arXiv subjects

N. Mizuno

Publications and source records attributed to N. Mizuno.

42 records · Page 3Linked to original sources

arXiv2007

12CO 4-3 and [CI] 1-0 at the centers of NGC 4945 and Circinus

Studying molecular gas in the central regions of the star burst galaxies NGC4945 and Circinus enables us to characterize the physical conditions and compare them to previous local and high-z studies. We estimate temperature, molecular density and column densities of CO and atomic carbon. Using model predictions we give a range of estimated CO/C abundance ratios. Using the new NANTEN2 4m sub-millimeter telescope in Pampa La Bola, Chile, we observed for the first time CO 4-3 and [CI] 3P1-3 P0 at the centers of both galaxies at linear scale of 682 pc and 732 pc respectively. We compute the cooling curves of 12CO and 13CO using radiative transfer models and estimate the physical conditions of CO and [CI]. The centers of NGC4945 and Circinus are very [CI] bright objects, exhibiting [CI] 3P1 - 3 P0 luminosities of 91 and 67Kkms-1kpc2, respectively. The [CI] 3P1-3 P0/CO 4-3 ratio of integrated intensities are large at 1.2 in NGC4945 and 2.8 in Circinus. Combining previous CO J= 1-0, 2-1 and 3-2 and 13CO J= 1-0, 2-1 studies with our new observations, the radiative transfer calculations give a range of densities, n(H2) = 10^3-3*104^cm-3, and a wide range of kinetic temperatures, Tkin = 20 - 100K, depending on the density. Future CO J= 7-6 and [CI] 2-1 observations will be important to resolve the ambiguity in the physical conditions and confirm the model predictions.

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arXiv2007

Clumpy photon-dominated regions in Carina. I. [CI] and mid-J CO lines in two 4'x4' fields

The Carina region is an excellent astrophysical laboratory for studying the feedback mechanisms of newly born, very massive stars within their natal giant molecular clouds (GMCs) at only 2.35 kpc distance. We use a clumpy PDR model to analyse the observed intensities of atomic carbon and CO and to derive the excitation conditions of the gas. The NANTEN2-4m submillimeter telescope was used to map the [CI] 3P1-3P0, 3P2-3P1 and CO 4-3, 7-6 lines in two 4'x4' regions of Carina where molecular material interfaces with radiation from the massive star clusters. One region is the northern molecular cloud near the compact OB cluster Tr14, and the second region is in the molecular cloud south of etaCar and Tr16. These data were combined with 13CO SEST spectra, HIRES/IRAS 60um and 100um maps of the FIR continuum, and maps of 8um IRAC/Spitzer and MSX emission. We used the HIRES far-infrared dust data to create a map of the FUV field heating the gas. The northern region shows an FUV field of a few 1000 in Draine units while the field of the southern region is about a factor 10 weaker. We constructed models consisting of an ensemble of small spherically symmetric PDR clumps within the 38" beam (0.43pc), which follow canonical power-law mass and mass-size distributions. We find that an average local clump density of 2x10**5 cm-3 is needed to reproduce the observed line emission at two selected interface positions. Stationary, clumpy PDR models reproduce the observed cooling lines of atomic carbon and CO at two positions in the Carina Nebula.

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arXiv2007

Sub-millimeter Observations of Giant Molecular Clouds in the Large Magellanic Cloud: Temperature and Density as Determined from J=3-2 and J=1-0 transitions of CO

We have carried out sub-mm 12CO(J=3-2) observations of 6 giant molecular clouds (GMCs) in the Large Magellanic Cloud (LMC) with the ASTE 10m sub-mm telescope at a spatial resolution of 5 pc and very high sensitivity. We have identified 32 molecular clumps in the GMCs and revealed significant details of the warm and dense molecular gas with n(H2) $\sim$ 10$^{3-5}$ cm$^{-3}$ and Tkin $\sim$ 60 K. These data are combined with 12CO(J=1-0) and 13CO(J=1-0) results and compared with LVG calculations. We found that the ratio of 12CO(J=3-2) to 12CO(J=1-0) emission is sensitive to and is well correlated with the local Halpha flux. We interpret that differences of clump propeties represent an evolutionary sequence of GMCs in terms of density increase leading to star formation.Type I and II GMCs (starless GMCs and GMCs with HII regions only, respectively) are at the young phase of star formation where density does not yet become high enough to show active star formation and Type III GMCs (GMCs with HII regions and young star clusters) represents the later phase where the average density is increased and the GMCs are forming massive stars. The high kinetic temperature correlated with \Halpha flux suggests that FUV heating is dominant in the molecular gas of the LMC.

