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Masao Mori

Publications and source records attributed to Masao Mori.

At least 55 records · Page 3Linked to original sources
arXiv2012

Astrophysical Particle Simulations on Heterogeneous CPU-GPU Systems

A heterogeneous CPU-GPU node is getting popular in HPC clusters. We need to rethink algorithms and optimization techniques for such system depending on the relative performance of CPU vs. GPU. In this paper, we report a performance optimized particle simulation code "OTOO", that is based on the octree method, for heterogenous systems. Main applications of OTOO are astrophysical simulations such as N-body models and the evolution of a violent merger of stars. We propose optimal task split between CPU and GPU where GPU is only used to compute the calculation of the particle force. Also, we describe optimization techniques such as control of the force accuracy, vectorized tree walk, and work partitioning among multiple GPUs. We used OTOO for modeling a merger of two white dwarf stars and found that OTOO is powerful and practical to simulate the fate of the process.

astro-ph.IM↗
arXiv2011

Sub-millimeter brightness of early star-forming galaxies

Based on a three-dimensional model of an early star-forming galaxy, we explore the evolution of the sub-millimeter brightness. The model galaxy is employed from an ultra-high-resolution chemodynamic simulation of a primordial galaxy by Mori & Umemura, where the SFR is ~ 10 Msun/yr at t<0.3 Gyr and several Msun/yr at t>0.3 Gyr. The former phase well reproduces the observed properties of LAEs and the latter does LBGs. We solve the three-dimensional radiative transfer in the clumpy interstellar media in this model galaxy, taking the size distributions of dust grains into account, and calculate the dust temperature as a function of galactic evolutionary time. We find that the clumpiness of interstellar media plays an important role for the sub-millimeter brightness. In the LAE phase, dust grains are concentrated on clumpy star-forming regions that are distributed all over the galaxy, and the grains can effectively absorb UV radiation from stars. As a result, the dust is heated up to T>35 K. In the LBG phase, the continuous supernovae drive dust grains far away from star-forming regions. Then, the grains cannot absorb much radiation from stars, and becomes into a cold state close to the CMB temperature. Consequently, the dust temperature decreases with the evolutionary time, where the mass-weighted mean temperature is T=26 K at t=0.1 Gyr and T=21 K at t=1.0 Gyr. By this analysis, it turns out that the sub-millimeter brightness is higher in the LAE phase than that in the LBG phase, although the dust-to-gas ratio increases monotonically as a function of time. We derive the spectral energy distributions by placing the model galaxy at a given redshift. The peak flux at 850 micron is found to be S_850 ~ 0.2 - 0.9 mJy if the model galaxy is placed at 6>z>2. This means that ALMA can detect an early star-forming galaxy with SFR of ~ 10 Msun/yr by less than one hour integration with 16 antennas.

astro-ph.CO↗
arXiv2011

Near-Infrared Reverberation by Dusty Clumpy Tori in Active Galactic Nuclei

According to recent models, the accretion disk and black hole in active galactic nuclei are surrounded by a clumpy torus. We investigate the NIR flux variation of the torus in response to a UV flash for various geometries. Anisotropic illumination by the disk and the torus self-occultation contrast our study with earlier works. Both the waning effect of each clump and the torus self-occultation selectively reduce the emission from the region with a short delay. Therefore, the NIR delay depends on the viewing angle (where a more inclined angle leads to a longer delay) and the time response shows an asymmetric profile with a negative skewness, opposing to the results for optically thin tori. The range of the computed delay coincides with the observed one, suggesting that the viewing angle is primarily responsible for the scatter of the observed delay. We also propose that the red NIR-to-optical color of type-1.8/1.9 objects is caused by not only the dust extinction but also the intrinsically red color. Compared with the modest torus thickness, both a thick and a thin tori display the weaker NIR emission. A selection bias is thus expected such that NIR-selected AGNs tend to possess moderately thick tori. A thicker torus shows a narrower and more heavily skewed time profile, while a thin torus produces a rapid response. A super-Eddington accretion rate leads to a much weaker NIR emission due to the disk self-occultation and the disk truncation by the self-gravity. A long delay is expected from an optically thin and/or a largely misaligned torus. A very weak NIR emission, such as in hot-dust-poor active nuclei, can arise from a geometrically thin torus, a super-Eddington accretion rate or a slightly misaligned torus.

