TIDAL DWARFS IN THE M81 GROUP: THE SECOND GENERATION?

Abstract. We present a quantitative star formation history derivation of the four suspected tidal dwarf galaxies in the M 81 group: Holmberg IX, BK3N, Arp-loop (A0952+69) and Garland us-ing HST/WFPC2 images of these galaxies. We construct a library of synthetic Colour-Magnitude Diagrams (CMDs) based on theoretical isochrones and data-derived determinations of photometric errors. These synthetic CMDs were combined linearly and χ2-compared to observed photometry. All the galaxies show continuous star formation between about 20 and 200 Myr ago with star formation rates between 7.5·10−3 M/yr and 7.6·10−4 M/yr. The metallicity of the detected stars is spanning rather a wide range, being lower than solar abundance. We suppose, that all the galaxies were formed out of material from metal-poor outer part of the giant spiral galaxy M81 after tidal interaction about 200 Myr ago. However, this suggestion requires significantly more deep color-magnitude diagrams to be sure with the scenario of the galaxy evolution