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We investigated the evolution of interacting disk galaxies using high-resolution $N$-body/SPH simulations, taking into account the multiphase nature of the interstellar medium (ISM). In our high-resolution simulations, a large-scale starburst occurred naturally at the collision interface between two gas disks at the first encounter, resulting in the formation of star clusters. This is consistent with observations of interacting galaxies. The probability distribution function (PDF) of gas density showed clear change during the galaxy-galaxy encounter. The compression of gas at the collision interface between the gas disks first appears as an excess at $n_{\rm H} \sim 10{\rm cm^{-3}}$ in the PDF, and then the excess moves to higher densities ($n_{\rm H} \gtrsim 100{\rm cm^{-3}}$) in a few times $10^7$ years where starburst takes place. After the starburst, the PDF goes back to the quasi-steady state. These results give a simple picture of starburst phenomena in galaxy-galaxy encounters., Comment: 6 pages, 6 figures, accepted to PASJ. For high resolution figures, see http://www.cfca.nao.ac.jp/~saitoh/Papers/2009/Saitoh+2009a.pdf

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 CMD is gravitationally unstable, energy feedback from supernovae (SNe) supports a geometrically thick, turbulent gas disk. The accretion in this mode is dominated by turbulent viscosity, and it is significantly larger than that in the mode (ii), i.e., the CMD is supported by gas pressure. Once the gas supply from the host is stopped, the high accretion phase ($\sim 0.01- 0.1 M_{\odot} {\rm yr}^{-1}$) changes to the low one (mode (ii), $\sim 10^{-4} M_{\odot} {\rm yr}^{-1}$), but there is a delay with $\sim 10^{8}$ yr. Through this evolution, the gas-rich CND turns into the gas poor stellar disk. We found that not all the gas supplied from the host galaxy accrete onto the SMBH even in the high accretion phase (mode (i)), because the part of gas is used to form stars. As a result, the final SMBH mass ($M_{\rm BH,final}$) is not proportional to the total gas mass supplied from the host galaxy ($M_{\rm sup}$); $M_{\rm BH,final}/M_{\rm sup}$ decreases with $M_{\rm sup}$.This would indicate that it is difficult to form a SMBH with $\sim 10^{9} M_{\odot}$ observed at high-$z$ QSOs. The evolution of the SMBH and CND would be related to the evolutionary tracks of different type of AGNs., Comment: 11 pages, 11 figures, accepted for publication in ApJ

We performed 3-dimensional N-body/SPH simulations to study how mass resolution and other model parameters such as the star formation efficiency parameter, C* and the threshold density, nth affect structures of the galactic gaseous/stellar disk in a static galactic potential. We employ 10^6 - 10^7 particles to resolve a cold and dense (T < 100 K & n_H > 100 cm^{-3}) phase. We found that structures of the ISM and the distribution of young stars are sensitive to the assumed nth. High-nth models with nth = 100 cm^{-3} yield clumpy multi-phase features in the ISM. Young stars are distributed in a thin disk of which half-mass scale height is 10 - 30 pc. In low-nth models with nth = 0.1 cm^{-3}, the stellar disk is found to be several times thicker, and the gas disk appears smoother than the high-nth models. A high-resolution simulation with high-nth is necessary to reproduce the complex structure of the gas disk. The global properties of the model galaxies in low-nth models, such as star formation histories, are similar to those in the high-nth models when we tune the value of C* so that they reproduce the observed relation between surface gas density and surface star formation rate density. We however emphasize that high-nth models automatically reproduce the relation, regardless of the values of C*. The ISM structure, phase distribution, and distributions of young star forming region are quite similar between two runs with values of C* which differ by a factor of 15. We also found that the timescale of the flow from n_H ~1 cm^{-3} to n_H > 100 cm^{-3} is about 5 times as long as the local dynamical time and is independent of the value of C*. The use of a high-nth criterion for star formation in high-resolution simulations makes numerical models fairy insensitive to the modelling of star formation. (Abridged), Comment: 15 pages, 14 figures, accepted for publication in PASJ. Abridged abstract. For high resolution figures, see http://www.cfca.nao.ac.jp/~saitoh/Papers/2008/Saitoh+2008a.pdf