astro-ph.CO↗
arXiv2011

The core-cusp problem in cold dark matter halos and supernova feedback: Effects of Mass Loss

The core-cusp problem remains as one of the unsolved discrepancies between observations and theories predicted by the standard paradigm of cold dark matter (CDM) cosmology. To solve this problem, we perform N-body simulations to study the nonlinear response of CDM halos to the variance of the gravitational potential induced by gas removal from galaxy centers. In this study, we focus on the timescale of the gas ejection, which is strongly correlated with stellar activities, and demonstrate that it is one of the key factors in determining the dynamical response of CDM halos. The results of simulations show that the power-low index of the mass-density profile of the dark matter halo correlated with the timescale of the mass loss, and it is flatter when the mass loss occurs over a short time than when it occurs over a long time. However, it is still larger than typical observational values; in other words, the central cusp remains for any mass loss model in the simulations. Moreover, for the slow mass-loss case, the final density profile of the dark matter halo recovers the universal density profiles predicted by the CDM cosmology. Therefore, mass loss driven by stellar feedback may not be an effective mechanism to flatten the central cusp.

astro-ph.CO↗
arXiv2010

Orientation Effects on the Inner Region of Dusty Torus of Active Galactic Nuclei

A sublimation process governs the innermost region of the dusty torus of active galactic nuclei. However, the observed inner radius of the torus is systematically smaller than the expected radius by a factor of ~ 1/3. We show that the anisotropy of the emission from accretion disks resolves this conflict naturally and quantitatively. An accretion disk emits lesser radiation in the direction closer to its equatorial plane (i.e., to the torus). We find that the anisotropy makes the torus inner region closer to the central black hole and concave. Moreover, the innermost edge of the torus may connect with the outermost edge of the disk continuously. Considering the anisotropic emission of each clump in the torus, we calculate the near-infrared flux variation in response to a UV flash. For an observer at the polar angle theta_obs = 25 deg, the centroid of the time delay is found to be 37% of the delay expected in the case of isotropic illumination, which explains the observed systematic deviation.

astro-ph.CO↗
arXiv2009

Global Radiation-Magnetohydrodynamic Simulations of Black Hole Accretion Flow and Outflow: Unified Model of Three States

Black-hole accretion systems are known to possess several distinct modes (or spectral states), such as low/hard state, high/soft state, and so on. Since the dynamics of the corresponding flows is distinct, theoretical models were separately discussed for each state. We here propose a unified model based on our new, global, two-dimensional radiation-magnetohydrodynamic simulations. By controlling a density normalization we could for the first time reproduce three distinct modes of accretion flow and outflow with one numerical code. When the density is large (model A), a geometrically thick, very luminous disk forms, in which photon trapping takes place. When the density is moderate (model B), the accreting gas can effectively cool by emitting radiation, thus generating a thin disk, i.e., the soft-state disk. When the density is too low for radiative cooling to be important (model C), a disk becomes hot, thick, and faint; i.e., the hard-state disk. The magnetic energy is amplified within the disk up to about twice, 30%, and 20% of the gas energy in models A, B, and C, respectively. Notably, the disk outflows with helical magnetic fields, which are driven either by radiation pressure force or magnetic pressure force, are ubiquitous in any accretion modes. Finally, our simulations are consistent with the phenomenological alpha-viscosity prescription, that is, the disk viscosity is proportional to the pressure.

