Keiichi Wada

Circumnuclear Multi-phase Gas in the Circinus Galaxy. I. Non-LTE Calculations of CO Lines

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 which non-equilibrium X-ray-dominated region chemistry is solved, we conduct post-processed, non-local thermodynamic equilibrium radiation transfer simulations for the CO lines. We obtain a spectral line energy distribution with a peak around J=6, and its distribution suggests that the lines are not thermalized. However, for a given line-of-sight, the optical depth distribution is highly non-uniform between

How to Determine the Smoothing Length in Sph

tau ll 1

Position-Velocity Diagrams as a Probe of the Bar in Edge-On Galaxies

and

A New Fueling Process in a Weak Bar

tau gg 1

Primeval Starburst and Bulge Formation

. The CO-to-H2 conversion factor (X_CO), which can be directly obtained from the results, is not a constant and depends strongly on the integrated intensity, and it differs from the fiducial value for local objects. X_CO exhibits a large dispersion of more than one order of magnitude, reflecting the non-uniform internal structure of a torus. We also found that the physical conditions differ between grid cells on a scale of a few parsecs along the observed lines of sight; therefore, a specific observed line ratio does not necessarily represent a single physical state of the ISM.

Rapid gas supply to a nuclear region by self-gravitational instability in a weak barred potential

In Smoothed Particle Hydrodynamics (SPH), many authors have recently used a spatially and temporally varying smoothing length, hi to achieve high spatial resolution and a wide dynamic range. Although there are many ways to determine hi, many of them are based on an approach introduced by either [1] Hernquist & Katz (1989) or [2] Benz (1990). In the former method, hi is determined so that each particle interacts with a roughly constant number of neighbors, Nnb, which helps achieving the same level of accuracy at all points (Hernquist & Katz 1989). This method, however, is time-consuming, and as pointed out by Steinmetz (1996), can lead to unphysical change in hi, especially in a clumpy medium, since the selection criteria are computational rather than physical. In method [2], on the other hand, hi is determined by the continuity equation and hi oc p i -1/3 in the method [2], where pi is the density at the location of particle i. Although this is a physically natural approach, there is no guarantee that all particles have nearly constant N nb. In this paper we compare the above two methods for a test problem: adiabatic collapse of an initially isothermal gas sphere, and investigate the reliability of these two methods for determining hi. We also propose here a modified method [3]: after hi is calculated by the method [2], it is corrected to ({h_i}) by using ({h_i}, = ,{1 over 2}{h_i}{left[ {1 + {{left( {{N_{min }}/{N_{nb}}} right)}^{1/3}}} right]_i}), if N nb N max.

Sustained Star Formation in the Central Molecular Zone of the Milky Way

Analyses of the distribution of far infrared point sources in the Galactic bulge have suggested that from a face-on perspective the bulge has a bar like shape. Here, we investigate how a rotating bar-like bulge affects the global gas dynamics in a disk and compare the longitude-velocity (LV) ;naps from selfgravitating hydrodynamical simulations) with observed maps of neutral hydrogen and carbon monoxide in the Galaxy. We found that the features on the numerical IV maps depend strongly on four factors: the pattern speed of the bar, the position angle of the Sun, the strength of the bar potential and the ratio of the gas mass to total dynamical mass. We conclude that our Galaxy has a rotating, weak, bar-like bulge (a/b ∼ 0.8) observed from nearly end on (θp < 20°). The allowed range of pattern speed of the bar is surprisingly narrow (~ 20 km s−1 kpc−1 ) and is consistent with recent observations of bulge stars. Selfgravity of the interstellar matter is needed to account for some of the observations.

Shock-Induced Starburst and Star Cluster Formation in Colliding Galaxies

A massive gaseous disk in the central region of a galaxy sensitively responds to a weakly distorted potential, and a large amount of gas can be fed into within 1/20 of the core radius of the potential in several 10 7 yr. The ILRs, the dissipative nature of the gas, and the self-gravity of the gas are essential for triggering this effective fueling. We also found that a counterrotating gaseous core can be formed as a result of the fueling. Our result suggests that the merger of galaxies is not the only way to form the observed counterrotating core in galaxies.

Gas Dynamics in the Central 100 pc of Galaxies

Starburst phenomena in galaxies should have been common in the epoch of galaxy formation. How did the primeval. starbursts affect the structure or the properties of the host galaxies?