We estimate the conversion factor relating CO emission to H 2 mass, α CO , in five Local Group galaxies that span approximately an order of magnitude in metallicity -M 31, M 33, the Large Magellanic Cloud (LMC), NGC 6822, and the Small Magellanic Cloud (SMC). We model the dust mass along the line of sight from infrared (IR) emission and then solve for the α CO that best allows a single gas-to-dust ratio (δ GDR ) to describe each system. This approach remains sensitive to COdark envelopes H 2 surrounding molecular clouds. In M 31, M 33, and the LMC we find α CO ≈ 3-9 M ⊙ pc −2 (K km s −1 ) −1 , consistent with the Milky Way value within the uncertainties. The two lowest metallicity galaxies in our sample, NGC 6822 and the SMC (12 + log(O/H) ≈ 8.2 and 8.0), exhibit a much higher α CO . Our best estimates are α NGC6822 CO ≈ 30 M ⊙ pc −2 (K km s −1 ) −1 and α SMC CO ≈ 70 M ⊙ pc −2 (K km s −1 ) −1 . These results are consistent with the conversion factor becoming a strong function of metallicity around 12 + log(O/H) ∼ 8.4 − 8.2. We favor an interpretation where decreased dust-shielding leads to the dominance of CO-free envelopes around molecular clouds below this metallicity. 11 Hubble Fellow 12 We work with α CO , the conversion from integrated CO intensity to mass of molecular gas. A linear scaling relates α CO

We are performing a uniform and unbiased imaging survey of the Large Magellanic Cloud (LMC; $7 ; 7) using the IRAC (3.6, 4.5, 5.8, and 8 m) and MIPS (24, 70, and 160 m) instruments on board the Spitzer Space Telescope in the Surveying the Agents of a Galaxy's Evolution (SAGE) survey, these agents being the interstellar medium (ISM) and stars in the LMC. This paper provides an overview of the SAGE Legacy project, including observing strategy, data processing, and initial results. Three key science goals determined the coverage and depth of the survey. The detection of diffuse ISM with column densities >1:2 ; 10 21 H cm À2 permits detailed studies of dust processes in the ISM. SAGE's point-source sensitivity enables a complete census of newly formed stars with masses >3 M that will determine the current star formation rate in the LMC. SAGE's detection of evolved stars with mass-loss rates >1 ; 10 À8 M yr À1 will quantify the rate at which evolved stars inject mass into the ISM of the LMC. The observing strategy includes two epochs in 2005, separated by 3 months, that both mitigate instrumental artifacts and constrain source variability. The SAGE data are nonproprietary. The data processing includes IRAC and MIPS pipelines and a database for mining the point-source catalogs, which will be released to the community in support of Spitzer proposal cycles 4 and 5. We present initial results on the epoch 1 data for a region near N79 and N83. The MIPS 70 and 160 m images of the diffuse dust emission of the N79/N83 region reveal a similar distribution to the gas emissions, especially the H i 21 cm emission. The measured point-source sensitivity for the epoch 1 data is consistent with expectations for the survey. The point-source counts are highest for the IRAC 3.6 m band and decrease dramatically toward longer wavelengths, A

We studied star formation activities in the molecular clouds in the Large Magellanic Cloud. We have utilized the second catalog of 272 molecular clouds obtained by NANTEN to compare the cloud distribution with signatures of massive star formation including stellar clusters, and optical and radio HII regions. We find that the molecular clouds are classified into three types according to the activities of massive star formation; Type I shows no signature of massive star formation, Type II is associated with relatively small HII region(s) and Type III with both HII region(s) and young stellar cluster(s). The radio continuum sources were used to confirm that Type I GMCs do not host optically hidden HII regions. These signatures of massive star formation show a good spatial correlation with the molecular clouds in a sense they are located within ∼100 pc of the molecular clouds. Among possible ideas to explain the GMC Types, we favor that the Types indicate an evolutionary sequence; i.e., the youngest phase is Type I, followed by Type II and the last phase is Type III, where the most active star formation takes place leading to cloud dispersal. The number of the three types of GMCs should be proportional to the time scale of each evolutionary stage if a steady state of massive star and cluster formation is a good approximation. By adopting the time scale of the youngest stellar clusters, 10 Myrs, we roughly estimate the timescales of Types I, II and III to be 6 Myrs, 13 Myrs and 7 Myrs, respectively, corresponding to a lifetime of 20-30 Myrs for the GMCs with a mass above the completeness limit, 5 × 10 4 M ⊙ .

We present Atacama Large Millimeter/submillimeter Array (ALMA) observations from the 2014 Long Baseline Campaign in dust continuum and spectral line emission from the HL Tau region. The continuum images at wavelengths of 2.9, 1.3, and 0.87 mm have unprecedented angular resolutions of 0″. 075 (10 AU) to 0″. 025 (3.5 AU), revealing an astonishing level of detail in the circumstellar disk surrounding the young solar analog HL Tau, with a pattern of bright and dark rings observed at all wavelengths. By fitting ellipses to the most distinct rings, we measure precise values for the disk inclination (46 .72 0 .05 ± • •) and position angle (138 .02 0 .07).

The second survey of the molecular clouds in 12 CO (J = 1-0) was carried out in the Large Magellanic Cloud 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 M CO 2 × 10 4 M ⊙ . 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, virial mass, of the 164 GMCs which 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 M VIR ∝ L 1.1±0.1 CO 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 CO -factor to be ∼ 7 × 10 20 cm −2 (K km s −1 ) −1 . The mass spectrum of the clouds is fitted well by a power law of N cloud (> M CO ) ∝ M −0.75±0.06 CO above the completeness limit of 5 × 10 4 M ⊙ . The slope of the mass spectrum becomes steeper if we fit only the massive clouds; e.g., N cloud (> M CO ) ∝ M −1.2±0.2 CO for M CO ≥ 3 × 10 5 M ⊙ .

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We have made CO(J = 2 − 1) observations towards the HII region RCW 49 and its ionizing source, the rich stellar cluster Westerlund 2, with the NANTEN2 sub-mm 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 argues 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 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 Myrs ago may have triggered the formation of the stellar cluster. Subject headings: ISM: clouds -open clusters and associations: individual (Westerlund 2) -HII regions: individual (RCW 49)