astro-ph.HE↗
arXiv2008

The Once and Future Andromeda Stream

The interaction between an accreting satellite and the Andromeda galaxy (M31) has been studied analytically and numerically, using a high-resolution N-body simulation with $4\times10^7$ particles. For the first time, we show the self-gravitating response of the disk, the bulge, and the dark matter halo of M31 to an accreting satellite. We reproduce the Stream and the shells at the East and West side of M31, by following the evolution of the collision 4 Gyr into the future, and show that recently discovered diffuse arcs on the south side of the minor axis are the remnants of a similar collision that occurred 3-4 Gyr earlier than the Stream event. The present day integrity of the M31 disk constrains the satellite mass to be $M \leq 5\times10^9 M_\odot$. The stars that were originally in the center of the satellite are now in the east shell. Therefore, observations in this region might reveal additional clues about the nature of satellite, such as the central core and any metallicity gradient.

astro-ph↗
arXiv2007

The evolution of galaxies from primeval irregulars to present-day ellipticals

The current understanding of galaxy formation is that it proceeds in a 'bottom up' way, with the formation of small clumps of gas and stars that merge hierarchically until giant galaxies are built up. The baryonic gas loses the thermal energy by radiative cooling and falls towards the centres of the new galaxies, while supernovae (SNe) blow gas out. Any realistic model therefore requires a proper treatment of these processes, but hitherto this has been far from satisfactory. Here we report an ultra-high-resolution simulation that follows evolution from the earliest stages of galaxy formation through the period of dynamical relaxation. The bubble structures of gas revealed in our simulation ($< 3\times10^8$ years) resemble closely the high-redshift Lyman $α$ emitters (LAEs). After $10^9$ years these bodies are dominated by stellar continuum radiation and look like the Lyman break galaxies (LBGs) known as the high-redshift star-forming galaxies at which point the abundance of elements heavier than helium ("metallicity") appears to be solar. After $1.3\times10^{10}$ years, these galaxies resemble present-day ellipticals.

astro-ph↗
arXiv2006

Chemodynamics of Lyman alpha emitters, Lyman break galaxies and elliptical galaxies

We report an ultra-high-resolution simulation that follows evolution from the earliest stages of galaxy formation through the period of dynamical relaxation. The bubble structures of gas revealed in our simulation ($< 3\times 10^8$ years) resemble closely the high-redshift Lyman $α$ emitters (LAEs). After $10^9$ years these bodies are dominated by stellar continuum radiation and look like the Lyman break galaxies (LBGs) known as the high-redshift star-forming galaxies at which point the abundance of elements appears to be solar. After $1.3\times10^{10}$ years, these galaxies resemble present-day ellipticals. The comparisons of simulation results with the observations of elliptical galaxies allow us to conclude that LAEs and LBGs are infants of elliptical galaxies or bulge systems in the nearby universe.

astro-ph↗
arXiv2005

QSO Formation under Coevolution of SMBH and Bulge

he formation and growth of supermassive black holes (SMBHs) physically linked with bulges are considered. We focus on the radiation hydrodynamic process for the growth of SMBH in the optically thick starburst phase, where radiation from bulge stars drives the mass accretion on to a galactic center through radiation drag effect. In the present scenario, the AGN luminosity-dominant phase (QSO phase) is preceded by the host luminosity-dominat phase, which is called ``proto-QSO phase''. In this phase, there exists the massive dusty disks within younger bulges. Also, the proto-QSO phase is anticipated by an optically-thick ultraluminous infrared galaxy (ULIRG) phase. Furthermore, such radiation hydrodynamic model has been also applied to disk galaxies. It turns out that the mass of a SMBH primarily correlates with a bulge component even in a disk galaxy. Thus, by analogy to proto-QSOs, the BH growing phase in disk galaxies may have massive dusty disks within younger bulges.

