Keiichi Wada

Keiichi Wada
Kagoshima University | Kadai · Graduate School of Science and Engineering

Doctor of Science

About

146
Publications
4,179
Reads
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4,106
Citations
Introduction
Additional affiliations
August 2009 - present
Kagoshima University
Position
  • Professor
March 1997 - July 2009
National Astronomical Observatory of Japan
Position
  • Professor (Associate)
January 1994 - April 1997
Hokkaido University
Position
  • Professor (Assistant)

Publications

Publications (146)
Preprint
Full-text available
A recent hydrodynamic model, "radiation-driven fountain model" (Wada et al. 2016), presented a dynamical picture that active galactic nuclei (AGNs) tori sustain their geometrical thickness by gas circulation around AGNs, and previous papers confirmed that this picture is consistent with multi-wavelength observations of nearby Seyfert galaxies. Rece...
Preprint
Full-text available
We studied the absorption features of CO lines against the continuum originating from the heated dust in the obscuring tori around active galactic nuclei (AGNs). We investigated the formation of absorption lines corresponding to the CO rotational transitions using three-dimensional non-LTE line transfer simulations considering the dust thermal emis...
Preprint
Full-text available
We present ALMA ~0.02"-resolution observations of the nucleus of the nearby (~14 Mpc) type-2 AGN NGC 1068 at HCN/HCO+/HNC J=3-2 lines, as well as at their 13C isotopologue and vibrationally excited lines, to scrutinize the morphological/dynamical/chemical/physical properties of dense molecular gas in the putative dusty molecular torus around a mass...
Preprint
We studied the magnetic field structures and dynamics of magnetized multi-phase gas on pc scales around supermassive black holes by using global 3D magnetohydrodynamics (MHD) simulations. We considered the effect of radiative cooling and X-ray heating due to active galactic nuclei (AGNs). The gas disk consists of a multi-phase gas with: (1) cold ($...
Preprint
In Wada, Tsukamoto, and Kokubo (2019), we proposed for the first time that a new class of planets, "blanets" (i.e., black hole planets), can be formed around supermassive black holes (SMBHs) in the galactic center. Here, we investigate the dust coagulation processes and physical conditions of the blanet formation outside the snowline ($r_{snow} \si...
Preprint
Submillimetre and millimetre observations are important in probing the properties of the molecular gas and dust around obscured active galactic nuclei (AGNs) and their feedback. With very high-resolution (0."02x0."03 (2x3 pc)) ALMA 345 GHz observations of CO 3-2, HCO$^+$ 4-3, HCN 4-3 $\nu_2$=1$f$, and continuum we have studied the molecular outflow...
Article
Submillimetre and millimetre line and continuum observations are important in probing the morphology, column density, and dynamics of the molecular gas and dust around obscured active galactic nuclei (AGNs) and their mechanical feedback. With very high-resolution (0. ″02 × 0. ″03 (2 × 3 pc)) ALMA 345 GHz observations of CO 3–2, HCO ⁺ 4–3, vibration...
Article
Full-text available
We study the obscuring structure of circumnuclear disks (CNDs) by considering supernova (SN) feedbacks from nuclear starburst and the effect of anisotropic radiative pressure from AGNs. We suppose that the mass accretion onto a central supermassive black hole (SMBH) is triggered by SN-driven turbulence within CNDs, and we explore how the structures...
Preprint
We studied the final phases of galactic mergers, focusing on interactions between supermassive black holes (SMBHs) and the interstellar medium in a central sub-kpc region, using an N-body/hydrodynamics code. This numerical experiment aims to understand the fate of the gas supplied by mergers of two or more galaxies with SMBHs, whose masses are $10^...
Preprint
Full-text available
We study the obscuring structure of circumnuclear disks (CNDs) by considering supernova (SN) feedbacks from nuclear starburst and the effect of anisotropic radiative pressure from AGNs. We suppose that the mass accretion onto a central supermassive black hole (SMBH) is triggered by SN-driven turbulence within CNDs, and we explore how the structures...