The ``galactic shocks'' \citep{fujimoto68,roberts69} is investigated using a full three-dimensional hydrodynamic simulations, taking into account self-gravity of the ISM, radiative cooling, and star formation followed by energy feedback from supernovae. This is an essential progress from the previous numerical models, in which 2-D isothermal, non-self-gravitating gas is assumed. We find that the classic galactic shocks appears is unstable and transient, and it shifts to a globally quasi-steady, inhomogeneous pattern due to non-linear development of instabilities in the disk. The spiral patterns consists of many GMC-like dense condensations, but those local structures are not steady, and they evolves into irregular spurs in the inter-arm regions. Energy feedback from supernovae do not destroy the quasi-steady spiral arms, but it mainly contributes to vertical motion and structures of the ISM., Comment: 8 pages, 6 figures, accepted by ApJ. Some minor changes are maid

We explore millimeter line diagnostics of an obscuring molecular torus modeled by a hydrodynamic simulation with three-dimensional nonLTE radiative transfer calculations. Based on the results of high-resolution hydrodynamic simulation of the molecular torus around an AGN, we calculate intensities of HCN and HCO^{+} rotational lines as two representative high density tracers. The three-dimensional radiative transfer calculations shed light on a complicated excitation state in the inhomogeneous torus, even though a spatially uniform chemical structure is assumed. Our results suggest that HCN must be much more abundant than HCO^{+} in order to obtain a high ratio ($R_{HCN/HCO+}\sim 2$) observed in some of the nearby galaxies. There is a remarkable dispersion in the relation between integrated intensity and column density, indicative of possible shortcomings of HCN(1-0) and HCO^{+}(1-0) lines as high density tracers. The internal structures of the inhomogeneous molecular torus down to subparsec scale in external galaxies will be revealed by the forthcoming Atacama Large Millimeter/submillimeter Array (ALMA). The three-dimensional radiative transfer calculations of molecular lines with high-resolution hydrodynamic simulation prove to be a powerful tool to provide a physical basis for molecular line diagnostics of the central regions of external galaxies., Comment: 29 pages, 13 figures, Accepted for publication in ApJ, For high resolution figures see http://alma.mtk.nao.ac.jp/~masako/MS72533v2.pdf

Possible origins of the molecular absorption discovered in some ULIRGs are investigated, based on a 3-D hydrodynamic model of star-forming interstellar gas in a galactic central region. The blue-shifted, warm ($\sim 200-300$ K), dense ($>10^6$ cm$^{-3}$) molecular gas suggested by CO absorption in IRAS 08752+3915 could be caused by the innermost region of the inhomogeneous inter-stellar medium (ISM) around a supermassive black hole. The infrequent observations of the dense gas with absorption in ULIRGs and Seyfert 2 galaxies could simply suggest that the high-density regions occupy only a very small volume fraction of the obscuring material. This is naturally expected if the inhomogeneous structure of the ISM is caused by non-linear development of instabilities. The model predicts a turbulent velocity field in the obscuring material, therefore blue- and red-shifted gases should be observable with nearly the same probability for the large enough statistical samples., Comment: 14 pages, 6 figures. Publications of Astronomical Society of Japan in press

The probability distribution functions (PDF) of density of the ISM in galactic disks and global star formation rate are discussed. Three-dimensional hydrodynamic simulations show that the PDFs in globally stable, inhomogeneous ISM in galactic disks are well fitted by a single log-normal function over a wide density range. The dispersion of the log-normal PDF (LN-PDF) is larger for more gas-rich systems, and whereas the characteristic density of LN-PDF, for which the volume fraction becomes the maximum, does not significantly depend on the initial conditions. %At the characteristic density, thermal pressure and kinetic %pressure due to turbulent motion are comparable, and the material is %statistically stagnated in a turbulent flow. Supposing the galactic ISM is characterized by the LN-PDF, we give a global star formation rate (SFR) as a function of average gas density, a critical local density for star formation, and star formation efficiency. We find that the observed SFR is well-fitted by the theoretical SFR in a wide range of the global gas density ($10 - 10^4 M_\odot$ pc$^{-2}$). Star formation efficiency (SFE) for high density gas ($n > 10^3$ cm$^{-3}$) is SFE $= 0.001 - 0.01$ for normal spiral galaxies, and SFE $= 0.01 - 0.1$ for starburst galaxies. The LN-PDF and SFR proposed here could be applicable for modeling star formation on a kpc-scale in galaxies or numerical simulations of galaxy formation, in which the numerical resolution is not fine enough to describe the local star formation., Comment: 20 pages with 19 figures, ApJ accepted

