The general structure of galaxies and their composition of stars of different ages as well as the existence of ``dark'' unseen matter are studied at Stockholm Observatory by deep CCD photometry in the optical and near-infrared regions, complemented by observations in the 21-cm line of interstellar hydrogen with large radio interferometers.
The spiral structure of galaxies is the manifestation of a density wave in the stellar population and in the interstellar matter. Star formation is triggered in interstaller matter compressed in this density wave. The case of so-called barred galaxies, where the spiral structure is caused by the gravitational influence of the bar, seems to be best understood. Mapping of the velocity field from interstellar emission lines for six isolated barred spiral (SB) galaxies, carried out at the observatory, compared with computer simulations of such systems, demonstrates how the spiral structure is created by the influence of a series of gravitational resonances caused by the rotation of the bar. The bar also forces interstellar matter into the nuclear region, thus seemingly providing fuel for nuclear activity and for extensive ejection of excited gas and relativistic plasma from the nucleus normal to the symmetry plane of the galaxy. No evidence is found supporting the hypothesis of a low amount of dark matter being characteristic of SB galaxies. The presence of companion galaxies is found to correlate with an increased statistical spread in the neutral hydrogen (HI) extent. It is concluded that the selection of galaxies with large HI extent may introduce a bias towards tidally interacting systems.
An in-depth study of the dynamics of the SB galaxy NGC 1300 is presented. Multi-wavelength data yield an estimate of the velocity field and gravitational potential. Subsequent hydrodynamical simulations are able to reproduce the morphology and kinematics in the bar region using a pure bar perturbing potential. To reproduce the spiral structure a weak spiral component has to be added, indicative of stellar spiral response to the bar and/or self-gravitating gas in the arms. Two separate models, differing mainly with respect to pattern speed and associated resonance structure, are found to reproduce the observations.