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OUTAP Colloquium 2016

ホーム > コロキウム & セミナー > 宇宙進化コロキウムのご案内 > 2016年度の宇宙進化コロキウム
・・・・・2016年度の宇宙進化コロキウム・・・・・


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Date : 3/13 (Mon) 16:15~
Location: F620
Speaker : Prof. Tomotsugu Goto (National Tsing Hua University)
Title : No Lya emitters detected around a QSO at z=6.4: Suppressed by the QSO?

Abstract: Understanding how QSO's UV radiation affects galaxy formation is vital to our understanding of reionization era. Using a custom made narrow-band filter, NB906, on Subaru/Suprime-Cam, we investigated the number density of Lya emitters (LAE) around a QSO at z=6.4. To date, this is the highest redshift narrow-band observation, where LAEs around a luminous QSO are investigated. Due to the large field-of-view of Suprime-Cam, our survey area is ~5400cMpc2, much larger than previously investigated QSO environments at z=5.7 (~200 cMpc2). In this field, we previously found a factor of 7 overdensity of Lyman break galaxies (LBGs). Based on this, we expected to detect ~200 LAEs down to NB906=25 ABmag in the 30' field of view. However, our 6.4 hour exposure with Subaru telescope found none. The obtained upper limit on the number density of LAEs is more than an order lower than the blank field. Furthermore, this lower density of LAEs spans a large scale of 10 pMpc across. A simple argument suggests a strong UV radiation from the QSO can suppress formation stars in halos with Mvir < 10^10 Msun near the QSO within a Mpc. This could explain the observed lack of LAEs if LAEs are in less massive, and detected LBGs are in more massive halos than 10^10Msun.


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Date : 2/22 (Wed) 15:00~
Location: F313
Speaker : Prof. Kaiki Inoue (Kindai University)
Title : Probing Dark Dwarf Galaxies with Gravitational Lensing

Abstract: The so-called 'missing satellite problem' is one of the most challenging problem in the modern astrophysics: Theory predicts an excess number of dark subhalos around the Milky Way than observed satellite galaxies, which are expected to trace them. If theory is correct, we expect a number of dark dwarf galaxies in the universe. Since dark dwarf galaxies are faint, the methods for detecting such objects are limited to either a direct detection of a faint signal or the gravitational lensing effect, bending of light rays passing by massive objects. In fact, several quasar-galaxy quadruply lensed systems have been known to show 'anomaly' in the flux ratios: The flux ratios of lensed images disagree with the prediction of best-fit lens models with a smooth potential whose fluctuation scale is larger than the separation between the lensed images. It has been considered as an imprint of cold dark matter subhalos, but recent works by our groups and others indicate that intervening halos and voids in the line-of-sight are important as well as subhalos. In this talk, I review previous our works on 'substructure lensing' caused by subhalos and intervening structures, and present our recent observational results obtained from ALMA submillimeter observations of a SMG-galaxy and a quasar-galaxy lens systems and their implications and future prospects.


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Date : 1/18 (Wed) 15:00~
Location: F313
Speaker : Prof. Cosimo Bambi (Fudan University)
Title : Testing the Kerr paradigm using X-ray reflection spectroscopy

Abstract: The spacetime metric around astrophysical black holes is supposed to be well described by the Kerr solution. However, deviations from the Kerr geometry are expected in a number of different scenarios beyond Einstein's gravity coupled to ordinary matter. In this talk, I will show that the study of the reflection spectrum of thin accretion disks is a promising approach to probe the metric around black holes and test the Kerr paradigm.


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Date : 11/16 (Wed) 15:00~
Location: F313
Speaker : Prof. Yasuhiko Sentoku (Osaka University)
Title : Enabling Numerical Modeling of Extreme-Intensity Laser Produced Dense Plasma

