Galactic Bars with Central Mass Concentrations: Three-dimensional Dynamics

The stellar dynamics at the transitions between a strongly barred and rotating potential and a spherical and dense central mass concentration is investigated in a model potential. Three different degrees of concentration of the central mass are studied. The main families of three-dimensional periodic orbit are found by numerical means. The structure of phase space is presented by bifurcation diagrams and surfaces of section. A large region of instability develops near the, inner Lindblad resonance (ILR) due to the strong bar potential. In such a mass configuration the radial ILR is typically near the corresponding vertical ILR, consequently the instability of orbits is radial as well as vertical, helping the heating of disk stars into a bulge. These secular evolution processes from barred to unbarred and in the direction of increasing the bulge-to-disk ratio should affect the evolution of galaxies over a Hubble time.