The dynamical mechanism to form rings at Lindblad resonances in a barred galaxy is now well-known: due to its dissipative character, the gas is forced in a spiral structure, and experiences torques from the bar potential. Angular momentum is transferred until gas accumulates in the resonant rings. Some problems remain however to account for all observations, such as the very different time-scales for nuclear, inner and outer ring formation, while the three are frequently observed in the same galaxy; the shapes, orientations and thickness of the rings, etc... The adequacy of the present gas dynamical modelizations is discussed.

Lenses are secondary components of barred galaxies that could originate from bar evolution. No model until now has met the observational constraints, in particular the sharp edge of the lenses, their strong velocity anisotropy, and their small thickness. We propose here that lenses are the result of partial bar destruction, a necessary step in a feedback cycle of bar formation-destruction, a cycle driven by gas accretion.

A review of the dynamics of the dissipative gas component in barred galaxies is given. The formation of large scale galactic shocks is described, and its implications for star formation and global mass inflow are discussed. Different approaches for modeling the gas will be contrasted, and it will be shown they all produce the same basic morphological picture. However, they do differ in their estimates of mass inflow rates. Some possible directions for future research are indicated.

I use N-body simulations to follow the evolution of bars in both isolated and interacting disk galaxies. The pattern speeds of bars evolving in isolated galaxies decline gradually with time, due to transfer of angular momentum from the bar to other components in the galaxy. Both the form and amount of this decline depend on the model used. The fate of a bar in an interacting disk galaxy depends on the mass, central concentration and orbit of the perturber. The pattern speed, form and amplitude of the bar may change, the bar can become off-centered, or, more drastically, it can disappear altogether. Finally I propose a scenario for the evolution of NGC 7217, which could, if proven correct, explain the formation of the rings in that galaxy and also, at least qualitatively, the existence of a retrograde population.

Ordered orbits in barred galaxies appear along the bar and between the −4/1 and −2/1 resonances of the outer spiral. Chaotic orbits appear mainly near corotation. Such orbits support the bar and the spiral for long times and they are important for self-consistency. There are three main mechanisms for transition from order to chaos: (a) infinite bifurcations, (b) infinite gaps, and (c) infinite spirals. The Lyapunov characteristic number is zero for ordered orbits and positive for chaotic orbits. But much more information is provided by the distribution of the stretching numbers (one-period Lyapunov characteristic numbers). The spectrum of stretching numbers is invariant with respect to initial conditions in a connected chaotic domain. We provide examples of such spectra for 2-D maps, plane galactic orbits, 2-D dissipative systems, 3-D systems (represented by 4-D maps), and systems depending periodically on the time.

A unified picture is presented for the formation of bars in galaxies of different morphological types based on a simple model of the growth of galactic disks by gas infall from haloes. The infall timescale is found to be a key parameter which determines the dynamical property of the resulting disk. It is suggested that the bars in early-type disk galaxies have been formed in tidal interactions with other galaxies whereas those in late-type galaxies formed spontaneously due to gravitational instability (bar instability) in their disks.

Spiral galaxies in the Virgo cluster appear to be barred more frequently than galaxies in less dense environments. Tidal forces due to the cluster mass concentration seem to be the most likely mechanism for inducing these bars.

We study the dynamics of spherical systems which consist of extended shells of relatively low-mass particles around and inside the orbit of two heavy centers of gravity (a binary with point-mass components) by computer simulations.

Observational evidence indicates that the frequency of internal structures (such as rings, bars, spirals, etc.) is preferentially enhanced for spirals in binary systems (e.g. Elmegreen et al. 1990). In this context we study the tidal effects produced due to two spiral galaxies in a near grazing relative orbit with a near parabolic relative velocity, as such barely bound encounters are the most frequent ones. Both the spirals are endowed with an inner and an outer ring.