In order to examine the ``giant impact hypothesis'' for the Moon formation, we run the first grid-based, high-resolution hydrodynamic simulations for an impact between proto-Earth and a proto-planet. The spatial resolution for the impact-generated disk is greatly improved from previous particle-based simulations. This allows us to explore fine structures of a circumterrestrial debris disk and its long-term evolution. We find that in order to form a debris disk from which a lunar-sized satellite can be accumulated, the impact must result in a disk of mostly liquid or solid debris, where pressure is not effective, well before the accumulation process starts. If the debris is dominated by vapor gas, strong spiral shocks are generated, and therefore the circumterrestrial disk cannot survive more than several days. This suggests that there could be an appropriate mass range for terrestrial planets to harbor a large moon as a result of giant impacts, since vaporization during an impact depends on the impact energy., Comment: ApJ 638, Feb. 20 in press

Tidal disruption of dark matter halos around proto-globular clusters in a halo of a small galaxy is studied in the context of the hierarchical clustering scenario by using semi-cosmological N-body/SPH simulations assuming the standard cold dark matter model ($\Omega_0 = 1$). Our analysis on formation and evolution of the galaxy and its substructures archives until $z = 2.0$. In such a high-redshift universe, the Einstein-de Sitter universe is still a good approximation for a recently favored $\Lambda$-dominated universe, and then our results does not depend on the choice of cosmology. In order to resolve small gravitationally-bound clumps around galaxies and consider radiative cooling below $T = 10^4 K$, we adopt a fine mass resolution ($m_{\rm SPH} = 1.12 \times 10^3 \Msun$). Because of the cooling, each clump immediately forms a `core-halo' structure which consists of a baryonic core and a dark matter halo. The tidal force from the host galaxy mainly strips the dark matter halo from clumps and, as a result, theses clumps get dominated by baryons. Once a clump is captured by the host halo, its mass drastically decreases each pericenter passage. At $z = 2$, more than half of the clumps become baryon dominated systems (baryon mass/total mass $> 0.5$). Our results support the tidal evolution scenario of the formation of globular clusters and baryon dominated dwarf galaxies in the context of the cold dark matter universe., Comment: 9page, 13 figures. Accepted for publication in ApJ. A high-resolution PDF of the paper can be obtained from http://th.nao.ac.jp/~takayuki/ApJ05/

Combining a theoretical model of mass accretion onto a galactic center with a high-resolution $N$-body/SPH simulation, we investigate the formation of an intermediate massive black hole (IMBH) during the hierarchical formation of a small spiral galaxy (with a total mass of $10^{10}M_{\odot}$) in the high-$z$ universe. We found that the rate of average mass accretion to the nucleus due to the radiation drag exerted by newly formed stars in the forming galaxy is $\approx 10^{-5}M_{\odot}$yr$^{-1}$. As a result of this accretion, an IMBH with $\approx 10^{4}M_{\odot}$ can be formed in the center of the spiral galaxy at $z\sim 4$. We found that a central BH coevolves with the dark matter halo from $z\sim 15$ to $z\sim 2$. The mass ratio of the BH to the dark matter halo is nearly constant $\approx (1-3) \times 10^{-6}$ from $z\sim 10$ to $z\sim 2$. This is because that change in the dark matter potential enhances star formation in the central part of the galaxy, and as a result the BH evolves due to mass accretion via the radiation drag. Therefore, our model naturally predicts a correlation between massive BHs and dark matter halos. Moreover, it is found that the final BH-to-bulge mass ratio ($\approx 5\times 10^{-5}$) in a small spiral galaxy at high-$z$ is much smaller than that in the large galaxies ($\approx 10^{-3}$). Our results also suggest that the scatter in the observed scaling relations between the bulge mass and black hole mass are caused by a time lag between BH growth and growth of bulge. We also predict that the X-ray luminosity of AGN is positively correlated with the CO luminosity in the central region. By comparing our results with the properties of Lyman break galaxies (LBGs), it is predicted that some LBGs have massive BHs of $\approx 10^{6}-10^{7}M_{\odot}$., Comment: 8pages, 7figures, accepted for publication in ApJ (Volume 628, 20 July 2005 issue)

This is the first attempt to construct detailed X-ray spectra of clusters of galaxies from the results of high-resolution hydrodynamic simulations and simulate X-ray observations in order to study velocity fields of the intracluster medium (ICM). The hydrodynamic simulations are based on the recently proposed tsunami model, in which cluster cores are affected by bulk motions of the ICM and turbulence is produced. We note that most other solutions of the cooling flow problem also involve the generation of turbulence in cluster cores. From the mock X-ray observations with Astro-E2 XRS, we find that turbulent motion of the ICM in cluster cores could be detected with the satellite. The Doppler shifts of the metal lines could be used to discriminate among turbulence models. The gas velocities measured through the mock observations are consistent with the line-emission weighted values inferred directly from hydrodynamic simulations., Comment: 13 pages, 2 figures, ApJL in press. Movies are available at http://th.nao.ac.jp/tsunami/index.htm

