Panos A. Patsis: Personal

THESES:

Nonlinear phenomena, like complex instability, inverse bifurcations, and collisions of bifurcations, dominate the dynamics of rotating triaxial potentials.
For Sb and Sc galaxies the best self-consistent orbital stellar model is a nonlinear one ending at the 4:1 resonance. For Sa galaxies the stellar spiral arms are weak and the best theoretical orbital model is a linear one ending at corotation.
The above results hold for gaseous SPH models as well. In addition the "4:1 resonance" models are also superior to models assuming the end of the strong, symmetric spirals at the OLR.
The use of color and rotation is an efficient method to visualize the four-dimensional "space" of section in 3D Hamiltonian systems. The neighborhood of stable, simple unstable, double unstable and complex unstable periodic orbits has in each case a characteristic fingerprint.
Stable 3D periodic orbits with elliptical projections on the equatorial plane can support a spiral pattern with variable thickness. The edge-on shapes of orbits of these thick spirals are determined by the vertical resonances existing in the potential.
The addition of an m=1 component in a bisymmetric spiral potential brings striking similarities between the morphology of open grand design spiral galaxies and the corresponding SPH, "4:1 resonance" models.
The bar of NGC4314 is built by nonperiodic orbits trapped around the stable periodic orbits of the x1 family. The boxy structure at the end of the bar is due to chaotic orbits.
The method of dynamical spectra is efficient in detecting tiny ordered domains embedded in big chaotic seas.
Many spiral galaxies show grand design spiral structure in K' although several have a flocculent appearance on blue images suggesting that stellar and gaseous disks are decoupled.
Comparing the extent of the spiral pattern in K' in the galaxies NGC3223, NGC5085, NGC5247, NGC5861, and NGC7083 with their angular velocity curves, the best agreement was obtained when the symmetric, two-armed spiral starts just outside ILR and terminates around the 4:1 resonance.
In N-body simulations, non-linear spiral modes last from four to seven pattern rotations and secure a grand-design morphology to the models. Their dynamical behavior includes the termination of the strong, symmetric, logarithmic stellar spirals inside their 4:1 resonance radius, as foreseen from the orbital theory and from SPH gasdynamical models.
In SPH models of barred galaxies, the longer lasting (for about 10 rotational periods of the bar) sharply described shocks, occur if we assume a sound speed 20 km/s. The sound speed influences the inflow rate of gas in barred galaxies. Strong shocks occur close to the ends of the bar and form "T-shaped" features.
The amount of light coming from Brgamma and H2 emission represents only a few percent of the observed K' light. A major contribution from young objects in K' comes from continuum radiation which in the arm regions can amount to 20%.
In a 3D bar, the backbone of the orbital structure is not just the x1 family, as in two-dimensional (2D) models, but a tree of 2D and 3D families bifurcating from x1.
The presence of a bar is not a sine qua non condition for the appearance of boxy bulges, as often assumed. A `boxy' or a `peanut' structure in the central parts of a model is due to the presence of vertical resonances at which stable families of periodic orbits bifurcate from the planar x1 family.
A boxy/peanut feature can be built by 3D families associated with 3D bifurcations of the x1 family. The `X' feature observed inside the boxy bulges of several edge-on galaxies can be attributed to the peaks of successive x1v1 orbits, provided their stability allows it.
3D orbits that are introduced due to vertical instabilities play a crucial role in the face-on profiles of bars and enhance their rectangularity.
Inner rings in barred spiral galaxies are associated with specific 2D and 3D families of periodic orbits located just beyond the end of the bar. These are families located between the inner radial ultraharmonic 4:1 resonance and corotation. The ring shapes that are favoured are mainly ovals, as well as polygons with `corners' on the minor axis, on the sides of the bar.
Deep surface photometry in the K band obtained for 54 normal spiral galaxies, shows that bulges are best represented by a Sersic law with an index in the typical range of 1-2. Bars down to the detection level at a relative amplitude of 3% were detected in 26 of 30 galaxies. The sample shows a lack of strong, tight spirals which could indicate that such patterns are damped by non-linear, dynamical effects due to their high radial force perturbations.
Different bar morphologies may have as a backbone the same set of x1 periodic orbits.
In two-dimensional models with sufficiently large bar axial ratios (a/c>6), stable orbits having propeller shapes play the dominant role.
Particle Swarm Optimization (PSO) is an efficient alternative method for locating periodic orbits in a three-dimensional (3D) model of barred galaxies.
The subtraction from the images of boxy edge-on galaxies of a model representing the axisymmetric component reveals the contribution of the non-axisymmetric terms. There is a direct correspondence between the orbital profiles of 3D bars in models and the observed main morphological features of the residuals.
The barred spiral galaxy NGC 3359 contains two pattern speeds. The best match of models with observed data indicates a pattern speed for the bar of 39.17 km/sec/kpc and a value between 10 and 16 km/sec/kpc for the spiral.
There are three types of orbits beyond corotation: Ordered (periodic or quasi periodic), Chaotic, and Escaping. The images of the x-axis on the (x, xdot) surface of section are along certain ``rays'' and ``bell-type'' curves.
The part of the spirals attached to the bar in many cases of barred spiral galaxies, is due to chaotic orbits. They are of the same type of orbits that is responsible for the boxiness of the outer isophotes of the bar in cases like NGC4314.