Article
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As a natural consequence of the elementary processes of dust growth, we discovered that a new class of planets can be formed around supermassive black holes (SMBHs). We investigated a growth path from submicron sized icy dust monomers to Earth-sized bodies outside the “snow line,” located several parsecs from SMBHs in low luminosity active galactic...
Preprint
As a natural consequence of the elementary processes of dust growth, we discovered that a new class of planets can be formed around supermassive black holes (SMBHs). We investigated a growth path from sub-micron sized icy dust monomers to Earth-sized bodies outside the "snow line'', located several parsecs from SMBHs in low luminosity active galact...
Article
Despite many theoretical studies and observations, we still do not fully understand the feeding mechanism in AGNs even in nearby galaxies, and how feedback from AGNs affects the gas dynamics itself in the galactic central regions. In this article, we summarize our recent theoretical studies and preliminary results in terms of the mass inflow and ou...
Article
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It is widely believed that, in active galactic nuclei (AGNs), a supermassive black hole with an accretion disk is surrounded by an optically and geometrically thick torus at sub-parsec scale. However, it is not clear how the mass supply is toward the central engine caused and how it is related with the internal structures of the tori. The magnetic...
Article
We used the Atacama Large Millimeter/Submillimeter Array (ALMA) to map the CO(3-2) and [C i](1-0) lines, as well as their underlying continuum emission, from the central ∼200 pc region of the Circinus galaxy that hosts the nearest type 2 Seyfert-class active galactic nucleus (AGN), with a spatial resolution of ∼6-15 pc. The lines and continuum-emit...
Article
We investigate the properties of the ionized gas irradiated by an active galactic nucleus (AGN) based on our "radiation-driven fountain" model for the nearest type-2 Seyfert galaxy, the Circinus galaxy. We conducted "quasi-three-dimensional" spectral analysis using the spectral synthesis code Cloudy and obtained the surface brightness distributions...
Preprint
We used the Atacama Large Millimeter/Submillimeter Array (ALMA) to map the CO(3-2) and [CI](1-0) lines, as well as their underlying continuum emission, from the central $\sim 200$ pc region of the Circinus galaxy that hosts the nearest type 2 Seyfert-class active galactic nucleus (AGN), with a spatial resolution of $\sim 6-15$ pc. The lines and con...
Preprint
We investigate the properties of the ionized gas irradiated by an active galactic nucleus (AGN) based on our "radiation-driven fountain" model for the nearest type-2 Seyfert galaxy, the Circinus galaxy (Wada et al. 2016). We conducted "quasi-three dimensional" spectral analysis using the spectral synthesis code CLOUDY and obtained the surface brigh...
Article
Full-text available
We present the results of our ALMA Cycle 4 high-spatial-resolution (0.04-0.07") observations, at HCN J=3-2 and HCO+ J=3-2 lines, of the nucleus of NGC 1068, the nearby prototypical type 2 active galactic nucleus (AGN). Our previous ALMA observations identified the compact emission of these lines at the putative location of the torus around a mass-a...
Article
In this study, we investigate the line emissions from cold molecular gas based on our previous "radiation-driven fountain model" (Wada 2016), which reliably explains the spectral energy distribution of the nearest type 2 Seyfert galaxy, the Circinus galaxy. Using a snapshot of the best-fit radiation-hydrodynamic model for the central r < 16 pc, in...
Article
High-resolution observations from the sub-mm to the optical wavelength regime resolve the central few 100pc region of nearby galaxies in great detail. They reveal a large diversity of features: thick gas and stellar discs, nuclear starbursts, in- and outflows, central activity, jet interaction, etc. Concentrating on the role circumnuclear discs pla...
Article
Full-text available
We used the Atacama Large Millimeter/submillimeter Array (ALMA) to map the CO(3-2) and the underlying continuum emissions around the type 1 low-luminosity active galactic nucleus (LLAGN; bolometric luminosity $\lesssim 10^{42}$ erg~s$^{-1}$) of NGC 1097 at $\sim 10$ pc resolution. These observations revealed a detailed cold gas distribution within...