Abstract: The sun gives enormous energy on the earth. Inside the sun charged particles and photons interact each others and form the complex states of matter. The physics in such extreme states of matter is called the high energy density physics (HEDP). The HEDP is considerable interest due to their relevance to inertial confinement fusion as well as astrophysical plasmas found in the stellar interiors, the cores of the giant planets, galactic nuclei and x-ray binaries. Due to the recent technological advances, lasers with sub-picosecond duration with petawatt power, which is a few order of magnitudes higher than the total electric consumption power on the globe, are now available. Such strong laser light is capable of producing a solid-state high temperature plasmas, which is equivalent to the states of matter inside the sun. So the powerful laser allows us study the physics inside the stars on the earth, namely, in laboratory. Although the intense short pulse can create the extreme states of matter, the physics in such states is actually very complicated because the plasma is non-thermal and no-equilibrated. It is also difficult to diagnose the high temperature plasmas with the fine spatiotemporal resolution since the experimental diagnostics are limited at this moment. So that it is not easy to see what is going on inside the plasma only with the experimental data. Numerical simulations on large computer system are used these days in many science researches since they are powerful tools to understand the physics behind. In the proposed work, a simulation code will be developed, which is capable to simulate the critical details of formation of extreme states of matter in the laser-matter interaction, including various atomic processes, such as collisions, ionizations, and radiations. The code will be used to explore the HEDP in various parameter regimes and also to optimize the fusion processes in the laser-produced plasmas. In this talk, I will introduce the laser-plasma simulation code, PICLS, which is based on a particle-in-cell scheme, capable to simulate plasma kinetics, particle acceleration, energy transport, x-ray radiation, and radiation transport in laser-produced dense plasmas.


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Date : 9/21 (Wed) 10:00~
Location: F620
Speaker : Dr. Kei Tanaka (University of Florida)
Title : Massive star formation by core accretion

Abstract: In this talk, I would like to introduce the theoretical view of massive star formation in various environment. Massive stars play important roles in many astrophysical fields: they are the main sources of mechanical, radiative and chemical feedback which regulate the evolution of interstellar medium and star formation activity in cosmic history. We are developing an analytical model of massive star formation by prestellar-cloud core collapse including the evolution of protostars and disks, and multiple feedback processes. Our model shows that the momentum by MHD disk wind drives strong outflow which sweeps up about 60% of the core material almost same as the case of low-mass star formation. In the formation of over-10Msun stars, the additional feedback by radiation, such as radiation pressure and photo-evaporation, stellar wind, reduces the core-to-star efficiency down to ~10% at minimum in our model. However, the dominant momentum output process is the MHD disk wind, and the radiation feedback do not limit the maximum mass which can formed by core accretion due to flashlight effect and dust absorption of EUV photons. Based on our accretion model, we also perform the radiative transfer calculation and make observational predictions to test theoretical model. We find that our model predictions are consistent with the properties of observed radio jets and winds. I'll also briefly explain similarities and differences of massive star formation processes at lower metallicities, e.g., metallicity dependence of radiation feedback and disk stability.


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Date : 8/1 (Mon) 15:00~
Location: F313
Speaker : Prof. Kohta Murase (The Pennsylvania State University)
Title : Mysteries of High-Energy Cosmic Neutrinos

Abstract: In this talk, we review the latest results of IceCube observations and discuss the origin of high-energy cosmic neutrinos.


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Date : 7/21 (Thu) 15:00~
Location: F227
Speaker : Prof. Kazunari Iwasaki (Doushisha University)
Title : A Mechanism for Turbulence Generation inside Interstellar Clouds

Abstract: It is well known that the interstellar medium consists of two thermal equilibrium states, i.e., a clumpy cold phase and a diffuse warm phase as a result of the balance of radiative cooling and heating. Koyama and Inutsuka (2002) and many authors have shown that cold clouds have supersonictranslational velocity in a surrounding diffuse warm gas. The supersonic translational motion of cold clouds can be origin of the observed "supersonic" turbulence. However, the detailed physical processes in the multi-phase turbulence are still unknown. In this study, we investigate the turbulence generation mechanism inside the cold clouds that is important in star formation. One of the promising mechanisms is the Kelvin-Helmholtz (KH) instability induced by a velocity shear between the cold clouds and surrounding warm gas. We performed multi-dimensional numerical simulations of the KH instability with/without a radiative cooling/heating and the thermal conduction. As an initial condition, a static cold layer sandwiched by a warm gas with velocity shear is considered. Without cooling/heating and thermal conduction, the KH instability drives only a weak turbulence with a velocity dispersion of 0.1-0.2 km/s because of the high density contrast. On the other hand, from the simulations with cooling/heating and thermal conduction, we found that the phase transition significantly enhances the velocity dispersion inside the cold layer. The mechanism is as follows: the roll-up of the interface induces the phase transition from the warm gas to the cold gas. The phase transition increases the velocity dispersion in the cold layer because the warm gas streams with high velocity. As a result, the KH instability with the phase transition drives a transonic turbulence.


