Dynamical Evolution of Gaseous Disks in Halo Potentials. I. Massless Rings in Static, Spheroidal Halos
Abstract
We have conducted an experimental study of gaseous galaxy disks that evolve under the influence of an external halo potential. Because our numerical experiments are the first to employ a fully three-dimensional, Eulerian, hydrodynamical computer code, we have chosen the simplest possible initial conditions: the disks are massless and the external potential is either oblate or prolate spheroidal and nontumbling. The outcome and the details of each model's evolution depend critically on the degree of distortion of the halo from spherical symmetry and on the initial inclination of the disk from the equatorial plane of the halo. For small halo distortions and low initial inclinations, the disk identifies the symmetry plane of the halo as the preferred plane and settles to it, developing in the process a warped and twisted structure. As disks with higher initial inclinations are considered, matter inflow to the nuclear region of the potential becomes increasingly important relative to settling toward the preferred plane, and it dominates at moderate inclinations. Highly inclined disks and polar rings are able to survive at their original inclinations for long periods of time while they continuously supply small amounts of gas to the nuclear region. On the other hand, as the quadrupole distortion of the potential increases, the evolutions become progressively more violent at all initial inclinations. Finally, for strong halo distortions, disk evolutions are catastrophic and the dynamics does not involve only the competition between settling and inflow; the models develop extremely distorted structures after only a few rotations. In retrospect, smoothly warped and twisted gaseous disks seen in real galaxies should be at most moderately inclined inside weakly distorted dark halos. Highly inclined and nearly flat rings observed in polar-ring galaxies can be interpreted as transient features that survive because the dark halos around them are only slightly distorted. Because matter inflow does occur during settling, nuclear disks should be a common feature of such evolutions. A nuclear disk that is observed in a galaxy whose outer regions are dominated by a smooth warp should be an indication of a moderately tilted disk that is settling in the presence of only a slightly distorted halo potential. Nuclear disks and enhanced nuclear activity seen in galaxies that are not dominated by smooth warps may reflect moderately strong quadrupole distortions in the underlying potentials. Finally, extreme nuclear activity, disks in a stage of disruption, or even the irregular morphology of some galaxies can be caused by strongly distorted halos.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- January 1993
- DOI:
- 10.1086/172186
- Bibcode:
- 1993ApJ...403..110C
- Keywords:
-
- Astronomical Models;
- Computational Astrophysics;
- Dynamic Models;
- Galactic Evolution;
- Galactic Halos;
- Computerized Simulation;
- Galactic Nuclei;
- Galactic Structure;
- Astrophysics;
- GALAXIES: ISM;
- GALAXIES: KINEMATICS AND DYNAMICS;
- HYDRODYNAMICS;
- METHODS: NUMERICAL