Article
High resolution submm observations are important in probing the morphology, column density and dynamics of obscured active galactic nuclei (AGNs). With high resolution (0.06 x 0.05) ALMA 690 GHz observations we have found bright (TB >80 K) and compact (FWHM 10x7 pc) CO 6-5 line emission in the nucleus of the extremely radio-quiet galaxy NGC1377. Th...
Article
The structures and dynamics of molecular, atomic, and ionized gases are studied around a low-luminosity active galactic nucleus (AGN) with a small ($2\times 10^6 M_\odot$) black hole using 3D radiation hydrodynamic simulations. We studied, for the first time, the non-equilibrium chemistry for the X-ray dominated region in the "radiation-driven foun...
Article
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Using two-dimensional radiation hydrodynamic simulations, we investigate the origin of the ultra-fast outflows (UFOs) that are often observed in luminous active galactic nuclei (AGNs). We found that the radiation force due to the spectral lines generates strong winds (line-driven disk winds) that are launched from the inner region of accretion disk...
Article
With high resolution (0"25 x 0"18) ALMA CO 3-2 observations of the nearby (D=21 Mpc), extremely radio quiet galaxy NGC1377, we have discovered a high velocity, very collimated molecular jet with a projected length of $\pm$160 pc. Along the jet axis we find strong velocity reversals swinging from -180 to +180 km/s. A simple model of a precessing mol...
Article
Active galactic nuclei (AGNs) are believed to be obscured by an optical thick "torus" that covers a large fraction of solid angles for the nuclei. However, the physical origin of the tori and the differences in the tori among AGNs are not clear. In a previous paper based on three-dimensional radiation-hydorodynamic calculations, we proposed a physi...
Article
Full-text available
The central engines of Seyfert galaxies are thought to be enshrouded by geometrically thick gas and dust structures. In this article, we derive observable properties for a self-consistent model of such toroidal gas and dust distributions, where the geometrical thickness is achieved and maintained with the help of X-ray heating and radiation pressur...
Article
Although the "donut-like" obscuring molecular torus is often postulated to explain the type-1 and -2 dichotomy in AGNs, its physical origin is still unclear. We propose a plausible mechanism to explain the formation of the geometrically and optically thick torus, i.e. radiation-driven fountain. Using 3-D hydrodynamic simulations including radiative...
Article
Full-text available
The disk wind, which is powered by the radiation force due to spectral lines (line force), has been studied concerning broad absorption line (BAL) quasars. We investigated the structure of the disk wind based on a non-hydrodynamic method, and compared it with wind properties inferred from X-ray observations of BAL quasars. In the present work, we a...
Article
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In order to understand the physical mechanisms underlying non-steady stellar spiral arms in disk galaxies, we analyzed the growing and damping phases of their spiral arms using three-dimensional $N$-body simulations. We confirmed that the spiral arms are formed due to a swing amplification mechanism that reinforces density enhancement as a seeded w...
Article
We propose a plausible mechanism to explain the formation of the so-called obscuring tori around active galactic nuclei (AGNs) based on three-dimensional hydrodynamic simulations including radiative feedback from the central source. The X-ray heating and radiation pressure on the gas are explicitly calculated using a ray-tracing method. This radiat...
Article
We investigate the conditions for which the line-driven disk wind model reproduces broad absorption line (BAL) features. We research the structure of the disk wind using non-hydrodynamic calculations for a wide range of the parameters. We compare the resulting wind structure with the wind properties of X-ray observations of BAL quasars. We find tha...
Article
Full-text available
It has been believed that spiral arms in pure stellar disks decay in several galactic rotations due to the heating by the spiral arms. However, it might be caused by a numerical heating. We performed a three-dimensional N-body simulations with a sufficiently large number of particles and found that stellar disks can maintain spiral arms for more th...
Article
Astrometry using H2O maser sources in star forming regions is expected to be a powerful tool to study the structures and dynamics of our Galaxy. Honma et al. (2007) (hereafter H2007) claimed that the annual parallax of S269 is determined within an error of 0.008 milliarcsec (mas), concluding that S269 is located at 5.3 kpc +- 0.2 kpc from the sun,...