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Date : 7/6 (Wed) 15:00~
Location: F313
Speaker : Prof. Hiroshi Nagai (National Astronomical Observatory of Japan)
Title : Ultra High Resolution Observation of Gamma-ray-loud Radio Galaxy 3C 84 with VLBI

Abstract: Abstract: Relativistic jets emanating from SMBHs are one of the primary targets of high energy astrophysics. VLBI is a unique tool to provide very high angular resolution, which allows to resolve the innermost region of relativistic jets where the acceleration, collimation, and production of high energy particles are took place. Due to its brightness and proximity, 3C84 (NGC1275) is one of the best-studied radio galaxies in history. The VLBI observations revealed that a recent increased activity in radio bands originated in the restarted jet activity starting in about 2005 (Nagai et al. 2010). The restarted jet is resolved in the transverse direction and found to be limb brightened (Nagai et al. 2014) as also seen in M87 and other nearby radio galaxies. In 1990s, the jet limb-brightening was not observed even with the observations with a similar spatial resolution and sensitivity (Dhawan et al. 1997). The jet morphology was rather ridge brightening. This change in the jet morphology shows an interesting agreement with the gamma-ray flux increase, i.e., the gamma-ray flux in the 1990s was at least 10 times lower than the current one. In Nagai et al. (2014), we proposed that the change from ridge brightening to limb brightening may be attributed to a change in the transverse velocity structure on the basis of a “spine-sheath” scenario. In this talk, I will review these results and introduce the prospects for upcoming millimeter VLBI observation (PI: H. Nagai) with a possible inclusion of ALMA. I also briefly mention an ongoing work with Fujita-san, as well as a very fresh result from ALMA Cycle 3 observation of another radio galaxy Cen A.


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Date : 6/15 (Wed) 15:00~
Location: F313
Speaker : Mr. Yuichi Harikane (Tokyo University)
Title : Evolution of Stellar-to-Halo Mass Ratio at z=0-7 Identified by Clustering Analysis with the Hubble Legacy Imaging and Early Subaru/Hyper Suprime-Cam Survey Data

Abstract: We present clustering analysis results from 10,381 Lyman break galaxies (LBGs) at z~ 4-7, identified in the Hubble legacy deep imaging and new complimentary large-area Subaru/Hyper Suprime-Cam data. We measure the angular correlation functions (ACFs) of these LBGs at z~4, 5, 6, and 7, and fit these measurements using halo occupation distribution (HOD) models that provide an estimate of halo masses, M_h~(1-20)x10^11 Msun. Our M_h estimates agree with those obtained by previous clustering studies in a UV-magnitude vs. M_h plane, and allow us to calculate stellar-to-halo mass ratios (SHMRs) of LBGs. By comparison with the z~0 SHMR, we identify evolution of the SHMR from z~0 to z~4, and z~4 to z~7 at the >98% confidence levels. The SHMR decreases by a factor of ~2 from z~0 to 4, and increases by a factor of ~4 from z~4 to 7. We compare our SHMRs with results of a hydrodynamic simulation and a semi-analytic model, and find that these theoretical studies do not predict the SHMR increase from z~4 to 7.


