Norikazu Mizuno

The goal of this paper is to present the results of a preliminary analysis of the extended infrared (IR) emission by dust in the interstellar medium (ISM) of the Large Magellanic Cloud (LMC). We combine Spitzer Surveying the Agents of Galaxy Evolution (SAGE) and Infrared Astronomical Satellite (IRAS) data and correlate the infrared emission with gas tracers of H I, CO, and H_α. We present a global analysis of the infrared emission as well as detailed modeling of the spectral energy distribution (SED) of a few selected regions. Extended emission by dust associated with the neutral, molecular, and diffuse ionized phases of the ISM is detected at all IR bands from 3.6 μm to 160 μm. The relative abundance of the various dust species appears quite similar to that in the Milky Way (MW) in all the regions we have modeled. We construct maps of the temperature of large dust grains. The temperature map shows variations in the range 12.1-34.7 K, with a systematic gradient from the inner to outer regions, tracing the general distribution of massive stars and individual H II regions as well as showing warmer dust in the stellar bar. This map is used to derive the far-infrared (FIR) optical depth of large dust grains. We find two main departures in the LMC with respect to expectations based on the MW: (1) excess mid-infrared (MIR) emission near 70 μm, referred to as the 70 μm excess, and (2) departures from linear correlation between the FIR optical depth and the gas column density, which we refer to as FIR excess emission. The 70 μm excess increases gradually from the MW to the LMC to the Small Magellanic Cloud (SMC), suggesting evolution with decreasing metallicity. The excess is associated with the neutral and diffuse ionized gas, with the strongest excess region located in a loop structure next to 30 Dor. We show that the 70 μm excess can be explained by a modification of the size distribution of very small grains with respect to that in the MW, and a corresponding mass increase of 13% of the total dust mass in selected regions. The most likely explanation is that the 70 μm excess is due to the production of large very small grains (VSG) through erosion of larger grains in the diffuse medium. This FIR excess could be due to intrinsic variations of the dust/gas ratio, which would then vary from 4.6 to 2.3 times lower than the MW values across the LMC, but X _CO values derived from the IR emission would then be about three times lower than those derived from the Virial analysis of the CO data. We also investigate the possibility that the FIR excess is associated with an additional gas component undetected in the available gas tracers. Assuming a constant dust abundance in all ISM phases, the additional gas component would have twice the known H I mass. We show that it is plausible that the FIR excess is due to cold atomic gas that is optically thick in the 21 cm line, while the contribution by a pure H₂ phase with no CO emission remains a possible explanation.

Most stars form in Giant Molecular Clouds (GMCs) and regulate the evolution of galaxies in various respects. The formed stars affect the surrounding materials strongly via their UV photons, stellar winds, and supernova explosions, which lead to trigger the formation of next-generations of stars in the GMCs. It is therefore crucial to reveal the distribution and properties of GMCs in a galaxy. The Magellanic System is a unique target to make such detailed comprehensive study of GMCs. This is because it is nearby and the LMC is nearly face-on, making it feasible to unambiguously identify associated young objects within GMCs. Recent millimeter and sub-millimeter observations in the Magellanic System have started to reveal the distribution and properties of the individual GMCs in detail and their relation to star formation activities. From the NANTEN CO surveys, three types of GMCs can be classified in terms of star formation activities; Type I is starless, Type II is with H ii regions only, and Type III is associated with active star formation indicated by huge H ii regions and young star clusters. The further observations to obtain detailed structure of the GMCs by Mopra and SEST and to search for the dense cores by ASTE and NANTEN2 in higher tansition lines of CO have been carried out with an angular resolution of about 5 to 10 pc. These observations revealed that the differences of the physical properties represent an evolutionary sequence of GMCs in terms of density increase leading to star formation. Type I and II GMCs are at the early phase of star formation where density does not yet become high enough to show active star formation, and Type III GMCs represent the later phase where the average density is increased and the GMCs are forming massive stars.

The central few hundred parsecs of the Milky Way host a massive black hole and exhibit very violent gas motion and high temperatures in molecular gas. The origin of these properties has been a mystery for the past four decades. Wide-field imaging of the ¹² CO (rotational quantum number J = 1 to 0) 2.6-millimeter spectrum has revealed huge loops of dense molecular gas with strong velocity dispersions in the galactic center. We present a magnetic flotation model to explain that the formation of the loops is due to magnetic buoyancy caused by the Parker instability. The model has the potential to offer a coherent explanation for the origin of the violent motion and extensive heating of the molecular gas in the galactic center.

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.