We have performed three-dimensional, non-LTE radiative transfer calculations for 12CO and 13CO lines, applying them to our high-resolution hydrodynamic models of the `torus' around a SMBH in an active galactic nucleus. The hydrodynamic simulations reveal inhomogeneous and turbulent gas structure on a sub-pc scale in a circum-nuclear starburst region. Thick disks interlaced with filaments, clumps and holes are naturally formed due to the interplay among energy feedback from supernovae, self-gravity of the gas, galactic rotation, and radiative cooling. The intensity maps of the molecular lines for the circum-nuclear disks show a clumpy structure reflecting the intrinsic inhomogeneity and turbulent motion of the gas disk. The fine structure of the `torus' could be resolved in the nearby active galaxies using ALMA. We also found that the X-factor is not uniformly distributed in the central 100 pc region. The X-factor derived for 12CO(J=1-0) intensity depends strongly on the intensity, whereas the X for 12CO(3-2) is nearly constant over two orders of magnitude of the intensity. The suggested conversion factor for the molecular gas mass is X_CO (3-2) ~ 0.27 x 10^{20} cm^{-2} (K km s^{-1})^{-1}., Comment: 11 pages, 10 figures. ApJ 20 Jan., 2005, in press

Using high-resolution N-body/SPH simulations with $2\times 10^6$ particles, we investigate the evolution of stellar and gaseous galactic cores during the hierarchical formation of a spiral galaxy. We find that the galactic core ($r < 300 $ pc) coevolves with the host galaxy. The average mass ratio between the baryonic core and the halo is nearly constant, $\sim $ 0.04 from $z \sim 10$ to $z \sim 2$. However, there are several `rapid-growing phases' during the evolution, in which the rate of mass accretion to the central sub-kpc region is ten times higher ($\sim 1 M_\odot$ yr pc$^{-1}$) than the average accretion rate. The rapid growth of the inner core is associated with the major merger events with a time-delay. We also find that the spin-axis of the gas core frequently changes. As a result, the angular momentum vector of the central part of the galaxy is independent of the rotation of the outer part. Our results suggest that if a constant fraction of the baryonic mass in the central several 100 pc of a galaxy is converted into a massive black hole, the black hole mass should correlate with the total mass of the galaxies., Comment: 4 pages, 4 figures, Accepted for publication in ApJL

We report on infrared K- (2-2.5 micron) and L-band (2.8-4.1 micron) slit spectroscopy of 23 Seyfert 1 galaxies in the CfA and 12 micron samples. A polycyclic aromatic hydrocarbon (PAH) emission feature at 3.3 micron in the L band is primarily used to investigate nuclear star-forming activity in these galaxies. The 3.3 micron PAH emission is detected in 10 sources (=43%), demonstrating that detection of nuclear star-formation in a significant fraction of Seyfert 1 galaxies is now feasible. For the PAH-detected nuclei, the surface brightness values of the PAH emission are as high as those of typical starbursts, suggesting that the PAH emission probes the putative nuclear starbursts in the dusty tori around the central active galactic nuclei (AGNs). The magnitudes of the nuclear starbursts are quantitatively estimated from the observed 3.3 micron PAH emission luminosities. The estimated starburst luminosities relative to some indicators of AGN powers in these Seyfert 1s are compared with 32 Seyfert 2s in the same samples that we have previously observed. We find that there is no significant difference in nuclear starburst to AGN luminosity ratios between Seyfert 1 and 2 galaxies, and that nuclear starburst luminosity positively correlates with AGN power in both types of Seyferts. Our results favor a slightly-modified AGN unification model, which predicts that nuclear starbursts occurring in the dusty tori of Seyferts are physically connected to the central AGNs, rather than the classical unification paradigm, in which the dusty tori simply hide the central AGNs of Seyfert 2s and reprocess AGN radiation as infrared dust emission in Seyferts. No significant differences in nuclear star formation properties are recognizable between Seyfert 1s in the CfA and 12 micron samples., Comment: 39 pages, 6 figures, Accepted for publication in ApJ (10 December 2004, vol 617 issue)

Based on high-resolution two-dimensional hydrodynamic simulations, we show that the bulk gas motions in a cluster of galaxies, which are naturally expected during the process of hierarchical structure formation of the universe, have a serous impact on the core. We found that the bulk gas motions represented by acoustic-gravity waves create local but strong turbulence, which reproduces the complicated X-ray structures recently observed in cluster cores. Moreover, if the wave amplitude is large enough, they can suppress the radiative cooling of the cores. Contrary to the previous studies, the heating is operated by the turbulence, not weak shocks. The turbulence could be detected in near-future space X-ray missions such as ASTRO-E2., Comment: Movies are available at http://th.nao.ac.jp/tsunami/index.htm