The spiral galaxies (of radius R) are modeled by an exponential disk of scale length α = 4/R, with a (static) thickness (Chatterjee 1990), and a spherical polytropic bulge (n=0, 3, 4 equally weighted combination) containing 1/3 of the mass; about 10% of the mass of the disk contains gas particles. Two polytropic rings (n=1; Ostriker 1964), are projected on the disk at 1/4 (inner) and 3/4 (outer) of its radius. Though the rings are gaseous, they are here treated as density enhancements. The gravitational potential is softened with softening constants of ∊ = r∘/5, r∘/3, and 0.8r∘, for the bulge, stellar and gaseous components of the disk, respectively. Here r∘ is the radius containing 75% of the total mass of the spiral. The softening constant for the polytropic rings is 0.35, while the mutual gravitational interaction is softened with a softening constant of r∘/4.

Weinberg’s (1985) perturbative calculation of the dynamical friction by halos on bars predicts that bars will be slowed down very quickly. Previous numerical experiments (e.g. Hernquist & Weinberg 1992) have obtained results that are in agreement with Weinberg’s calculation.

Observations, however, suggest that most bars are rotating rapidly. (In this context, a bar is slow if the major axis Lagrange point is far outside the bar, while a fast bar ends at, or near, this Lagrange point.) Neither Weinberg’s calculation nor the numerical experiments have, to the present date, been fully self-consistent, so possibly important effects such as resonance capture and the back reaction on the bar were not included.

Both active and starburst galactic nuclei are best understood if they are fuelled by gas inflow to the nucleus (e.g. Simkin, Su & Schwarz 1980; Hutchings & Campbell 1983).

There is observational evidence that the occurrence of Seyfert and QSO activity is enhanced in the presence of nearby companions (Dahari 1984; Keel et al 1985; Hummel et al. 1987). At least some samples of Seyfert host galaxies show more close companions than would be expected for a random sample of similar galaxies, though the situation for tidal disturbances themselves is more complex. The selection effects modelled by Byrd et al. (1987) suggest that virtually all the Seyfert galaxies in Dahari’s sample have a companion which is (in principle) physically sufficient to provoke gas inflow in the disk.

We present a mechanism to explain the permanent excitation of warps in spiral or barred galaxies. We show that non-linear mode coupling with the spiral wave can be very efficient to drive two warp waves. We derive an expression of the coupling coefficient in the WKB approximation. We show that the coupling is too weak in the stellar disk, except at the Outer Lindblad Resonance where the spiral slows down and is efficiently coupled to warp waves. There, the spiral can almost completely transfer its energy and angular momentum to a ”transmitted“ warp, which we can observe in HI, and a ”reflected“ one, which we can observe as a corrugation. Our mechanism reproduces the observed amplitudes of the warp and of the corrugation.

We first present side results concerning the propagation of warps in moderately thick disks.

To investigate bar destruction by a vertical head-on galaxy encounter, the dynamical evolution after the instantaneous momentum change of the impulsive approximation of a vertical head-on galaxy encounter, and the instability of finite amplitude one-armed perturbations to barred structure, are investigated by 2-dimensional N-body simulations.

Preliminary results show that off-center vertical head-on collisions induce bar destruction but collisions in which the colliding galaxy passes through the very center of the target galaxy do not induce bar destruction.

Rings are frequently observed in barred galaxies (e.g. Buta & Crocker 1993). Their origin is attributed to gas accumulation near resonances, which is due to a combination of the gravitational torques of a bar component acting on inelastically colliding clouds: outer rings are presumably associated with the Outer Lindblad Resonance (OLR), inner rings with the 4/1 resonance, and nuclear rings with the Inner Lindblad Resonance (ILR).

However, multiple rings are also observed in normal non-barred, or not obviously barred galaxies (e.g. NGC 7217, Verdes-Montenegro et al. 1995) so that we may ask what is the mechanism of their formation. We argue in this contribution that even a very weak bar or a triaxial distortion in the galaxy center, which would hardly be identified from observations, leads to long-lasting ring-like distributions of gaseous clouds. The formation time-scales of rings associated with different resonances are similar in our model so that all types of rings can coexist in galaxies with a high degree of axial symmetry in their central parts, such as SA galaxies.

