An Observational Study of Arm Structure in Normal Spiral Galaxies.

Multivariate data have been obtained and analyzed in an effort to develop a new classification system for spiral galaxies, one which is not necessarily based on morphological properties. By comparing morphological measures (such as arm delineation) with the intrinsic parameters on which the new system is based, it can be seen just how fundamentally important a galaxian property arm structure truly is. A sample of 492 moderately bright (B_ {rm T} <=q 13.2) northern Sa, SBa, Sc, and SBc spirals was chosen for statistical analysis. The literature was searched for redshifts, optical and near-infrared photometric data, 20 cm fluxes, and H I 21 cm line parameters. New observations were made at 20 and 21 cm; in addition, IRAS fluxes were obtained from archival data. These data were subjected to principal components analysis, modified in that non-detections were explicitly accounted for. Spiral galaxies have two fundamental properties, confirming previous work done with fewer data. While "scale" (bulk) is most important, galaxies of given scale can vary in "form" (color, bulge/disk ratio, H I surface density). Arm strength is unrelated to other properties. Forty-five spatially isolated spirals were chosen for two-color optical CCD imaging, for 20 cm continuum imaging, and for CO 2.6 mm spectroscopy. Isolated Sa's are often found to be misclassified; they are either armless SO's are else peculiar victims of collisions. Isolated Sc's are surprising in that all but the most isolated examples have symmetric arms over at least part of the disk, contrary to the predictions of stochastic models for arm formation. Given the extreme isolation of all highly chaotic disks, the presence of tidal companions (or at least candidates) for the other normal isolated spirals, the apparent prevalence of violent processes in isolated Sa's, and recent N-body simulations of tidal encounters, it is likely that arm structure in most spirals is produced by tidal encounters with small companions. Arm structure is not an intrinsic property of a given galaxy. All of these results can be explained by supposing that protogalactic environment is the determining influence for spirals.