astro-ph↗
arXiv2004

The Nature of Lya Blobs: Supernova-Dominated Primordial Galaxies

We consider a forming galaxy undergoing multitudinous supernova (SN) explosions, as a possible model of \lya blobs (LABs). For this purpose, an ultra--high resolution hydrodynamic simulation is performed using $1024^3$ grid points, where SN remnants are resolved with sufficient accuracy. It is found that multiple SN explosions produce kpc--size expanding hot bubbles, which drive cool, dense shells by strong shock. The colliding high--density cooling shells radiate intensive \lya emission, resulting in a high \lya luminosity of $\sim10^{43}$ erg s$^{-1}$, comparable to the observed level in LABs. Also, recently discovered bubbly features in some LABs are quite similar to the structure predicted in the present simulation. Furthermore, the result demonstrates that LABs are representative of evolving primordial galaxies; they could hold direct information on the early chemical enrichment of galaxies, contrary to present--day galaxies which have undergone intense recycling of interstellar matter, thus erasing most of the early chemical history. It turns out that the metal mixing proceeds in a very inhomogeneous fashion, so that there appears a large spread of metallicity, that is,$[{\rm Fe/H}] \approx 0 {\rm ~to} -5$ or $[{\rm O/H}] \approx 1 {\rm ~to} -4$. Hence, the early galactic chemical evolution may have proceeded in a different manner from that hitherto considered in one--zone models.

astro-ph↗
arXiv2002

Proto-Quasars:Physical States and Observable Properties

Based on the radiation hydrodynamical model for the black hole(BH) growth,incorporated with the chemical evolution of the early-type host galaxy, we construct the coevolution model of a QSO BH and the host galaxy. As a result, it is found that after a galactic wind epoch,the luminosity is shifted from the host-dominant phase to the AGN-dominant phase (QSO phase) in the timescale of a few $10^{8}$ years.The former phase corresponds to the early stage of growing BH, and can be regarded as a ``proto-QSO'' phase. It has observable characteristic properties (detail inthis paper).By comparing these predictions with recent observations, radio galaxies are a possible candidate for proto-QSOs.Also, it is anticipated that the proto-QSO phase is preceded by an optically thick phase, which may correspond to ULIRGs.In this phase, $M_{\rm BH}/M_{\rm bulge}$ is predicted to be much less than $10^{-3}$ and grow with metallicity.Moreover, as precursors of ULIRGs, optically-thin star-forming galaxies are predicted. These may be in the assembly phase of Lyman break galaxies (LBGs) or Ly$α$ emitters.

astro-ph↗
arXiv2002

Does the Slim-Disk Model Correctly Consider Photon-Trapping Effects?

We investigate the photon-trapping effects in the super-critical black hole accretion flows by solving radiation transfer as well as the energy equations of radiation and gas. It is found that the slim-disk model generally overestimates the luminosity of the disk at around the Eddington luminosity (L_E) and is not accurate in describing the effective temperature profile, since it neglects time delay between energy generation at deeper inside the disk and energy release at the surface. Especially, the photon-trapping effects are appreciable even below L ~ L_E, while they appear above ~ 3L_E according to the slim disk. Through the photon-trapping effects, the luminosity is reduced and the effective temperature profile becomes flatter than r^{-3/4} as in the standard disk. In the case that the viscous heating is effective only around the equatorial plane, the luminosity is kept around the Eddington luminosity even at very large mass accretion rate, Mdot>>L_E/c^2. The effective temperature profile is almost flat, and the maximum temperature decreases in accordance with rise in the mass accretion rate. Thus, the most luminous radius shifts to the outer region when Mdot/(L_E/c^2) >> 10^2. In the case that the energy is dissipated equally at any heights, the resultant luminosity is somewhat larger than in the former case, but the energy-conversion efficiency still decreases with increase of the mass accretion rate, as well. The most luminous radius stays around the inner edge of the disk in the latter case. Hence, the effective temperature profile is sensitive to the vertical distribution of energy production rates, so is the spectral shape. Future observations of high L/L_E objects will be able to test our model.

