greater than 1.2. The highest ratio of 2.5 is found in the most developed foot points between loops 1 and 2. This is interpreted that the foot points indicate strongly shocked conditions as inferred from their large linewidths of 50-100 km s-1, confirming the suggestion by Torii et al. (2010b). The other two regions outside the foot points suggest that the molecular gas is heated up by some additional heating mechanisms possibly including magnetic reconnection. A detailed analysis of four foot points have shown a U shape, an L shape or a mirrored-L shape in the b-v distribution. It is shown that a simple kinematical model which incorporates global rotation and expansion of the loops is able to explain these characteristic shapes., Comment: 59 pages, accepted to PASJ

Large-scale CO surveys have revealed that the central molecular zone (CMZ) of our Galaxy is characterized by a number of expanding shells/arcs, and by a peculiar population of compact clouds with large velocity widths -- high-velocity compact clouds (HVCCs). We have performed a large-scale CO J=3-2 survey of the CMZ from 2005 to 2008 with the Atacama Submillimeter-wave Telescope Experiment (ASTE). The data cover almost the full extent of the CMZ and the Clump 2 with a 34 arcsec grid spacing. The CO J=3-2 data were compared with the CO J=1--0 data taken with the Nobeyama Radio Observatory 45 m telescope. Molecular gas in the CMZ exhibits higher J=3-2/J=1-0 intensity ratios [R(3-2/1-0)=0.7] than that in the Galactic disk [R(3-2/1-0)=0.4]. We extracted highly excited, optically thin gas by the criterion, R(3-2/1-0)>=1.5. Clumps of high R(3-2/1-0) gas were found in the Sgr A and Sgr C regions, near SNR G 0.9+0.1, and three regions with energetic HVCCs; CO 1.27+0.01, CO -0.41-0.23, and CO -1.21-0.12. We also found a number of small spots of high R(3-2/1-0) gas over the CMZ. Many of these high R(3-2/1-0) clumps and spots have large velocity widths, and some apparently coincides with HVCCs, suggesting that they are spots of shocked molecular gas. Their origin should be local explosive events, possibly supernova explosions. These suggest that the energetic HVCCs are associated with massive compact clusters, which have been formed by microbursts of star formation. Rough estimates of the energy flow from large to small scale suggest that the supernova shocks can make a significant contribution to turbulence activation and gas heating in the CMZ., Comment: 10 pages, 6 figures, Proceedings of the Galactic Center Workshop 2009, Shanghai

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., Comment: 42 pages, 16 figures, ApJ in press, version with high resolution figures is available via http://www.nro.nao.ac.jp/~kmuraoka/m83paper/m83aste-otf.pdf

Fukui et al. (2006) discovered two molecular loops in the Galactic center and argued that the foot points of the molecular loops, two bright spots at both loops ends, represent the gas accumulated by the falling motion along the loops, subsequent to magnetic flotation by the Parker instability. We have carried out sensitive CO observations of the foot points toward l=356 deg at a few pc resolution in the six rotational transitions of CO; 12CO(J=1-0, 3-2, 4-3, 7-6), 13CO(J=1-0) and C18O(J=1-0). The high resolution image of 12CO (J=3-2) has revealed the detailed distribution of the high excitation gas including U shapes, the outer boundary of which shows sharp intensity jumps accompanying strong velocity gradients. An analysis of the multi-J CO transitions shows that the temperature is in a range from 30-100 K and density is around 10^3-10^4 cm^-3, confirming that the foot points have high temperature and density although there is no prominent radiative heating source such as high mass stars in or around the loops. We argue that the high temperature is likely due to the shock heating under C-shock condition caused by the magnetic flotation. We made a comparison of the gas distribution with theoretical numerical simulations and note that the U shape is consistent with numerical simulations. We also find that the region of highest temperature of ~100 K or higher inside the U shape corresponds to the spur having an upward flow, additionally heated up either by magnetic reconnection or bouncing in the interaction with the narrow neck at the bottom of the U shape. We note these new findings further reinforce the magnetic floatation interpretation., Comment: 40 pages, 23 figures, accepted by PASJ on Vol.62 No.3