Using an analytical model and numerical simulations, we show that acoustic waves generated by turbulent motion in intracluster medium effectively heat the central region of a so-called ``cooling flow'' cluster. We assume that the turbulence is generated by substructure motion in a cluster or cluster mergers. Our analytical model can reproduce observed density and temperature profiles of a few clusters. We also show that waves can transfer more energy from the outer region of a cluster than thermal conduction alone. Numerical simulations generally support the results of the analytical study., Comment: 18 pages, accepted for publication in ApJ

We found that loosely wound spiral shocks in an isothermal gas disk caused by a non-axisymmetric potential are hydrodynamically unstable, if the shocks are strong enough. High resolution, global hydrodynamical simulations using three different numerical schemes, i.e. AUSM, CIP, and SPH, show similarly that trailing spiral shocks with the pitch angle of larger than ~10 deg wiggle, and clumps are developed in the shock-compressed layer. The numerical simulations also show clear wave crests that are associated with ripples of the spiral shocks. The spiral shocks tend to be more unstable in a rigidly rotating disk than in a flat rotation. This instability could be an origin of the secondary structures of spiral arms, i.e. the spurs/fins, observed in spiral galaxies. In spite of this local instability, the global spiral morphology of the gas is maintained over many rotational periods. The Kelvin-Helmholtz (K-H) instability in a shear layer behind the shock is a possible mechanism for the wiggle instability. The Richardson criterion for the K-H stability is expressed as a function of the Mach number, the pitch angle, and strength of the background spiral potential. The criterion suggests that spiral shocks with smaller pitch angles and smaller Mach numbers would be more stable, and this is consistent with the numerical results., Comment: 11 pages, 14 figures, to be published in MNRAS, high quality figures can be downloaded from http://th.nao.ac.jp/~wada/paperlist.html

Supplying gas to the galactic central regions is one of key ingredients for AGN activity. I will review various fueling mechanisms for a R ~ 1 kpc region, determined mainly by numerical simulations over the last decade. I will also comment on the bars-within-bars mechanism. Observations suggest that the stellar bar is not a sufficient condition for gas fueling. Moreover, considering the various factors for the onset of gas accretion, stellar bars would not even be a necessary condition. I introduce recent progress obtained through our two- and three-dimensional, high resolution hydrodynamical simulations of the ISM in the central 0.1--1 kpc region of galaxies. Possible structure of the obscuring molecular tori around AGNs is also shown. The nuclear starburst is an important factor in determining the structure of the molecular tori and the mass accretion rate to the nucleus. It is natural that the ISM in the central 100 pc region is a highly inhomogeneous and turbulent structure. As a result, gas accretion to the central parsec region should be time dependent and stochastic. The conventional picture of gas fueling and the AGN unified model may be modified in many respects., Comment: 16 pages, 7 figures, Carnegie Observatories Astrophysics Series, Vol. 1: Coevolution of Black Holes and Galaxies, ed. L. C. Ho (Cambridge: Cambridge Univ. Press), high resolution version is available at http://www.ociw.edu/ociw/symposia/series/symposium1/proceedings.html

Rotation curves are often used to estimate the mass distribution of spiral galaxies, assuming circular rotation of disks. However, non-circular motions caused by a non-axisymmetric gravitational potential, such as a stellar bar, may disturb the velocity field, resulting in errors in mass estimation, especially in the central regions of galaxies, because the line-of-sight velocity depends on the viewing angles in a non-axisymmetric flow. Observing rotation curves of edge-on galaxies in time-dependent numerical simulations from different viewing angles, we obtain errors in the estimation of galactic mass from the rotation curves. In the most extreme case, the ellipticity of gas orbits is as high as 0.8 in the central regions, even if the bar potential is weak. When rotation curves are defined as the highest velocity envelope of position-velocity diagrams, the mass estimated from the rotation curves is larger than the true mass by a factor of five for 15% of the viewing angles, and the ratio between the apparent mass and true mass is less than six for any viewing angle. The overestimation in mass occurs more frequently than the underestimation., Comment: 11 pages, 10 figures, accepted by A&A

