A method for studying the star formation history of dwarf irregular galaxies: Pt. 1

In order to study the star formation history of dwarf irregular galaxies, we have obtained B and V photometry of 2250 stars down to a limiting magnitude B=24.5 on CCD images of two regions in the Local Group galaxy WLM. The resulting colour-magnitude diagrams have been compared with synthetic diagrams generated with the aid of a simulation code based on theoretical models of stellar evolution. A significant difference is found between the star formation activities in the two regions, one of which (Region 1) shows a young population due to star formation processes which stopped a few million years ago; this population is totally absent in Region 2. Both regions have an underlying population at least 109 yr old. (author)