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arXiv2007

Star-forming Regions in the Small Magellanic Cloud Multi-wavelength Properties of Stellar Complexes

We trace the star formation regions in the SMC and study their properties. The size and spatial distribution of these regions is found to support the hierarchical scenario of star formation, whereas, the evaluation of their intensity, contributes to the understanding of the various stages of star formation. Their connection to the LMC-SMC close encounter, about $(0.9-2) \times 10^{8}$ years ago, is investigated as well. The SMC, being almost edge-on, does not easily reveal these areas, as is the case with the LMC. However, a study through multi-wavelength images such as optical, IR and radio has been proved very useful. A selection of areas, with enhanced 60 and 100-$μ$m infrared flux and emission in all IRAS bands, identifies the star forming regions. All of the identified regions are dominated by early-type stars and considering their overall size (increasing order) a total of 24 aggregates, 23 complexes, and 3 super-complexes were found. We present their coordinates, dimensions, and IR fluxes. Moreover, we correlate their positions with known associations, SNRs, and \hii regions and discuss their activity.

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arXiv2006

Spitzer SAGE survey of the Large Magellanic Cloud II: Evolved Stars and Infrared Color Magnitude Diagrams

Color-magnitude diagrams (CMDs) are presented for the Spitzer SAGE (Surveying the Agents of a Galaxy's Evolution) survey of the Large Magellanic Cloud (LMC). IRAC and MIPS 24 um epoch one data are presented. These data represent the deepest, widest mid-infrared CMDs of their kind ever produced in the LMC. Combined with the 2MASS survey, the diagrams are used to delineate the evolved stellar populations in the Large Magellanic Cloud as well as Galactic foreground and extragalactic background populations. Some 32000 evolved stars brighter than the tip of the red giant branch are identified. Of these, approximately 17500 are classified as oxygen-rich, 7000 carbon-rich, and another 1200 as ``extreme'' asymptotic giant branch (AGB) stars. Brighter members of the latter group have been called ``obscured'' AGB stars in the literature owing to their dusty circumstellar envelopes. A large number (1200) of luminous oxygen--rich AGB stars/M supergiants are also identified. Finally, there is strong evidence from the 24 um MIPS channel that previously unexplored, lower luminosity oxygen-rich AGB stars contribute significantly to the mass loss budget of the LMC (1200 such sources are identified).

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arXiv2006

A multi-resolution analysis of the radio-FIR correlation in the Large Magellanic Cloud

We investigate the local correlation betwen the 1.4 GHz radio continuum and 60 micron far-infrared (FIR) emission within the Large Magellanic Cloud (LMC) on spatial scales between 0.05 and 1.5 kpc. On scales below ~1 kpc, the radio-FIR correlation is clearly better than the correlation of the cold gas tracers with either the radio or the FIR emission. For the LMC as a whole, there is a tight correlation between the radio and FIR emission on spatial scales above ~50 pc. By decomposing the radio emission into thermal and non-thermal components, however, we show that the scale on which the radio-FIR correlation breaks down is inversely proportional to the thermal fraction of the radio emission: regions that show a strong correlation to very small scales are the same regions where the thermal fraction of the radio emission is high. Contrary to previous studies of the local radio-FIR correlation in the LMC, we show that the slope of the relation between the radio and FIR emission is non-linear. In bright star-forming regions, the radio emission increases faster than linearly with respect to the FIR emission (power-law slope of ~1.2), whereas a flatter slope of ~0.6-0.9 applies more generally across the LMC. Our results are consistent with a scenario in which the UV photons and cosmic rays in the LMC have a common origin in massive star formation, but the cosmic rays are able to diffuse away from their production sites. Our results do not provide direct evidence for coupling between the magnetic field and the local gas density, but we note that synchrotron emission may not be a good tracer of the magnetic field if cosmic rays can readily escape the LMC.

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