Panos A. Patsis

Propeller Orbits in Barred Galaxy Models

The central, or x1, family of periodic orbits is the most important one in almost all two-dimensional numerical models of galactic bars in the literature. However, we present evidence that in two-dimensional models with sufficiently large bar axial ratios (a/c 6), stable orbits having propeller shapes play the dominant role. In our models this propeller family is in fact a distant relative of the x1 family. There are also intermediate cases in which both families are important. The dominance of one family over the other may have direct consequences on the morphological properties of the bars that can be constructed from them, properties such as face-on bar thinness and strength as well as the boxiness of the outer isophotes.

Dust Lanes in Spiral Galaxies

Many grand design spiral galaxies exhibit long, narrow dust lanes along the inner edge of their main arms. The extent of such dust lanes suggests that they are caused by a global mechanism such as large-scale shocks in the gas flowing through potential minima of a density wave (Fujimoto, 1966, Roberts, 1969).Their exact location relative to the density wave potential is of significant interest as it can be used to calibrate hydrodynamic calculations of the shock. It is easy to trace the dust on blue images but the underlying perturbation of the disk cannot be derived from maps in visual bands due to the high extinction at these wavelengths. With large near-infrared detectors, it has become possible to determine the spiral perturbations in the old stellar disk population with sufficient accuracy.

Periodic Orbits in Three-Armed Galaxies

Although interarm features in the Population I disk of spiral galaxies frequently give an impression of a three-fold symmetry (Patsis et al., 1997), true three-armed spiral structures in the old stellar disk are seldomly seen. Such systems are of special interest since they display unique conditions which favor the growth of m=3 modes. The face-on spiral NGC 7137 shows a clear three-armed pattern on K-band images and was used as a prototype for the potential of these systems.