High-resolution, 2-D hydrodynamical simulations with a large dynamic range are performed to study the turbulent nature of the interstellar medium (ISM) in galactic disks. The simulations are global, where the self-gravity of the ISM, realistic radiative cooling, and galactic rotation are taken into account. In the analysis undertaken here, feedback processes from stellar energy source are omitted. We find that the velocity field of the disk in a non-linear phase shows a steady power-law energy spectrum over three-orders of magnitude in wave number. This implies that the random velocity field can be modeled as fully-developed, stationary turbulence. Gravitational and thermal instabilities under the influence of galactic rotation contribute to form the turbulent velocity field. The Toomre effective Q value, in the non-linear phase, ranges over a wide range, and gravitationally stable and unstable regions are distributed patchily in the disk. These results suggest that large-scale galactic rotation coupled with the self-gravity of the gas can be the ultimate energy sources that maintain the turbulence in the local ISM. We find that our models of turbulent rotating disks are consistent with the velocity dispersion of an extended HI disk in the dwarf galaxy, NGC 2915, where there is no prominent active star formation. Numerical simulations show that the stellar bar in NGC 2915 enhances the velocity dispersion, and it also drives spiral arms as observed in the HI disk., Comment: 22 pages, 12 figures; to apper in ApJ 577, Sep.20; high resolution figures are available at http://th.nao.ac.jp/~wada/paperlist.html

The structure of obscuring matter in the environment of active galactic nuclei with associated nuclear starbursts is investigated using 3-D hydrodynamical simulations. Simple analytical estimates suggest that the obscuring matter with energy feedback from supernovae has a torus-like structure with a radius of several tens of parsecs and a scale height of about 10 pc. These estimates are confirmed by the fully non-linear numerical simulations, in which the multi-phase inhomogeneous interstellar matter and its interaction with the supernovae are consistently followed. The globally stable, torus-like structure is highly inhomogeneous and turbulent. To achieve the high column densities (> 10^{24} cm^{-2}) as suggested by observations of some Seyfert 2 galaxies with nuclear starbursts, the viewing angle should be larger than about 70 degree from the pole-on for a 10^8 solar mass massive black hole. Due to the inhomogeneous internal structure of the torus, the observed column density is sensitive to the line-of-sight, and it fluctuates by a factor of order 100. The covering fraction for N > 10^{23} cm^{-2} is about 0.4. The average accretion rate toward R < 1 pc is 0.4 solar mass/yr, which is boosted to twice that in the model without the energy feedback., Comment: ApJL in press (4 pages, 3 figures) A gziped ps file with high resolution figures is available at http://th.nao.ac.jp/~wada/AGN/

Using high-resolution, three-dimensional hydrodynamical simulations, we investigate the structure of the interstellar medium in the central hundred pc region in galaxies, taking into account self-gravity of the gas, radiative cooling from 10 K to $10^8$ K, and energy feedback from supernovae. Similar to the previous two-dimensional results produced by Wada and Norman, we find that a gravitationally and thermally unstable ISM evolves, in a self-stabilizing manner, into a quasi-stable thin disk, which is characterized by a network of cold ($T < 100$ K), dense clumps and filaments, and hot ($T > 10^6$ K), diffuse medium. Supernova explosions blow the diffuse gases from the disk, and as a result, a quasi-steady diffuse halo, which is not uniform but has a plume-like structure, is formed. The density probability distribution function (PDF) in a quasi-steady state is well fitted by a Log-Normal function over about seven orders of magnitude., Comment: to be published in ApJL, 559 (Sep.20) high quality figures are available at http://th.nao.ac.jp/~wada/paperlist.html

The central kiloparsec of M81 has been observed in the CO(J=1-0) line and the 3 mm continuum at 100 pc resolution in an attempt to probe molecular gas, and to search for the nuclear inner Lindblad resonance (NILR), around the low-luminosity AGN M81*. We found the following. (1) Molecular gas in the central kpc is mainly on a ``pseudoring'' or a spiral arm at a radius of about 500 pc. (2) The region within ~300 pc from the nucleus is mostly devoid of molecular gas except for diffuse one; in particular, there is neither a giant molecular cloud that is now accreting on the nucleus nor a conspicuous gas feature that can be identified as an NILR. (3) The 3 mm continuum emission shows significant intraday variation, suggesting an emitting region of ~100 AU. (4) The 3-sigma upper limit for CO absorption toward the continuum source is tau_CO(0->1) dV < 0.1 for a linewidth of 10 km/s. The dearth of accreting molecular gas in the vicinity of the nucleus may explain the low luminosity of M81*., Comment: 25 pages, 8 figures (2 of them bitmapped), to appear in AJ (Sept. 2001), for full ps file see http://th.nao.ac.jp/~wada/m81/

