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Sawada T.,Joint ALMA Office | Hasegawa T.,NAOJ Chile Observatory | Koda J.,State University of New York at Stony Brook
Astrophysical Journal Letters | Year: 2012

We explore the development of structures in molecular gas in the Milky Way by applying the analysis of the brightness distribution function and the brightness distribution index (BDI) in the archival data from the Boston University-Five College Radio Astronomy Observatory 13CO J = 1-0 Galactic Ring Survey. The BDI measures the fractional contribution of spatially confined bright molecular emission over faint emission extended over large areas. This relative quantity is largely independent of the amount of molecular gas and of any conventional, pre-conceived structures, such as cores, clumps, or giant molecular clouds. The structured molecular gas traced by higher BDI is located continuously along the spiral arms in the Milky Way in the longitude-velocity diagram. This clearly indicates that molecular gas changes its structure as it flows through the spiral arms. Although the high-BDI gas generally coincides with H II regions, there is also some high-BDI gas with no/little signature of ongoing star formation. These results support a possible evolutionary sequence in which unstructured, diffuse gas transforms itself into a structured state on encountering the spiral arms, followed by star formation and an eventual return to the unstructured state after the spiral arm passage. © 2012. The American Astronomical Society. All rights reserved. Source


Higdon J.L.,Georgia Southern University | Higdon S.J.U.,Georgia Southern University | Ruiz S.M.,Joint ALMA Office | Rand R.J.,University of New Mexico
Astrophysical Journal Letters | Year: 2015

Atacama Large Millimeter/submillimeter Array 12CO(J = 1-0) observations are used to study the cold molecular ISM of the Cartwheel ring galaxy and its relation to H i and massive star formation (SF). CO moment maps find (2.69 ± 0.05) × 109 Mo of H2 associated with the inner ring (72%) and nucleus (28%) for a Galactic ICO-to- conversion factor (αCO). The spokes and disk are not detected. Analysis of the inner ring's CO kinematics shows it to be expanding (Vexp = 68.9 ± 4.9 km s-1), implying an ≈70 Myr age. Stack averaging reveals CO emission in the starburst outer ring for the first time, but only where H i surface density (ΣH i) is high, representing Mo for a metallicity-appropriate αCO, giving small (3.7 Mo pc-2), molecular fraction (fmol = 0.10), and H2 depletion timescales (τmol ≈ 50-600 Myr). Elsewhere in the outer ring Mo pc-2, fmol ≲ 0.1 and τmol ≲ 140-540 Myr (all 3σ). The inner ring and nucleus are H2 dominated and are consistent with local spiral SF laws. ΣSFR in the outer ring appears independent of ΣH i, or The ISM's long confinement in the robustly star-forming rings of the Cartwheel and AM0644-741 may result in either a large diffuse H2 component or an abundance of CO-faint low column density molecular clouds. The H2 content of evolved starburst rings may therefore be substantially larger. Due to its lower ΣSFR and age, the Cartwheel's inner ring has yet to reach this state. Alternately, the outer ring may trigger efficient SF in a H i-dominated ISM. © 2015. The American Astronomical Society. All rights reserved. Source


Krips M.,Institut Universitaire de France | Martin S.,European Southern Observatory | Eckart A.,University of Cologne | Neri R.,Institut Universitaire de France | And 10 more authors.
Astrophysical Journal | Year: 2011

We present high angular resolution (05-20) observations of the millimeter continuum and the 12CO(J = 3-2), 13CO(J = 3-2), 13CO(J = 2-1), C18O(J = 2-1), HCN(J = 3-2), HCO +(J = 4-3), and HCO+(J = 3-2) line emission in the circumnuclear disk (r ≲ 100pc) of the prototypical Seyfert 2 galaxy NGC1068, carried out with the Submillimeter Array. We also include in our analysis new 13CO(J = 1-0) and improved 12CO(J = 2-1) observations of NGC1068 at high angular resolution (10-20) and sensitivity, conducted with the Institute de Radioastronomie Millimetrique Plateau de Bure Interferometer. Based on the complex dynamics of the molecular gas emission indicating non-circular motions in the central 100pc, we propose a scenario in which part of the molecular gas in the circumnuclear disk of NGC1068 is blown radially outward as a result of shocks. This shock scenario is further supported by quite warm (Tkin ≥ 200K) and dense (n(H2)≃ 104cm-3) gas constrained from observed molecular line ratios. The HCN abundance in the circumnuclear disk is found to be [HCN]/[ 12CO] ≈ 10-3.5. This is slightly higher than the abundances derived for Galactic and extragalactic star-forming/starbursting regions. This result lends further support to X-ray-enhanced HCN formation in the circumnuclear disk of NGC1068 as suggested by earlier studies. The HCO + abundance ([HCO+]/[12CO] ≈ 10 -5) appears to be somewhat lower than that of Galactic and extragalactic star-forming/starbursting regions. When trying to fit the centimeter-to-millimeter continuum emission by different thermal and non-thermal processes, it appears that electron-scattered synchrotron emission yields the best results while thermal free-free emission seems to overpredict the millimeter continuum emission. © 2011. The American Astronomical Society. All rights reserved. Source


