arXiv subjects

Norikazu Mizuno

Publications and source records attributed to Norikazu Mizuno.

40 records · Page 3Linked to original sources

arXiv2006

Spitzer Survey of the Large Magellanic Cloud, Surveying the Agents of a Galaxy's Evolution (SAGE) I: Overview and Initial Results

We are performing a uniform and unbiased, ~7x7 degrees imaging survey of the Large Magellanic Cloud (LMC), using the IRAC and MIPS instruments on board the Spitzer Space Telescope in order to survey the agents of a galaxy's evolution (SAGE), the interstellar medium (ISM) and stars in the LMC. The detection of diffuse ISM with column densities >1.2x10^21 H cm^-2 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 solar masses that will determine the current star formation rate in the LMC. SAGE's detection of evolved stars with mass loss rates >1x10^-8 solar masses per year 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 three months, that both mitigate instrumental artifacts and constrain source variability. The SAGE data are non-proprietary. 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 with a special focus on the N79 and N83 region. The SAGE epoch 1 point source catalog has ~4 million sources. The point source counts are highest for the IRAC 3.6 microns band and decrease dramatically towards longer wavelengths consistent with the fact that stars dominate the point source catalogs and that the dusty objects, e.g. young stellar objects and dusty evolved stars that detected at the longer wavelengths, are rare in comparison. We outline a strategy for identifying foreground MW stars, that may comprise as much as 18% of the source list, and background galaxies, that may comprise ~12% of the source list.

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arXiv2006

Giant Molecular Clouds in Local Group Galaxies

We present the first comparative study of extragalactic GMCs using complete data sets for entire galaxies and a uniform set of reduction and analysis techniques. We present results based on CO observations for the LMC, SMC, M33, M31, IC10 and the nucleus of M64, and make comparisons with archival Milky Way observations. Our sample includes large spirals and dwarf irregulars with metallicities that vary by an order of magnitude. GMCs in HI rich galaxies are seen to be well-correlated with HI filaments that pervade the galactic disks, suggesting that they form from pre-existing HI structures. Virial estimates of the ratio of CO line strength to H2 column density, XCO, suggests that a value of 4 x 10^20 cm^-2 (K km/s)^-1 is a good value to use in most galaxies (except the SMC) if the GMCs are virialized. However, if the clouds are only marginally self-gravitating, as appears to be the case judging from their appearance, half the virial value may be more appropriate. There is no clear trend of XCO with metallicity. The clouds within a galaxy are shown to have the about the same H2 surface density and differences between galaxies seem to be no more than a factor of ~2. We show that hydrostatic pressure appears to be the main factor in determining what fraction of atomic gas is turned into molecules. In the high-pressure regions often found in galactic centers, the observed properties of GMCs appear to be different from those in the found in the Local Group. From the association of tracers of star formation with GMCs in the LMC, we find that about 1/4 of the GMCs exhibit no evidence of star formation and we estimate that the lifetime of a typical GMC in these galaxies is 20--30 Myr.

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arXiv2006

Large Scale CO Observations of a Far-Infrared Loop in Pegasus; Detection of a Large Number of Very Small Molecular Clouds Possibly Formed via Shocks

We have carried out large scale 12CO and 13CO observations with a mm/sub-mm telescope NANTEN toward a loop-like structure in far infrared whose angular extent is about 20x20 degrees around (l, b) ~ (109, -45) in Pegasus. The 12CO distribution is found to consist of 78 small clumpy clouds whose masses range from 0.04 Mo to 11 Mo. About 83% of the 12CO clouds have very small masses less than 1.0 Mo. 13CO emission shown in the 19 of the 78 12CO clouds was detected in the region where the column density of H2 derived from 12CO is greater than 5x10(20) cm(-2), corresponding to Av of ~ 1 mag, which takes into account that of HI. We find no indication of star formation in these clouds in IRAS and 2MASS Point Source Catalogs. The very low mass clouds, M < 1 Mo, identified are unusual in the sense that they have very weak 12CO peak temperature of 0.5 K to 2.7 K and that they aggregate in a region of a few pc with no main massive clouds of ~ 100 Mo. A comparison with a theoretical work on molecular cloud formation (Koyama & Inutsuka 2002) suggests that the very low-mass clouds may have been formed in the shocked layer through the thermal instability. The star HD886 (B2IV) may be the source of the mechanical luminosity via stellar winds to create shocks, forming the loop-like structure where the very low-mass clouds are embedded.

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arXiv2002

Centrally Peaked X-Ray Supernova Remnants in the Small Magellanic Cloud Studied with ASCA and ROSAT

This paper presents ASCA/SIS and ROSAT/HRI results of three supernova remnants (SNRs) in the Small Magellanic Cloud: 0103-726, 0045-734, and 0057-7226. The ROSAT/HRI images of these SNRs indicate that the most of the X-ray emissions are concentrated in the center region. Only from 0103-726 are faint X-rays along the radio shell also detected. The ASCA/SIS spectra of 0103-726 and 0045-734 exhibit strong emission lines from highly ionized metals. The spectra were well-fitted with non-equilibrium ionization (NEI) plasma models. The metal abundances are found to be larger than the mean chemical compositions in the interstellar medium (ISM) of the SMC. Thus, X-rays from these two SNRs are attributable to the ejecta gas, although the ages estimated from the ionization timescale are significantly large, ~> 10^4 yr. The chemical compositions are roughly consistent with the type-II supernova origin of a progenitor mass ~< 20 M_solar. The SIS spectrum of 0057-7226 was also fitted with an NEI model of an estimated age ~> 6 x 10^3 yr. Although no constraint on the metal abundances was obtained, the rather weak emission lines are consistent with the low metal abundances in the ISM of the SMC. A possible scenario for the evolution of the morphologies and spectra of SNRs is proposed.

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