The second survey of the molecular clouds in the Large Magellanic Cloud in super(12)CO was carried out by NANTEN. The sensitivity of this survey is twice as high as that of the previous NANTEN ...survey, leading to a detection of molecular clouds with image M sub(image). We identified 272 molecular clouds, 230 of which are detected at three or more observed positions. We derived the physical properties, such as size, line width, and virial mass, of the 164 GMCs that have an extent more than the beam size of NANTEN in both the major and minor axes. The CO luminosity and virial mass of the clouds show a good correlation of image, with a Spearman rank correlation of 0.8, suggesting that the clouds are in nearly virial equilibrium. Assuming the clouds are in virial equilibrium, we derived an X sub(CO)-factor of image cm super(-2) (K km s super(-1)) super(-1). The mass spectrum of the clouds is fitted well by a power law of image above the completeness limit of image M sub(image). The slope of the mass spectrum becomes steeper if we fit only the massive clouds, e.g., image for image M sub(image).
We have made CO(J = 2-1) observations toward the H II region RCW 49 and its ionizing source, the rich stellar cluster Westerlund 2, with the NANTEN2 submillimeter telescope. These observations have ...revealed that two molecular clouds in velocity ranges of -11 to +9 km s-1 and 11 to 21 km s-1, respectively, show remarkably good spatial correlations with the Spitzer IRAC mid-infrared image of RCW 49, as well a velocity structures indicative of localized expansion around the bright central regions and stellar cluster. This strongly suggests that the two clouds are physically associated with RCW 49. We obtain a new kinematic distance estimate to RCW 49 and Wd2 of 5.4+1.1 -1.4 kpc, based on the mean velocity and velocity spread of the associated gas. We argue that the acceleration of the gas by stellar winds from Westerlund 2 is insufficient to explain the entire observed velocity dispersion of the molecular gas, and suggest a scenario in which a collision between the two clouds ~4 Myr ago may have triggered the formation of the stellar cluster.
Conditions in super star clusters (SSCs) lead to the formation of dozens of massive stars in close proximity. However, SSCs are rare in the local universe. H72.97-69.39, located in the N79 region of ...the Large Magellanic Cloud (LMC), is an SSC candidate. In this paper we report the ALMA observations of the potential SSC. ALMA reveals colliding filaments, outflows, an H ii region, and a C ii region associated with this cluster. The timescale of the outflow is 65,000 yr, which is consistent with this being a young cluster. The molecular gas around this potential early-stage SSC candidate is complex in nature on small scales (as seen with ALMA) and large scales (as seen with Herschel).
Context. When modeling infrared or γ-ray data as a linear combination of observed gas tracers, excess emission has been detected compared to expectations from known neutral and molecular gas traced ...by HI and CO measurements, respectively. This excess might correspond to additional gas component. This so-called “dark gas” (DG) has been observed in our Galaxy, as well as the Magellanic Clouds. Aims. For the first time, we investigate the correlation between visible extinction (AV) data and gas tracers on large scales in the solar neighborhood, to detect DG and to verify our compatibility with previous studies. Methods. Our work focuses on both the solar neighborhood (|b| > 10°), and the inner and outer Galaxy, as well as on four individual regions: Taurus, Orion, Cepheus-Polaris, and Aquila-Ophiuchus. Thanks to the recent production of an all-sky AV map, we first perform the correlation between AV and both HI and CO emission over the most diffuse regions (with low-to-intermediate gas column densities), to derive the optimal (AV/NH)ref ratio. We then iterate the analysis over the entire regions (including low and high gas column densities) to estimate the CO-to-H2 conversion factor, as well as the DG mass fraction. Results. The average extinction to gas column-density ratio in the solar neighborhood is found to be (AV/NH)ref = 6.53 × 10-22 mag cm2, with significant differences between the inner and outer Galaxy, of about 60%. We derive an average value of the CO-to-H2 conversion factor of XCO = 1.67 × 1020 H2 cm-2/(K km s-1), with significant variations between nearby clouds. In the solar neighborhood, the gas mass in the dark component is found to be 19% relative to that in the atomic component and 164% relative to the one traced by CO. These results are compatible with a recent analysis of Planck data within the uncertainties of our measurements. We estimate the ratio of dark gas to total molecular gas to be 0.62 in the solar neighborhood. The HI-to-H2 and H2-to-CO transitions appear for AV ≃ 0.2 mag and AV ≃ 1.5 mag, respectively, in agreement with theoretical models of dark-H2 gas.
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N132D is the brightest gamma-ray supernova remnant (SNR) in the Large Magellanic Cloud (LMC). We carried out 12CO(J = 1-0, 3-2) observations toward the SNR using the Atacama Large ...Millimeter/submillimeter Array (ALMA) and Atacama Submillimeter Telescope Experiment. We find diffuse CO emission not only at the southern edge of the SNR as previously known, but also inside the X-ray shell. We spatially resolved nine molecular clouds using ALMA with an angular resolution of 5″, corresponding to a spatial resolution of ∼1 pc at the distance of the LMC. Typical cloud sizes and masses are ∼2.0 pc and ∼100 M , respectively. High intensity ratios of CO J = 3-2/1-0 > 1.5 are seen toward the molecular clouds, indicating that shock heating has occurred. Spatially resolved X-ray spectroscopy reveals that thermal X-rays in the center of N132D are produced not only behind a molecular cloud but also in front of it. Considering the absence of a thermal component associated with the forward shock toward one molecular cloud located along the line of sight to the center of the remnant, this suggests that this particular cloud is engulfed by shock waves and is positioned on the near side of the remnant. If the hadronic process is the dominant contributor to the gamma-ray emission, the shock-engulfed clouds play a role as targets for cosmic rays. We estimate the total energy of cosmic-ray protons accelerated in N132D to be ∼0.5-3.8 × 1049 erg as a conservative lower limit, which is similar to that observed in Galactic gamma-ray SNRs.