Article
Full-text available
Ultraluminous infrared galaxies (ULIRGs) with multiple (≥3) nuclei are frequently observed. It has been suggested that these nuclei are produced by multiple major mergers of galaxies. The expected rate of such mergers is, however, too low to reproduce the observed number of ULIRGs with multiple nuclei. We have performed high-resolution simulations...
Article
Full-text available
We propose a new dynamical picture of galactic stellar and gas spirals, based on hydrodynamic simulations in a "live" stellar disk. We focus especially on spiral structures excited in an isolated galactic disk without a stellar bar. Using high-resolution, three-dimensional N-body/smoothed particle hydrodynamic simulations, we found that the spiral...
Article
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We present hydrodynamic simulations of gas clouds in the central kpc region of the Milky Way that is modeled with a three-dimensional bar potential. Our simulations consider realistic gas cooling and heating, star formation, and supernova feedback. A ring of dense gas clouds forms as a result of X 1-X 2 orbit transfer, and our potential model resul...
Article
Full-text available
Gas materials in the inner Galactic disk continuously migrate toward the Galactic center (GC) due to interactions with the bar potential, magnetic fields, stars, and other gaseous materials. Those in forms of molecules appear to accumulate around 200 pc from the center (the central molecular zone, CMZ) to form stars there and further inside. The ba...
Article
Gas materials in the inner Galactic disk continuously migrate toward the Galactic center (GC) due to interactions with the bar potential, magnetic fields, stars, and other gaseous materials. Those in forms of molecules appear to accumulate around 200 pc from the center (the central molecular zone, CMZ) to form stars there and further inside. The ba...
Article
A 3D hydrodynamical model of an AGN torus (Wada, Papadopoulos & Spaans, 2009), which follows the formation of H2 out of the neutral hydrogen phase according to the evolving ambient ISM conditions (at a resolution of 0.25 pc), is used to estimate the abundance of different atomic and molecular species from a chemical model driven by X-rays. The abun...
Article
Full-text available
We propose a new dynamical picture of galactic stellar and gas spirals, based on hydrodynamic simulations in a `live' stellar disk. We focus especially on spiral structures excited in a isolated galactic disk without a stellar bar. Using high-resolution, 3-dimensional N-body/SPH simulations, we found that the spiral features of the gas in galactic...
Article
Several attempts have been made to model the mass distribution and dynamical evolution of the circumnuclear gas in active galactic nuclei (AGNs). However, chemical evolution is not included in detail in three-dimensional (3D) hydrodynamic simulations. The X-ray radiation from the AGN can drive the gas chemistry and affect the thermodynamics, as wel...
Article
Full-text available
Many efforts have been made to model the mass distribution and dynamical evolution of the circumnuclear gas in active galactic nuclei (AGNs). However, chemical evolution is not included in detail in three-dimensional (3-D) hydrodynamic simulations. The X-ray radiation from the AGN can drive the gas chemistry and affect the thermodynamics, as well a...
Article
Full-text available
We studied the formation process of star clusters using high-resolution N-body/smoothed particle hydrodynamcs simulations of colliding galaxies. The total number of particles is 1.2x10^8 for our high resolution run. The gravitational softening is 5 pc and we allow gas to cool down to \sim 10 K. During the first encounter of the collision, a giant f...
Article
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We modeled the gas dynamics of barred galaxies using a three-dimensional, high-resolution, $N$-body $+$ hydrodynamical simulation, and applied it to the Milky Way in an attempt to reproduce both the large-scale structure and the clumpy morphology observed in galactic HI and CO $l$–$v$ diagrams. Owing to including of the multi-phase interstellar med...
Article
In order to clarify the physics of AGN feedback and feeding, we need to understand the interstellar medium (ISM) in the central several tens of parsecs in galaxies where our observational and theoretical knowledge is relatively poor. Here we discuss feedback processes due to strong far UV and X-ray radiation, as well as the dynamical effect of a su...
Article
Full-text available
It has been believed that spirals in pure stellar disks, especially the ones spontaneously formed, decay in several galactic rotations due to the increase of stellar velocity dispersions. Therefore, some cooling mechanism, for example dissipational effects of the interstellar medium, was assumed to be necessary to keep the spiral arms. Here we show...
