Using self-consistent three-dimensional (3D) N-body simulations, we investigate the physical properties of non-axisymmetric features in a disk galaxy created by a tidal interaction with its companion. The primary galaxy consists of a stellar disk, a
bugle, and a live halo, corresponding to Milky-Way type galaxies, while the companion is represented by a halo alone. We vary the companion mass and the pericenter distance to explore situations with differing tidal strength parameterized by either the relative tidal force P or the relative imparted momentum S. We find that the formation of a tidal tail in the outer parts requires P > 0.05 or S > 0.07. A stronger interaction results in a stronger, less wound tail that forms earlier. Similarly, a stronger tidal forcing produces stronger, more loosely wound spiral arms in the inner parts. The arms are approximately logarithmic in shape, with both amplitude and pitch angle decaying with time. The derived pattern speed decreases with radius and is close to the Omega-kappa/2 curve at late time, with Omega and kappa denoting the angular and epicycle frequencies, respectively. This suggests that the tidally-induced spiral arms are most likely kinematic density waves weakly modified by self-gravity. Compared to the razor-thin counterparts, arms in the 3D models are weaker, have a smaller pitch angle, and wind and decay more rapidly. The 3D density structure of the arms is well described by the concentrated and sinusoidal models when the arms are in the nonlinear and linear regimes, respectively. We demonstrate that dynamical friction between interacting galaxies transfers the orbital angular momentum of one galaxy to the spin angular momentum of the companion halo.
An ionization front (IF) surrounding an H II region is a sharp interface where a cold neutral gas makes transition to a warm ionized phase by absorbing UV photons from central stars. We investigate the instability of a plane-parallel D-type IF thread
ed by parallel magnetic fields, by neglecting the effects of recombination within the ionized gas. We find that weak D-type IFs always have the post-IF magnetosonic Mach number $mathcal{M}_{rm M2} leq 1$. For such fronts, magnetic fields increase the maximum propagation speed of the IFs, while reducing the expansion factor $alpha$ by a factor of $1+1/(2beta_1)$ compared to the unmagnetized case, with $beta_1$ denoting the plasma beta in the pre-IF region. IFs become unstable to distortional perturbations due to gas expansion across the fronts, exactly analogous to the Darrieus-Landau instability of ablation fronts in terrestrial flames. The growth rate of the IF instability is proportional linearly to the perturbation wavenumber as well as the upstream flow speed, and approximately to $alpha^{1/2}$. The IF instability is stabilized by gas compressibility and becomes completely quenched when the front is D-critical. The instability is also stabilized by magnetic pressure when the perturbations propagate in the direction perpendicular to the fields. When the perturbations propagate in the direction parallel to the fields, on the other hand, it is magnetic tension that reduces the growth rate, completely suppressing the instability when $mathcal{M}_{rm M2}^2 < 2/(beta_1 - 1)$. When the front experiences an acceleration, the IF instability cooperates with the Rayleigh-Taylor instability to make the front more unstable.
Gas in disk galaxies interacts nonlinearly with an underlying stellar spiral potential to form galactic spiral shocks. While numerical simulations typically show that spiral shocks are unstable to wiggle instability (WI) even in the absence of magnet
ic fields and self-gravity, its physical nature has remained uncertain. To clarify the mechanism behind the WI, we conduct a normal-mode linear stability analysis as well as nonlinear simulations assuming that the disk is isothermal and infinitesimally thin. We find that the WI is physical, originating from the generation of potential vorticity at a deformed shock front, rather than Kelvin-Helmholtz instabilities as previously thought. Since gas in galaxy rotation periodically passes through the shocks multiple times, the potential vorticity can accumulate successively, setting up a normal mode that grows exponentially with time. Eigenfunctions of the WI decay exponentially downstream from the shock front. Both shock compression of acoustic waves and a discontinuity of shear across the shock stabilize the WI. The wavelength and growth time of the WI depend on the arm strength quite sensitively. When the stellar-arm forcing is moderate at 5%, the wavelength of the most unstable mode is about 0.07 times the arm-to-arm spacing, with the growth rate comparable to the orbital angular frequency, which is found to be in good agreement with the results of numerical simulations.
The neutral component of the interstellar medium is segregated into the cold neutral medium (CNM) and warm neutral medium (WNM) as a result of thermal instability. It was found that a plane-parallel CNM-WNM evaporation interface, across which the CNM
undergoes thermal expansion, is linearly unstable to corrugational disturbances, in complete analogy with the Darrieus-Landau instability (DLI) of terrestrial flames. We perform a full linear stability analysis as well as nonlinear hydrodynamic simulations of the DLI of such evaporation fronts in the presence of thermal conduction. We find that the DLI is suppressed at short length scales by conduction. The length and time scales of the fastest growing mode are inversely proportional to the evaporation flow speed of the CNM and its square, respectively. In the nonlinear stage, the DLI saturates to a steady state where the front deforms to a finger-like shape protruding toward the WNM, without generating turbulence. The evaporation rate at nonlinear saturation is larger than the initial plane-parallel value by a factor of 2.4 when the equilibrium thermal pressure is 1800 k_B cm^-3 K. The degrees of front deformation and evaporation-rate enhancement at nonlinear saturation are determined primarily by the density ratio between the CNM and WNM. We demonstrate that the Field length in the thermally unstable medium should be resolved by at least four grid points to obtain reliable numerical outcomes involving thermal instability.