A simple formula is derived for the force of dynamical friction acting on a satellite galaxy which is moving on a nearly circular orbit around its parent galaxy. Using this formula, estimates of the decay time are computed. The results are then compared with the corresponding results from numerical simulations, and are found to be in good agreement.

We study the effects of different collision laws and of recycling on the results of simulations using sticky particles to describe the gas clouds in a disc galaxy.

Numerous galaxies clearly show two misaligned nested stellar bars or one bar plus one structure with twisted isophotes. Some galaxies seem even to have three distinct triaxial structures. Recent optical and near-infrared multi-band observations show that the angle between the primary and secondary bars does not take any peculiar value. The best way to reconcile such observations with theory certainly consists in postulating that the secondary bar rotates faster than the primary one. Numerical simulations have demonstrated the viability of such systems, and both direct and counterrotating models with bars within bars can be computed. Bar-within-bar systems could play a significant role in fueling AGN’s and/or circumnuclear starbursts, since AGN’s and/or blue or Hα nuclear rings are frequently hosted by such galaxies.

Detailed studies of stellar and gas dynamics have been carried out primarily in model galaxies which are not good approximations to the shapes of real galaxies (Athanassoula 1991). The high quality of recently available near-IR images of galaxies makes it possible to make realistic mass models of the matter that is traced by starlight. The gravitational potential derived from the mass model of a galaxy can be used to conduct stellar and gas dynamical studies in the galaxy, and the results can be compared to observed velocity fields and other observations of these galaxies. In this report we discuss how accurately the gravitational potential can be estimated based on an infrared image of the galaxy. We describe two uses of such an estimate of a gravitational potential: stellar orbits integrated in the barred galaxy NGC 4314 and an estimate of the gas inflow rate along the bar in NGC 7479.

For more than a decade, a group at the University of Florida has been constructing models of barred spiral galaxies that we have observed at the VLA. Group members include: S. T. Gottesman, G. Contopoulos, J. H. Hunter, several Ph.D. students, and collaborators at other institutions. The HI distributions in our galaxies are either gas rich, or nearly devoid of gas, in their bar regions. Initially, we deduced density distributions and potentials of the stellar disks and bars from near infrared observations, modeled the gas response assuming different pattern speeds, etc., and compared the models with our HI observations. This simple approach is successful in some cases, such as NGC 3359, but less successful in others. More recently, our approach has been toward selfconsistent models, including stars and gas. Our techniques range from the Contopoulos-Grosbøl-Kaufmann stellar dynamical approach to methods exploiting grid codes and SPH tree codes. Some of our observations and models will be reviewed, and some new results will be presented.

The in-plane appearance of barred galaxies is formed in either of two ways: 1) the usual bar-mode instability in a fast rotating disk forms a fast bar, while 2) the radial orbit instability (or its modifications) leads to a slow bar. The vertical structure depends on the universal mechanism of the bending instability both in fast and slow bars. Among all the relevant topics, I chose (i) a critical review of the work on the bending instability, and (ii) some comments concerning the local and global stability criteria for non-axisymmetric disturbances in galactic disks; in particular, a new approximate local dispersion relation for a star disk is presented. Also shown is a range of accuracy for the intuitional global criterion of the slow bar instability of the Ostriker-Peebles type (on the example of the exactly-solved model).

Several authors have explored the field of gas dynamics in barred systems. One of the aims of these investigations was to compare the model gaseous response, due to some assumed underlying stellar gravitational field, with observed gas density distribution and kinematics of barred galaxies. The gas is known to respond in a highly non-linear way, and therefore should give clues to dynamical parameters like the mass distribution, positions and existence of principal resonances and thereby the pattern speed.

High resolution HI data now exist for NGC 1365 (Jörsäter & van Moorsel 1995), and the kinematical HI data have been combined with optical long slit measurements to obtain the velocity field (Lindblad et al. 1995) used for extracting the rotation curve, representing the axisymmetric forces in NGC 1365, and for comparisons with models. A mosaic image of NGC 1365 in the J-band was used to compute the perturbing potential used in the models.