Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Physics and Astronomy

First Advisor

Joel E. Tohline


We model the structure, kinematics, and dynamics of neutral hydrogen gas in galaxies. Our attention is focused on specific features that are commonly observed mainly in the 21-cm line, such as twists, warps and polar rings. The intimate connection between these features and the presumed dark halos of galaxies is analyzed with the intention to discover ways to probe the dark matter of individual galaxies by simply observing the distribution and kinematics of the luminous matter. An important, intermediate step in this analysis is to demonstrate that the twists that appear in the isovelocity maps of many spiral galaxies (and that are reproduced by the corresponding kinematical models) have a dynamical origin, owing their existence to the differential precession of gas-particle orbits. The main conclusions from this work are: (a) The kinematics of normal spiral galaxies can be modeled satisfactorily by a tilted-ring model. If, in addition, the observed velocity field of a galaxy shows a strongly twisted structure, then unambiguous results can be derived about the structure of the implied warp and the geometry of the superimposed dark halo. (b) The galaxies NGC5033 and NGC5055 show the signature of a prolate halo in their kinematics. This result supports the "Dark Matter Hypothesis" while it seems to have no explanation in the framework of dynamical theories that modify the Newtonian gravitational force in order to reproduce the observed flat rotation curves of spiral galaxies. (c) Strongly twisted warps form naturally when a gaseous disk attempts to settle toward a preferred orientation in the external potential well of a weakly distorted halo, but only if the disks start from low or moderate inclinations relative to the preferred orientation. (d) Highly and moderately inclined disks, as well as disks at all inclinations under the influence of a strongly non-spherically-symmetric halo, show significant matter inflow. This results suggests that warps should only develop at relatively low inclinations and in weakly distorted halos. (e) Polar rings and highly inclined disks can only survive for a Hubble time if the superimposed halo is nearly spherical. This conclusion is in agreement with the results of recent observation of polar-ring galaxies.