STELLAR DISKS OF OPTICALLY FLOCCULANT AND GRAND DESIGN SPIRALS - DECOUPLING OF STELLAR AND GASEOUS DISKS

Accurate surface photometry of five spiral galaxies (NGC 3223, NGC 508 5, NGC 5247, NGC 5861 and NGC7083) in visual and near-infrared bands i s presented. All galaxies show grand design spiral structure in K' alt hough several have a flocculent appearance on blue images suggesting t hat stellar and gaseous disks are decoupled. The decomposition of the K' maps in axisymmetric components suggests that both a spherical bulg e and a flat exponential disk are required to explain the light distri bution in the bulge regions. The central disk has a short scale length and sky projection parameters similar to the main disk. The scale len gth of the main exponential disk measured in the interarm region is co nsistently smaller than the value determined from the arms for three o f the galaxies. Weak oval distortions were found in three galaxies whi le only one was classified as such. The galaxies with oval distortions have less concentrated and fainter bulges than those in the two galax ies without such ovals. Model rotation curves were constructed for eac h galaxy based on the axisymmetric decomposition of their K' surface b rightness maps and observed velocity data. The mass-to-light ratio est imated in K' for the disk component was around 0.7 in solar units for all galaxies using the disk scale length derived from the interarm reg ions. Pitch angles of the main two-armed spiral estimated in B,V,I and K' show a systematic trend of arms being tighter in bluer colors whic h suggests the presence of density waves in the galaxies. Comparing th e extent of the spiral pattern in the galaxies with their angular velo city curves, the best agreement was obtained when the symmetric, two-a rmed spiral starts just outside ILR and terminates around the 4:1 reso nance. The spirals continue beyond this region but are weaker and more fragmented. The pattern speed estimated for the two Sb galaxies was s ignificantly higher than that for the Sc galaxies. Several galaxies ha ve dust spirals inside the main stellar spiral. In most cases, the mai n spiral pattern is more open in the inner region than further out whe re it is well approximated with a logarithmic spiral. This may support a scenario where a central oval distortion drives the spiral, which t hen would correspond to the long waved solution of the dispersion rela tion.