astro-ph↗
arXiv2001

Early Metal Enrichment by Pregalactic Outflows: II. Simulations of Blow-away

We present results from three-dimensional numerical simulations of the dynamics of SN-driven bubbles as they propagate through and escape the grasp of subgalactic halos with masses M ~ 10^8 Msun at redshift z=9. The hydrodynamic simulations use a nested grid method to follow the evolution of explosive multi-SN events operating on the characteristic timescale of a few x 10^7 yr, the lifetime of massive stars. The results confirm that, if the star formation efficiency of subgalactic halos is close to 10%, a significant fraction of the halo gas will be lifted out of the potential well (`blow-away'), shock the intergalactic medium, and pollute it with metal-enriched material, a scenario recently advocated by Madau, Ferrara & Rees (2001). The volume filling factor of the ejecta is close to unity. Depending on the stellar distribution, we find that less than 30% of the available SN energy is converted into kinetic energy of the blown away material, the remainder being radiated away. It appears that mechanical feedback is less efficient than expected from simple energetic arguments, as off-nuclear SN explosions drive inward-propagating shocks that tend to collect and pile up cold gas in the central regions of the host halo. Low-mass galaxies at early epochs may survive multiple SN events and continue forming stars.

astro-ph↗
arXiv2000

Numerical Simulations of Globular Cluster Formation

We examine various physical processes associated with the formation of globular clusters by using the three-dimensional Smoothed Particle Hydrodynamics (SPH) code. Our code includes radiative cooling of gases, star formation, energy feedback from stars including stellar winds and supernovae, and chemical enrichment by stars. We assume that, in the collapsing galaxy, isothermal cold clouds form through thermal condensations and become proto-globular clouds. We calculate the size of proto-globular clouds by solving the linearized equations for perturbation. We compute the evolution of the inner region of the proto-cloud with our SPH code for various initial radius and initial composition of gases. When the initial gases contain no heavy elements, the evolution of proto-clouds sensitively depends on the initial radius. For a smaller initial radius, the initial star burst is so intense that the subsequent star formation occurs in the central regions to form a dense star cluster as massive as the globular cluster. When the initial gases contain some heavy elements, the metallicity of gases affects the evolution and the final stellar mass. If the initial radius of the proto-globular clouds was relatively large, the formation of a star cluster as massive as the globular clusters requires the initial metallicity as high as [Fe/H] $\geq -2$. The self-enrichment of heavy elements in the star cluster does not occur in all cases.

astro-ph↗
arXiv2000

Gas Stripping of Dwarf Galaxies in Clusters of Galaxies

Many clusters of galaxies contain an appreciable amount of hot gas, the intracluster medium. As a consequence, gas will be stripped from galaxies that move through the inter cluster medium, if the ram pressure exceeds the internal gravitational force. We study the interaction between the intracluster medium and an extended gas component of dwarf galaxies confined by a surrounding cold dark matter halo analytically and numerically, using axisymmetric two-dimensional hydrodynamical simulations at high resolution. The results show that the gas within the dark matter halo is totally stripped in a typical galactic cluster. The process of ram pressure stripping therefore must have played an important role during the chemo-dynamical evolution of dwarf galaxies in galactic clusters. Our results predict that most of the chemical evolution of dwarf galaxies must have occurred at high redshift, before the virialized cluster had formed.

astro-ph↗
arXiv1998

Dissipative Process as a Mechanism of Differentiating Internal Structures between Dwarf and Normal Elliptical Galaxies in a CDM Universe

We simulate the dynamical, chemical, and spectro-photometric evolution of dwarf and normal elliptical galaxies embedded in a dark matter halo, using a three-dimensional N-body/SPH simulation code. For the forming dwarf elliptical galaxies, supernova-driven winds propagating outwards from inside the system collide with the infalling gas and produce the super-shell in which stars are formed. The resulting stellar system forms a loosely bound virialized system due to the significant mass loss and has a large velocity dispersion and a large core. Consequently the surface brightness distribution shows an exponential profile and the color distribution shows a positive gradient such that the colors become redder away from the galaxy center in agreement with observations. On the other hand, for the normal elliptical galaxies with deep gravitational potential, the mass loss out of the system does not have a significant dynamical effect on the stellar system. The resulting surface brightness distribution has a large central concentration like de Vaucouleurs' $r^{1/4}$-profile and the color distribution shows a negative gradient as observed. Our simulation shows that different features between dwarf and normal elliptical galaxies stem from different cooling efficiencies for their respective protogalaxies in a standard CDM universe.

astro-ph↗