Nuclear rings in barred galaxies are sites of active star formation. We use hydrodynamic simulations to study temporal and spatial behavior of star formation occurring in nuclear rings of barred galaxies where radial gas inflows are triggered solely
by a bar potential. The star formation recipes include a density threshold, an efficiency, conversion of gas to star particles, and delayed momentum feedback via supernova explosions. We find that star formation rate (SFR) in a nuclear ring is roughly equal to the mass inflow rate to the ring, while it has a weak dependence on the total gas mass in the ring. The SFR typically exhibits a strong primary burst followed by weak secondary bursts before declining to very small values. The primary burst is associated with the rapid gas infall to the ring due to the bar growth, while the secondary bursts are caused by re-infall of the ejected gas from the primary burst. While star formation in observed rings persists episodically over a few Gyr, the duration of active star formation in our models lasts for only about a half of the bar growth time, suggesting that the bar potential alone is unlikely responsible for gas supply to the rings. When the SFR is low, most star formation occurs at the contact points between the ring and the dust lanes, leading to an azimuthal age gradient of young star clusters. When the SFR is large, on the other hand, star formation is randomly distributed over the whole circumference of the ring, resulting in no apparent azimuthal age gradient. Since the ring shrinks in size with time, star clusters also exhibit a radial age gradient, with younger clusters found closer to the ring. The cluster mass function is well described by a power law, with a slope depending on the SFR. Giant gas clouds in the rings have supersonic internal velocity dispersions and are gravitationally bound.
We investigate gravitational instability (GI) of rotating, vertically-stratified, pressure-confined, polytropic gas disks using linear stability analysis as well as analytic approximations. The disks are initially in vertical hydrostatic equilibrium
and bounded by a constant external pressure. We find that GI of a pressure-confined disk is in general a mixed mode of the conventional Jeans and distortional instabilities, and is thus an unstable version of acoustic-surface-gravity waves. The Jeans mode dominates in weakly confined disks or disks with rigid boundaries. When the disk has free boundaries and is strongly pressure-confined, on the other hand, the mixed GI is dominated by the distortional mode that is surface-gravity waves driven unstable under own gravity and thus incompressible. We demonstrate that the Jeans mode is gravity-modified acoustic waves rather than inertial waves and that inertial waves are almost unaffected by self-gravity. We derive an analytic expression for the effective sound speed c_eff of acoustic-surface-gravity waves. We also find expressions for the gravity reduction factors relative to a razor-thin counterpart, appropriate for the Jeans and distortional modes. The usual razor-thin dispersion relation after correcting for c_eff and the reduction factors closely matches the numerical results obtained by solving a full set of linearized equations. The effective sound speed generalizes the Toomre stability parameter of the Jeans mode to allow for the mixed GI of vertically-stratified, pressure-confined disks.
Using hydrodynamic simulations, we investigate the physical properties of gaseous substructures in barred galaxies and their relationships with the bar strength. The gaseous medium is assumed to be isothermal and unmagnetized. The bar potential is mo
deled as a Ferrers prolate with index n. To explore situations with differing bar strength, we vary the bar mass fbar relative to the spheroidal component as well as its aspect ratio. We derive expressions as functions of fbar and the aspect ratio for the bar strength Qb and the radius r(Qb) where the maximum bar torque occurs. When applied to observations, these expressions suggest that bars in real galaxies are most likely to have fbar=0.25-0.5 and n<1. Dust lanes approximately follow one of x1-orbits and tend to be more straight under a stronger and more elongated bar, but are insensitive to the presence of self-gravity. A nuclear ring of a conventional x2 type forms only when the bar is not so massive or elongated. The radius of an x2-type ring is generally smaller than the inner Lindblad resonance, decreases systematically with increasing Qb, and slightly larger when self-gravity is included. This evidences that the ring position is not determined by the resonance but by the amount of angular momentum loss at dust-lane shocks. Nuclear spirals exist only when the ring is of the x2-type and sufficiently large in size. Unlike the other features, nuclear spirals are transient in that they start out as being tightly-wound and weak, and then due to the nonlinear effect unwind and become stronger until turning into shocks, with an unwinding rate higher for larger Qb. The mass inflow rate to the galaxy center is found to be less than 0.01 Msun/yr for models with Qb<0.2, while becoming larger than 0.1 Msun/yr when Qb>0.2 and self-gravity is included.
