Large scale star formation in galaxies I. The spirals NGC 7217, NGC 1058 and UGC 12732. Young star groupings in spirals

We present multiband photometric observations of three spiral galaxies sele cted in a sample suited for the study of young stellar groupings and their relationship with the parent galaxy and the galactic environment. Star form ing regions have been identified using an objective technique based on a mu ltivariate statistical analysis. Maps of young star groupings are given for each galaxy. The luminosity functions of the young star group populations show a remarkable similarity with a slope in the range -1.52 to -1.33. The size distributions peak around the classical 100 pc value of the Local Grou p associations for two out of the three galaxies. NGC 1058 shows smaller as sociations (peak at similar to 50 pc). The total number of young groups per unit B absolute luminosity of the galaxy is significantly greater in UGC 1 2732. The activity of star formation is in all three galaxies clearly stron ger in the central regions.