Contribution of the Large Magellanic Cloud to the Galactic Warp

Multi-scale interaction between the LMC, Galactic halo, and the disk is examined with N-body simulations, and precise amplitudes of the Galactic warp excitation are obtained. The Galactic models are constructed most realistically to satisfy available observational constraints on the local circular velocity, the mass, surface density and thickness of the disk, the mass and size of the bulge, the local density of the halo matter at the solar radius, and the mass and orbit of the LMC. By using hybrid algorithm (SCF-TREE) I have succeeded to follow the evolution with millions of particles. The orbiting satellite excites density enhancement as a wake, and the wake exerts a tidal force on the disk. Because of the additional torque from the wakes in the halo, the amplitudes of the induced warps are much larger than the classical estimate by [hunt1969], who considered only the direct torque from the LMC. The obtained amplitudes of m=0, 1, 2 warps in the larger halo model show very good agreement with the observed amplitude in the Milky Way. This result revives the LMC as a possible candidate of the origin of the Galactic warp. Our smaller halo model, however, yield only weak warps in all the harmonic modes. Therefore the halo still has significant influence on excitation of warp even in the interaction scenario for excitation of warps.Comment: 36 pages, including 20 EPS figures. Final revision for the publicatio

Time evolution of galactic warps in prolate haloes

A recent observation with the Hipparcos satellite and some numerical simulations imply that the interaction between an oblate halo and a disc is inappropriate for the persistence of galactic warps. Then, we have compared the time evolution of galactic warps in a prolate halo with that in an oblate halo. The haloes were approximated as fixed potentials, while the discs were represented by N-body particles. We have found that the warping in the oblate halo continues to wind up, and finally disappears. On the other hand, for the prolate halo model, the precession rate of the outer disc increases when the precession of the outer disc recedes from that of the inner disc, and vice versa. Consequently, the warping in the prolate halo persisted to the end of the simulation by retaining the alignment of the line of nodes of the warped disc. Therefore, our results suggest that prolate haloes could sustain galactic warps. The physical mechanism of the persistence of warp is discussed on the basis of the torque between a halo and a disc and that between the inner and outer regions of the disc.Comment: 8 pages, 8 figures. Accepted for publication in MNRA

大規模な海底変動による海中音速場の変化 : 3.11巨大地震は海洋の音響環境に影響を与えたか？

発表番号: 12-27 / 海洋音響学会2012年度研究発表会（2012年5月28日～29日, 東京工業大学） / 海洋音響学会2012年度研究発表会講演論文集から転

海洋酸性化による音波伝搬損失への影響

発表番号: 10-29 / 海洋音響学会2010年度研究発表会（2010年5月27日～28日, 東京工業大学） / 海洋音響学会2010年度研究発表会講演論文集から転

Deep sea exploration survey technology by JAMSTEC and results of sunken object search

Japan Agency for Marine-Earth Science and Technology (JAMSTEC) is the research center established by Japanese Government in 1970 to study about the ocean and earth science and technology. Especially，technology of the field of the deep sea survey will be the top level in the world. JAMSTEC was developing many deep sea survey equipment from around 1990 and was improving survey technology. These survey systems include many sonars, deep tow system, ROV (Remotely Operated Vehicle) and AUV (Autonomous Underwater Vehicle) etc. And we have succeeded in a survey and collection of many sunken ships and objects which sank in a deep sea from 1995. This lecture would be explained about the outline of the deep sea survey technology in JAMSTEC and the following examples which succeeded in survey. (1) Survey of the Russian tanker "Nakhodka" sunk by bad weather into the Sea of Japan. At depth about 2500m in 1997. (2) Survey of the "Tsushimamaru" of the evacuated pupil ship sunk by an attack of a U.S. Navy submarine during the World War II. At depth about 900m in1997. (3) Search of a main engine of the large rocket which has fallen by a breakdown at Pacific Ocean. At depth about 3000m in 1999. (4) Survey and collection of many momentos of the "Ehimemaru" of the fisheries training ship sunk by collision with a nuclear submarine of U.S. Navy. At depth 1800m in 2001. (5) Survey of the "Seitokumaru" which is a sunk fishing boat by collision with Aegis destroyer of Japan Self-Defense Forces. At depth about 1800m in 2008 During these searches, the comprehensive use of a multi-beam echo-sounder, a side-scan sonar and a narrow-beam sub-bottom profiler, deep tow system and ROV.The Preservation and Management of Underwater Cultural Heritage- Theories, Methods and Practices / 2015年水下文化資?保存與再利用研討會 -理論、方法與實務 (4-6 October 2015, Taichung, Taiwan

Constraints of the Clumpyness of Dark Matter Halos Through Heating of the Disk Galaxies

Motivated by the presence of numerous dark matter clumps in the Milky Way's halo as expected from the cold dark matter cosmological model, we conduct numerical simulations to examine the heating of the disk. We construct an initial galaxy model in equilibrium, with a stable thin disk. The disk interacts with dark matter clumps for about 5 Gyr. Three physical effects are examined : first the mass spectrum of the dark matter clumps, second the initial thickness of the galactic disk, and third the spatial distribution of the clumps. We find that the massive end of the mass spectrum determines the amount of disk heating. Thicker disks suffer less heating. There is a certain thickness at which the heating owing to the interaction with the clumps becomes saturates. We also find that the heating produced by the model which mimics the distribution found in Standard CDM cosmology is significant and too high to explain the observational constraints. On the other hand, our model that corresponds to the clump distribution in a $\Lambda$CDM cosmology produces no significant heating. This result suggests that the $\Lambda$CDM cosmology is preferable with respect to the Standard CDM cosmology in explaining the thickness of the Milky Way.Comment: 28 pages, 13 figures, submitted to Ap