Barred galaxies are known to possess magnetic fields that may affect the properties of bar substructures such as dust lanes and nuclear rings. We use two-dimensional high-resolution magnetohydrodynamic (MHD) simulations to investigate the effects of
magnetic fields on the formation and evolution of such substructures as well as on the mass inflow rates to the galaxy center. The gaseous medium is assumed to be infinitesimally-thin, isothermal, non-self-gravitating, and threaded by initially uniform, azimuthal magnetic fields. We find that there exists an outermost x1-orbit relative to which gaseous responses to an imposed stellar bar potential are completely different between inside and outside. Inside this orbit, gas is shocked into dust lanes and infalls to form a nuclear ring. Magnetic fields are compressed in dust lanes, reducing their peak density. Magnetic stress removes further angular momentum of the gas at the shocks, temporarily causing the dust lanes to bend into an L shape and eventually leading to a smaller and more centrally distributed ring than in unmagnetized models. The mass inflow rates in magnetized models correspondingly become larger, by more than two orders of magnitude when the initial fields have an equipartition value with thermal energy, than in the unmagnetized counterparts. Outside the outermost x1-orbit, on the other hand, an MHD dynamo due to the combined action of the bar potential and background shear operates near the corotation and bar-end regions, efficiently amplifying magnetic fields. The amplified fields shape into trailing magnetic arms with strong fields and low density. The base of the magnetic arms has a thin layer in which magnetic fields with opposite polarity reconnect via a tearing-mode instability. This produces numerous magnetic islands with large density which propagate along the arms to turn the outer disk into a highly chaotic state.
The inner regions of barred galaxies contain substructures such as off-axis shocks, nuclear rings, and nuclear spirals. These substructure may affect star formation, and control the activity of a central black hole (BH) by determining the mass inflow
rate. We investigate the formation and properties of such substructures using high-resolution, grid-based hydrodynamic simulations. The gaseous medium is assumed to be infinitesimally-thin, isothermal, and non-self-gravitating. The stars and dark matter are represented by a static gravitational potential with four components: a stellar disk, the bulge, a central BH, and the bar. To investigate various galactic environments, we vary the gas sound speed c_s as well as the mass of the central BH M_BH. Once the flow has reached a quasi-steady state, off-axis shocks tend to move closer to the bar major axis as c_s increases. Nuclear rings shrink in size with increasing c_s, but are independent of M_BH, suggesting that ring position is not determined by the Lindblad resonances. Rings in low-c_s models are narrow since they are occupied largely by gas on x2-orbits and well decoupled from nuclear spirals, while they become broad because of large thermal perturbations in high-c_s models. Nuclear spirals persist only when either c_s is small or M_BH is large; they would otherwise be destroyed completely by the ring material on eccentric orbits. The shape and strength of nuclear spirals depend sensitively on c_s and M_BH such that they are leading if both c_s and M_BH are small, weak trailing if c_s is small and M_BH is large, and strong trailing if both c_s and M_BH are large. While the mass inflow rate toward the nucleus is quite small in low-c_s models because of the presence of a narrow nuclear ring, it becomes larger than 0.01 Msun/yr when c_s is large, providing a potential explanation of nuclear activity in Seyfert galaxies.
We use three-dimensional hydrodynamic simulations to investigate the nonlinear gravitational responses of gas to, and the resulting drag forces on, very massive perturbers moving on circular orbits. This work extends our previous studies that explore
d the cases of low-mass perturbers on circular orbits and massive perturbers on straight-line trajectories. The background medium is assumed to be non-rotating, adiabatic with index 5/3, and uniform with density rho0 and sound speed a0. We model the gravitating perturber using a Plummer sphere with mass Mp and softening radius rs in a uniform circular motion at speed Vp and orbital radius Rp, and run various models with differing R=rs/Rp, Mach=Vp/a0, and B=G*Mp/(a0^2*Rp). A quasi-steady density wake of a supersonic model consists of a hydrostatic envelope surrounding the perturber, an upstream bow shock, and a trailing low-density region. The continuous change in the direction of the perturber motion makes the detached shock distance reduced compared to the linear-trajectory cases, while the orbit-averaged gravity of the perturber gathers the gas toward the center of the orbit, modifying the background preshock density to rho1=(1 + 0.46B)*rho0 depending weakly on Mach. For sufficiently massive perturbers, the presence of a hydrostatic envelope makes the drag force smaller than the prediction of the linear perturbation theory, resulting in F = 4*pi*rho1*(G*Mp/Vp)^2 * (0.7/etaB) for etaB = B/(Mach^2 -1) > 0.1; the drag force for low-mass perturbers with etaB < 0.1 agrees well with the linear prediction. The nonlinear drag force becomes independent of R as long as R < etaB/2, which places an upper limit on the perturber size for accurate evaluation of the drag force in numerical simulations.