Lyman-alpha emitters are thought to be young, low-mass galaxies with ages of ~10^8 yr. An overdensity of them in one region of the sky (the SSA 22 field) traces out a filamentary structure in the early Universe at a redshift of z = 3.1 (equivalent to 15 per cent of the age of the Universe) and is believed to mark a forming protocluster. Galaxies that are bright at (sub)millimetre wavelengths are undergoing violent episodes of star formation, and there is evidence that they are preferentially associated with high-redshift radio galaxies, so the question of whether they are also associated with the most significant large-scale structure growing at high redshift (as outlined by Lyman-alpha emitters) naturally arises. Here we report an imaging survey of 1,100-um emission in the SSA 22 region. We find an enhancement of submillimetre galaxies near the core of the protocluster, and a large-scale correlation between the submillimetre galaxies and the low-mass Lyman-alpha emitters, suggesting synchronous formation of the two very different types of star-forming galaxy within the same structure at high redshift. These results are in general agreement with our understanding of the formation of cosmic structure., Comment: Published in Nature (7th May 2009 issue). The astro-ph paper includes the main text (10 pages, 2 figures, 1 table) and supplementary material (6 pages, 4 figures, 1 table)

We report the results of molecular line observations toward the l=1.3 deg complex, ananomalous cloud complex in the central molecular zone of the Galaxy. We have taken 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. The complex is found to be rich in shells and arcs of dense molecular gas. We have identified 9 expanding shells in HCN maps and compact SiO features associated to 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 kms^-1. The high-velocity gas has a high density [n_H ~ 10^4.5 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 an 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 - 52.5) erg and 10^(4.6 - 5.3) yr, respectively. The kinetic energy could be furnished by multiple supernova and/or hypernova explosions with 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 be originated by a massive cluster formation which took place 10^(6.8 - 7.6) yr ago., Comment: 19 pages, 8 figures, to appear in PASJ

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 high J=3-2/J=1-0 intensity ratio (~ 0.9) while gas in the spiral arms 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 J=3-2/J=1-0 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., Comment: 17 pages, 10 figures, to appear in PASJ

We estimated 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 12CO and 13CO data previously observed with the NRO 45m telescope. The large velocity gradient (LVG) approximation was employed. Distributions of gas density, kinetic temperature, and CO column density are derived as functions of position and velocity for the entire coverage of the CO J=3-2 data. We fairly determined physical conditions for 69 % of data points in the CMZ with >= 1 sigma CO detections. Kinetic temperature was found to be roughly uniform in the CMZ, while gas density is higher in the 120-pc star forming ring than in the outer dust lanes. Physical conditions of high J=3-2/J=1-0 features are also discussed., Comment: 8 pages, 6 figures, to appear in PASJ

We present CO(J=3-2) emission observations with the Atacama Submillimeter Telescope Experiment (ASTE) toward the 5' x 5' (or 6.6 x 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. We found a global CO(J=3-2) luminosity L'_CO(3-2) of 5.1 x 10^8 K km s^-1 pc^2 within the observed region. We also found L'_CO(3-2) in the disk region (0.5 < r < 3.5 kpc) of 4.2 x 10^8 K km s^-1 pc^2, indicating that CO(J=3-2) emission in the disk region significantly contributes to the global L'_CO(3-2). From a comparison of a 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 R_3-2/1-0 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 R_3-2/1-0. 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 R_3-2/1-0 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 R_3-2/1-0 governs the spatial variation of SFE in M 83., Comment: 13 pages, 11 figures, PASJ in press, version with high resolution figures is available via http://www.nro.nao.ac.jp/~z5001km/m83-aste.pdf