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 intersteller 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 a few selected cases, 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.
The evolution of galaxies over cosmic time scales is studied by deep observations of distant galaxy clusters and selected background fields. >From these observations important information on luminosity functions, populations, merger frequency etc. can be obtained. Investigations of colours will reveal very distant galaxies (with redshift z > 3).
The local burst of star formation in our Galaxy, giving rise to the phenomenon called Gould's Belt, is studied by means of observations by the HIPPARCOS satellite.