A Physical Model of Warped Galaxy Disks

Warped H I gas layers in the outer regions of spiral galaxies usually display a noticeably twisted structure. This structure almost certainly arises primarily as a result of differential precession in the H I disk as it settles toward a preferred orientation in an underlying dark halo potential well that is not spherically symmetric. In an attempt to better understand the structure and evolution of these twisted, warped disk structures, we have adopted the “twist–equation” formalism originally developed by Petterson (1977) to study accretion onto compact objects. Utilizing more recent treatments of this formalism, we have generalized the twist–equation to allow for the treatment of non–Keplerian disks and from it have derived a steady–state structure of twisted disks that develops from free precession in a nonspherical, logarithmic halo potential. We have used this steady–state solution to produce H I maps of five galaxies (M83, NGC 300, NGC 2841, NGC 5033, NGC5055), which match the general features of the observed maps of these galaxies quite well. In addition, the model provides an avenue through which the kinematical viscosity of the H I disk and the quadrupole distortion of the dark halo in each galaxy can be quantified. This generalized equation can also be used to examine the time-evolutionary behavior of warped galaxy disks. Subject headings: galaxies: kinematics and dynamics — galaxies: structure — radio lines: galaxies