A large-scale study of the molecular clouds toward the Trifid Nebula, M20, has been made in the J = 2-1 and J = 1-0 transitions of 12CO and 13CO. M20 is ionized predominantly by an O7.5 star ...HD164492. The study has revealed that there are two molecular components at separate velocities peaked toward the center of M20 and that their temperatures--30-50 K as derived by a large velocity gradient analysis--are significantly higher than the 10 K of their surroundings. We identify the two clouds as the parent clouds of the first generation stars in M20. The mass of each cloud is estimated to be ~103 M and their separation velocity is ~8 km s--1 over ~1-2 pc. We find that the total mass of stars and molecular gas in M20 is less than ~3.2 X 103 M , which is too small by an order of magnitude to gravitationally bind the system. We argue that the formation of the first generation stars, including the main ionizing O7.5 star, was triggered by the collision between the two clouds in a short timescale of ~1 Myr, a second example alongside Westerlund 2, where a super-star cluster may have been formed due to cloud-cloud collision triggering.
We present new large field observations of molecular clouds with NANTEN2 toward the super star cluster NGC 3603 in the transitions super(12)CO(J = 2-1, J = 1-0) and super(13)CO(J = 2-1, J = 1-0). We ...suggest that two molecular clouds at 13 km s super(-1) and 28 km s super(-1) are associated with NGC 3603 as evidenced by higher temperatures toward the H II region, as well as morphological correspondence. The mass of the clouds is too small to gravitationally bind them, given their relative motion of ~20 km s super(-1). We suggest that the two clouds collided with each other 1 Myr ago to trigger the formation of the super star cluster. This scenario is able to explain the origin of the highest mass stellar population in the cluster, which is as young as 1 Myr and is segregated within the central sub-pc of the cluster. This is the second super star cluster along with Westerlund 2 where formation may have been triggered by a cloud-cloud collision.
Abstract
We present the first compelling evidence of shock-heated molecular clouds associated with the supernova remnant (SNR) N49 in the Large Magellanic Cloud (LMC). Using
12
CO(
J
= 2–1, 3–2) and
...13
CO(
J
= 2–1) line emission data taken with the Atacama Large Millimeter/Submillimeter Array, we derived the H
2
number density and kinetic temperature of eight
13
CO-detected clouds using the large velocity gradient approximation at a resolution of 3.″5 (∼0.8 pc at the LMC distance). The physical properties of the clouds are divided into two categories: three of them near the shock front show the highest temperatures of ∼50 K with densities of ∼500–700 cm
−3
, while other clouds slightly distant from the SNR have moderate temperatures of ∼20 K with densities of ∼800–1300 cm
−3
. The former clouds were heated by supernova shocks, but the latter were dominantly affected by the cosmic-ray heating. These findings are consistent with the efficient production of X-ray recombining plasma in N49 due to thermal conduction between the cold clouds and hot plasma. We also find that the gas pressure is roughly constant except for the three shock-engulfed clouds inside or on the SNR shell, suggesting that almost no clouds have evaporated within the short SNR age of ∼4800 yr. This result is compatible with the shock-interaction model with dense and clumpy clouds inside a low-density wind bubble.
The Magellanic Mopra Assessment (MAGMA) is a high angular resolution 12CO (J= 1 → 0) mapping survey of giant molecular clouds (GMCs) in the Large Magellanic Cloud (LMC) and Small Magellanic Cloud ...using the Mopra Telescope. Here we report on the basic physical properties of 125 GMCs in the LMC that have been surveyed to date. The observed clouds exhibit scaling relations that are similar to those determined for Galactic GMCs, although LMC clouds have narrower linewidths and lower CO luminosities than Galactic clouds of a similar size. The average mass surface density of the LMC clouds is 50 M⊙ pc−2, approximately half that of GMCs in the inner Milky Way. We compare the properties of GMCs with and without signs of massive star formation, finding that non-star-forming GMCs have lower peak CO brightness than star-forming GMCs. We compare the properties of GMCs with estimates for local interstellar conditions: specifically, we investigate the H i column density, radiation field, stellar mass surface density and the external pressure. Very few cloud properties demonstrate a clear dependence on the environment; the exceptions are significant positive correlations between (i) the H i column density and the GMC velocity dispersion, (ii) the stellar mass surface density and the average peak CO brightness and (iii) the stellar mass surface density and the CO surface brightness. The molecular mass surface density of GMCs without signs of massive star formation shows no dependence on the local radiation field, which is inconsistent with the photoionization-regulated star formation theory proposed by McKee. We find some evidence that the mass surface density of the MAGMA clouds increases with the interstellar pressure, as proposed by Elmegreen, but the detailed predictions of this model are not fulfilled once estimates for the local radiation field, metallicity and GMC envelope mass are taken into account.
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