Spiral galaxies observed in the near-infrared K band - I. Data analysis and structural parameters

Deep surface photometry in the K band was obtained for 54 normal spiral galaxies, with the aim of quantifying the percentage of faint bars and studying the morphology of spiral arms. The sample was chosen to cover a wider range of morphological types while inclination angles and distances were limited to allow a detailed investigation of the internal structure of their disks and future observations and studies of the disk kinematics. An additional constraint for a well defined subsample was that no bar structure was seen on images in the visual bands. Accurate sky projection parameters were determined from the K maps comparing several different methods. The surface brightness distribution was decomposed into axisymmetric components while bars and spiral structures were analyzed using Fourier techniques. Bulges were best represented by a Sersic r(1/n) law with an index in the typical range of 1-2. The central surface brightness of the exponential disk and bulge-to-disk ratio only showed weak correlation with Hubble type. Indications of a central point source were found in many of the galaxies. An additional central, steep, exponential disk improved the fit for more than 80% of the galaxies suggesting that many of the bulges are oblate. Bars down to the detection level at a relative amplitude of 3% were detected in 26 of 30 galaxies in a subsample classified as ordinary SA spirals. This would correspond to only 5% of all spiral galaxies being non-barred at this level. In several cases, bars are significantly offset compared to the starting points of the main spiral pattern which indicates that bar and spiral have different pattern speeds. A small fraction (similar to 10%) of the sample has complex central structures consisting of several sets of bars, arcs or spirals. A majority of the galaxies (similar to60%) displays a two-armed, grand-design spiral pattern in their inner parts which often breaks up into multiple arms in their outer regions. Phase shifts between the inner and outer patterns suggest in some cases that they belong to different spiral modes. The pitch angles of the main two-armed symmetric spiral pattern in the galaxies have a typical range of 5-30degrees. The sample shows a lack of strong, tight spirals which could indicate that such patterns are damped by non-linear, dynamical effects due to their high radial force perturbations.