Based on our high resolution, two-dimensional hydrodynamical simulations, we propose that large cavities may be formed by the nonlinear development of the combined thermal and gravitational instabilities, without need for stellar energy injection in a galaxy modeling the Large Magellanic Clouds (LMC). Our numerical model of the star formation allows us to follow the evolution of the blastwaves due to supernovae in the inhomogeous, multi-phase, and turbulent-like media self-consistently. Formation of kpc-scale inhomogeneity, such as cavities, observed HI map of the LMC, is suppressed by frequent supernovae (average supernova rate for the whole disk is ~0.001/yr). However the supernova explosions are necessary for the hot component (T_g > 10^{6-7} K). Position-velocity maps show that kpc-scale shells/arcs formed through the nonlinear evolution in a model without stellar energy feedback has similar kinematics to explosional phenomena, such as supernovae. We also find that dense clumps and filamentary structure are formed due to a natural consequence of the non-linear evolution of the multi-phage ISM. Although the ISM in a small scale looks turbulent-like and transient, the global structure of the ISM is quasi-stable. We compare the observations of HI and molecular gas of the LMC with the numerically obtained HI and CO map. The morphology and statistical properties of the numerical HI and CO maps are discussed., Comment: 12 pages, 10 figures. high resolution figures are available at http://th.nao.ac.jp/~wada/paperlist.html ApJ, 540, in press

We investigate the origin of a unified scaling relation in spiral galaxies. Observed spiral galaxies are spread on a plane in the three-dimensionallogarithmic space of luminosity L, radius R and rotation velocity V. The plane is expressed as $L \propto (V R)^{\alpha}$ in I-passband, where $\alpha$ is a constant. On the plane, observed galaxies are distributed in an elongated region which looks like the shape of a surfboard. The well-known scaling relations, L-V (Tully-Fisher relation), V-R (also the Tully-Fisher relation) and R-L (Freeman's law), can be understood as oblique projections of the surfboard-like plane into 2-D spaces. This unified interpretation of the known scaling relations should be a clue to understand the physical origin of all the relations consistently. Furthermore, this interpretation can also explain why previous studies could not find any correlation between TF residuals and radius. In order to clarify the origin of this plane, we simulate formation and evolution of spiral galaxies with the N-body/SPH method, including cooling, star formation and stellar feedback. Initial conditions are set to isolated 14 spheres with two free parameters, such as mass and angular momentum. The CDM (h=0.5, $\Omega_0=1$) cosmology is considered as a test case. The simulations provide the following two conclusions: (a) The slope of the plane is well reproduced but the zero-point is not. This zero-point discrepancy could be solved in a low density ($\Omega_0<1$) and high expansion (h>0.5) cosmology. (b) The surfboard-shaped plane can be explained by the control of galactic mass and angular momentum., Comment: Accepted for publication in ApJ Letters. 6 pages including 2 figures

Using high resolution, two-dimensional hydrodynamical simulations, we investigate the evolution of a self-gravitating multi-phase interstellar medium in the central kiloparsec region of a galactic disk. We find that a gravitationally and thermally unstable disk evolves, in a self-stabilizing manner, into a globally quasi-stable disk that consists of cold (T < 100 K), dense clumps and filaments surrounded by hot (T > 10^4 K), diffuse medium. The quasi-stationary, filamentary structure of the cold gas is remarkable. The hot gas, characterized by low-density holes and voids, is produced by shock heating. The shocks derive their energy from differential rotation and gravitational perturbations due to the formation of cold dense clumps. In the quasi-stable phase where cold and dense clouds are formed, the effective stability parameter, Q, has a value in the range 2-5. The dynamic range of our multi-phase calculations is 10^6 - 10^7 in both density and temperature. Phase diagrams for this turbulent medium are analyzed and discussed., Comment: 10 pages, 3 figures, ApJ Letters in press (vol. 516)

In our previous paper, we have shown that a gas disk in the nuclear region of a barred galaxy which contains a central supermassive black hole (SMBH) rapidly evolves into a nuclear gas ring by the effect of an additional inner Lindblad resonance caused by the SMBH. In this paper, we investigate the fate of the gas ring, involving self-gravity of gas, using two-dimensional hydrodynamical simulations. We find that the gas ring becomes gravitationally unstable for a gas surface density of gas above a critical value, and fragments into several gas clumps. Some denser clumps increase their mass via the accretion of the surrounding gas and collisions with other clumps, and finally a very massive gas clump (10^7 M_sun) is formed. Due to the torque from the most massive clump, a part of the gas in the ring loses its angular momentum and falls into the galactic center. As a result, a nuclear gas disk (50 pc) is formed around the SMBH. The accretion rate for $R<50$ pc attains about 1 M_sun/yr for 3.5*10^7 yr. At the final phase of the bar-driven fueling, self-gravity is crucial for the angular momentum transfer of the gas. This is a new mechanism for gas fueling to the vicinity of the SMBH., Comment: 14 pages, 7 figures, AASTeX, submitted to ApJ