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Date : 5/25 (Wed) 15:00~
Location: F313
Speaker : Dr. Gen Chiaki (Konan University)
Title : Chemo-thermal evolution of collapsing gas clouds and metal-poor star formation

Abstract: The gravitational collapse of metal-poor gas clouds and star formation are investigated. Recently, the long-lived and metal-poor stars have been observed in the Galactic halo. Yet, the origin of these stars is still unknown. We perform three-dimensional hydrodynamics simulations to see the critical condition for which cloud fragmentation occurs and low-mass stars are formed for four clouds with various metallicities. We first consider accretion of gas-phase metal molecules onto grains as well as the all relevant chemical reactions. The metal and dust amounts and dust size distribution are consistently calculated in the models of Population III supernova, which mainly supply grains in the early Universe. Also, radiative cooling by chemical species such as metal ions, atoms, and molecules is included. We make use of smoothed hydrodynamics (SPH) scheme, and the mass resolution in the collapse center is refined by a novel particle splitting technique based on the Voronoi diagrams created by SPH particles. The mass resolution becomes from hundreds of solar masses to tens of Earth masses when the first protostellar core appears in the collapse center. We find that, several decades after the first protostar formation, fragmentation does not occur in the vast majority of clouds even with metallicity higher than 10^{-4} Zsun (solar metallicity), where dust cooling is effective. With 10^{-5} Zsun, two of four clouds undergo enough dust cooling. In one of these two clouds, however, rapid gas heating along with H_2 molecular formation prevents cloud elongation, which is the precursor of fragmentation. With 10^{-4} Zsun, H_2 formation heating is important for all clouds, but OH/H2O molecular cooling enhances cloud elongation in one of four cloud. If the cloud elongation is sufficiently promoted before dust cooling is important, the cloud becomes unstable again to fragment. We also find that he different heating/cooling processes become important for different clouds because the overall thermal evolution of these clouds is different. The collapse timescale of these clouds is important to determine the collapse timescale.


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Date : 5/18 (Wed) 15:00~
Location: F313
Speaker : Mr. Tomohiro Ono ( Kyoto University )
Title : Critical Condition and Physical Mechanism of The Rossby Wave Instability

Abstract: Protoplanetary disks with non-axisymmetric structures have been observed. It is often considered that the non-axisymmetric structures are formed by the Rossby wave instability (RWI). The RWI is a non-axisymmetric hydrodynamic instability in differentially rotating disks. It is known that the RWI is unstable and forms large-scale vortexes when the disk has a rapid radial variation. However, the critical condition and the physical mechanism of the RWI have not been investigated well. We perform linear stability analyses to explore them. As a result, we find that the co-rotation radius is located at the background vortensity minimum with large concavity if the system is marginally stable to the RWI, and this allows us to easily check the stability against the RWI. By using the Sommerfeld-Wilson quantization condition, which is known well in quantum mechanics, we newly derive the necessary and sufficient condition for the onset of the RWI in semi-analytic form. We also realize that the RWI is one of the shear instabilities, and explain the physical mechanism of the RWI by the wave interaction. In my talk, I will report these results about the RWI. More detailed information is provided in our new paper (Ono et al. in press; http://arxiv.org/abs/1603.09225).


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Date : 4/21 (Thu) 11:00~
Location: F227
Speaker : Dr. Kohei Inayoshi (Columbia University)
Title : Hyper-Eddington accretion flows onto massive black holes

Abstract: How fast can black holes (BHs) grow? The existence of bright quasars at high-redshift provides a challenging puzzle about the origin of supermassive BHs. To form such massive objects within a billion year, rapid growth of seed BHs is required. We study very-high rate, spherically symmetric accretion flows onto massive BHs embedded in dense metal-poor clouds. We find solutions from outside the Bondi radius at hyper-Eddington rates, unimpeded by radiation feedback. Accretion rates in this regime are steady, and larger than 5000 L_Edd/c^2. At lower rates, the accretion is episodic due to radiative feedback and the average rate is below the Eddington rate. The hyper-Eddington accretion solution is maintained as long as the emergent luminosity is limited to < (10-30) L_Edd because of photon trapping due to electron scattering. We apply our result to the rapid formation of massive BHs in protogalaxies. Once a seed BH forms at the center of the galaxy, it can grow to a maximum ~ 10^5 Msun via gas accretion independent of the initial BH mass.


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Date : 4/13 (Wed)15:00~
LOCATION:F313
Speaker :Prof. Luca Baiotti (大阪大学)
TITLE :General-relativistic astrophysics on supercomputers

ABSTRACT:
I will introduce some basic ideas about how to solve numerically the Einstein equations and other relativistic equations. I will also talk about computational methods and codes for such simulations. Finally, I will speak about my current research on binary neutron-star mergers and their gravitational waves.


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