Koda J.,State University of New York at Stony Brook | Koda J.,California Institute of Technology | Sawada T.,Joint ALMA Office | Sawada T.,Japan National Astronomical Observatory | And 9 more authors.
Astrophysical Journal, Supplement Series | Year: 2011

We report the CO(J = 1-0) observations of the Whirlpool Galaxy M51 using both the Combined Array for Research in Millimeter Astronomy (CARMA) and the Nobeyama 45 m telescope (NRO45). We describe a procedure for the combination of interferometer and single-dish data. In particular, we discuss (1) the joint imaging and deconvolution of heterogeneous data, (2) the weighting scheme based on the root-mean-square (rms) noise in the maps, (3) the sensitivity and uv coverage requirements, and (4) the flux recovery of a combined map. We generate visibilities from the single-dish map and calculate the noise of each visibility based on the rms noise. Our weighting scheme, though it is applied to discrete visibilities in this paper, should be applicable to grids in uv space, and this scheme may advance in future software development. For a realistic amount of observing time, the sensitivities of the NRO45 and CARMA visibility data sets are best matched by using the single-dish baselines only up to 4-6 kλ (about 1/4-1/3 of the dish diameter). The synthesized beam size is determined to conserve the flux between the synthesized beam and convolution beam. The superior uv coverage provided by the combination of CARMA long baseline data with 15 antennas and NRO45 short spacing data results in the high image fidelity, which is evidenced by the excellent overlap between even the faint CO emission and dust lanes in an optical Hubble Space Telescope image and polycyclicaromatichydrocarbon emission in a Spitzer 8 μm image. The total molecular gas masses of NGC 5194 and 5195 (d = 8.2 Mpc) are 4.9 × 10 9 M ⊙ and 7.8 × 107 M ⊙, respectively, assuming the CO-to-H2 conversion factor of X CO = 1.8 × 1020 cm-2(K km s-1)-1. The presented images are an indication of the millimeter-wave images that will become standard in the next decade with CARMA and NRO45, and the Atacama Large Millimeter/Submillimeter Array. © 2011. The American Astronomical Society. All rights reserved. Source


Liu G.,University of Massachusetts Amherst | Liu G.,Johns Hopkins University | Calzetti D.,University of Massachusetts Amherst | Kennicutt R.C.,University of Cambridge | And 5 more authors.
Astrophysical Journal | Year: 2013

The H II region luminosity function (LF) is an important tool for deriving the birthrates and mass distribution of OB associations and is an excellent tracer of the newly formed massive stars and associations. To date, extensive work (predominantly in Hα) has been done from the ground, which is hindered by dust extinction and the severe blending of adjacent (spatially or in projection) H II regions. Reliably measuring the properties of H II regions requires a linear resolution <40 pc, but analyses satisfying this requirement have been done only in a handful of galaxies, so far. As the first space-based work using a galaxy sample, we have selected 12 galaxies from our HST/NICMOS Paα survey and studied the LF and size distribution of H II regions both in individual galaxies and cumulatively, using a virtually extinction-free tracer of the ionizing photon rate. The high angular resolution and low sensitivity to diffuse emission of NICMOS also offer an advantage over ground-based imaging by enabling a higher degree of de-blending of the H II regions. We do not confirm the broken power-law LFs found in ground-based studies. Instead, we find that the LFs, both individual and co-added, follow a single power law dN(L)/dln LL -1, are consistent with the mass function of star clusters in nearby galaxies, and are in agreement with the results of the existing analyses with Hubble Space Telescope (HST) data. The individual and co-added size distributions of H II regions are both roughly consistent with dN(D)/dln DD -3, but the power-law scaling is probably contaminated by blended regions or complexes. © 2013. The American Astronomical Society. All rights reserved. Source

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