Article
We performed high resolution simulations of galaxy-galaxy merging in order to investigate the mechanism of a starburst. The multiphase nature of the interstellar medium is correctly taken into account in our model, which allows the use of a realistic star formation model. In these simulations, we for the first time found that the shock-induced star...
Article
Full-text available
Spiral structures in the disk galaxies have been extensively studied by many theoretical papers, but conventional steady-state models are not consistent with what we observe in time-dependent, multi-dimensional numerical simulations and also in real galaxies. Here we review recent progress in numerical modeling of stellar and gas spirals in disk ga...
Article
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Based on the physical model of a supermassive black hole (SMBH) growth via gas accretion in a circumnuclear disk (CND) proposed by Kawakatu & Wada (2008), we describe the formation of high-$z$ ($z > 6$) quasars (QSOs) whose BH masses are M_{BH}> 10^{9} M_{\odot}. We derive the necessary conditions to form QSOs at z > 6 by only gas accretion: (i) A...
Article
We propose a new evolutionary model of a supermassive black hole (SMBH) and a circumnuclear disk (CND), taking into account the mass-supply from a host galaxy and the physical states of CND. In the model, two distinct accretion modes depending on gravitational stability of the CND play a key role on accreting gas to a SMBH. (i) If the CND is gravit...
Article
We present new high-resolution numerical simulations of the interstellar medium (ISM) in a central R ≤ 32 parsecs region around a supermassive black hole (1.3 × 107M ☉) at a galactic center. Three-dimensional hydrodynamic modeling of the ISM (Wada & Norman 2002) with the nuclear starburst now includes tracking of the formation of molecular hydrogen...
Article
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We present new high resolution numerical simulations of the ISM in a central R ~32 parsecs region around a supermassive black hole at a galactic center. Three-dimensional hydrodynamic modeling of the ISM (Wada & Norman 2002) with the nuclear starburst now includes tracking of the formation of molecular hydrogen out of the neutral hydrogen phase. In...
Article
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Recent VLBI (Very Long Baseline Interferometer) observations determined the distances and proper motions of star-forming regions in spiral arms directly. They showed that star-forming regions and young stars have large peculiar motions, as large as 30 km/s with complex structures. Such a large peculiar motion is incompatible with the prediction of...
Article
Hydrodynamic simulations have been compared with high-resolution CO (J = 1-0) observations of the central region (R < 1.5 kpc) of NGC 4321. In the simulations, the evolution of a self-gravitating gas disk in gravitational potentials that suitably match the nuclear bar of NGC 4321 has been followed. Postulating that the nuclear bar of R ~ 16'' gover...
Article
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Recent observations suggest molecular line ratios in millimeter and submillimeter bands may be a good tool to reveal the long-standing question on the origin of energy sources in obscured active galaxies -- AGN and/or starburst. Observations of actual molecular medium show in general inhomogeneous structures as well as high-resolution hydrodynamic...
Article
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A combination of Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) and Near-Infrared Camera Multiobject Spectrograph (NICMOS) images are used to investigate the gas/dust and stellar structure inside the central 300 pc of the nearby active galaxy NGC 4303. The NICMOS H-band (F160W) image reveals a bright core and a nuclear-elongated barli...
Article
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"Galactic shocks" (Fujimoto; Roberts) are investigated using full three-dimensional hydrodynamic simulations that take into account the self-gravity of the ISM, radiative cooling, and star formation followed by energy feedback from supernovae. This is an essential improvement over previous numerical models, in which two-dimensional isothermal, non-...
Article
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We discuss the origin of the Tully-Fisher (TF) relation using the N-body/smoothed particle hydrodynamics method, which includes cooling, star formation, and stellar feedback of energy, mass, and metals. We consider initially rotating overdense spheres and trace formation processes of disk galaxies from z = 25 to z = 0 in the cold dark matter cosmol...
Article
Full-text available
We have performed three-dimensional, non-LTE (non-local thermodynamic equilibrium) radiative transfer calculations for 12CO and 13CO lines, applying them to our high-resolution hydrodynamic models of the torus around a supermassive black hole in an active galactic nucleus. The hydrodynamic simulations reveal inhomogeneous and turbulent gas structur...