The Distribution of Maximum Relative Gravitational Torques in Disk Galaxies

The maximum ratio of the tangential force to the mean background radial forceis a useful quantitative measure of the strength of nonaxisymmetricperturbations in disk galaxies. Here we consider the distribution of thisratio, called Qg, for a statistically well-defined sample of 180 spiralgalaxies from the Ohio State University Bright Galaxy Survey and the Two MicronAll-Sky Survey. Qg is derived from gravitational potentials inferred fromnear-infrared images under the assumptions of a constant mass-to-light ratioand an exponential vertical density law. In order to derive the most reliablemaximum relative torques, orientation parameters based on blue-light isophotesare used to deproject the galaxies, and the more spherical shapes of bulges aretaken into account using two-dimensional decompositions which allow foranalytical fits to bulges, disks, and bars. Also, vertical scaleheights hz arederived by scaling the radial scalelengths hR from the two-dimensionaldecompositions allowing for the type dependence of hR/hz indicated by opticaland near-infrared studies of edge-on spiral galaxies. The impact of dark matteris assessed using a "universal rotation curve" parametrization, and is found tobe relatively insignificant for our sample. In agreement with a previous studyby Block et al. (2002), the distribution of maximum relative gravitationaltorques is asymmetric towards large values and shows a deficiency of low Qggalaxies. However, due to the above refinements, our distribution shows morelow Qg galaxies than Block et al. We also find a significant type-dependence inmaximum relative gravitational torques, in the sense that Qg is lower onaverage in early-type spirals compared to late-type spirals.Comment: Accepted for publication in the Astronomical Journal, January 2004 issue (Latex, 39 pages + 17 figures, uses aastex.cls