Using N-body and SPH modeling we perform 3D numerical simulations of head-on collisions between gas rich disk galaxies, including collisions between counter-rotating disks and off-center collisions. Pure stellar intruders do not produce gaseous plumes similar to those seen in the Cartwheel and VII Zw466 complexes of interacting galaxies; the presence of gas in an intruder galaxy and radiative cooling are important for the formation of a gaseous plume extending from the disk of a target galaxy. A noticeable plume structure can be formed if the mass of an intruder is a few percent of the mass of the primary. The halo of the intruder is stripped in the collision, and dispersed particles form a broad stellar bridge connecting the two galaxies. The fraction of the intruder's halo dispersed in the collision depends on the total mass of the intruder, and low-mass intruders lose most of their mass., Comment: 15 pages, 14 figures in GIF. To appear ApJ. Vol. 505 #2

When a supermassive black hole exists in the centre of a galaxy, an additional inner Lindblad resonance (ILR) exists inside the usual ILRs. We study gas dynamics in a weakly barred potential with a central supermassive black hole by using 2D numerical simulations, and we investigate the effect of the additional ILR on fuelling gas into nuclear starburst regions or AGNs. Our numerical results show that strong trailing spiral shocks are formed at the resonance region, and the gas in the shock region is rapidly fuelled into a central region and make a nuclear gas ring. As a result, a large amount of gas is concentrated in the nuclear region beyond the ILR in a dynamical time scale., Comment: 7 pages, 14 Postscript figures, LaTeX 2.09, requires mn.sty, to be published in MNRAS

We have found an effective fueling process to a central region of galaxies with weak bar-like distortion by two dimensional hydrodynamical simulations. Gravitational instability of an elongated gas ring at the inner Lindblad resonance (ILR), which has been reported as an effective fueling mechanism, are not needed for this fueling process. A massive gaseous disk in a central region of galaxies sensitively responds to the weakly distorted potential, and a large amount of gas can be fed into within $1/20$ of a core radius of the potential in several $10^7$ yr. The ILRs, the dissipative nature of the gas, and self-gravity of the gas are essential for triggering this effective fueling. The accumulation process has not been ever known: the gas accumulates to form a dense `linear' structure inclined at about 45 degree with respect to the bar potential in a {\it leading sense}. We also found that a counter rotating gaseous core can be formed as a result of the fueling. The sense of the rotation of the core depends on a fraction of the gas mass to the background mass. Physical mechanism of the fueling process can be understood using a linear theory of gaseous orbits in a weak barred potential., Comment: 8 pages, uuencoded gziped Postscript file without figures. Figures will be sent upon request (wada@hipecs.hokudai.ac.jp). Accepted by MNRAS

We have investigated the effects of primeval starbursts on the galactic structure, showing that the dynamical structure of young galaxies is changed by relaxation of a stellar system formed by a starburst-driven outflowing gas (superwind). When a superwind interacts with halo gas, an expanding dense supershell of shocked gas is formed. Using a similarity solution for the expanding shell and a condition for the gravitational instability of a gaseous shell, we show that an expanding gaseous shell with a mass of several $10^{10} M_\odot$ and a radius of several kpc becomes gravitationally unstable to form stars. A stellar shell is thus formed, relaxes and will evolve into a fat stellar system. \par In order to investigate the fate of the stellar shell and its dynamical influence on the host galaxy, we performed three dimensional $N$-body simulations of a stellar shell plus disk system. The evolution of the stellar shell was computed under two types of environment: an external flattened potential and a live stellar disk. We find that the relaxation process and resultant structure are significantly affected by the external disk potential, and also depend on the velocity dispersion of the initial shell stars and the mass of the shell. The final galactic structure ranges from a thick disk and a compact bulge with a high density core to a thin disk with a diffuse bulge. Isodensity maps of the computed disk plus bulge system resemble isophotes of S0 galaxies. We suggest that the effects of primeval starbursts would explain some observational properties of S0 galaxies., Comment: 10 pages, uuencoded compressed Postscript file, Hard copy figures will be sent upon request (wada@hipecs.hokudai.ac.jp)