We derive quantitative star formation histories of the four suspected tidal dwarf galaxies in the M81 group , Holmberg IX , BK3N , Arp-loop ( A0952+69 ) , and Garland , using Hubble Space Telescope/Wide Field Planetary Camera 2 images in F606W and F814W obtained as part of a Snapshot survey of dwarf galaxies in the Local Universe .
We consider the spatial distribution and ages of resolved stellar populations in these dwarf irregular galaxies .
We use synthetic color-magnitude diagrams to derive the ages of the major star formation episodes , star formation rates , and approximate metallicity ranges .
All the galaxies show evidence of continuous star formation between about 20 and 200 Myr ago with star formation rates in the range 7.5 \cdot 10 ^ { -3 } – 7.6 \cdot 10 ^ { -4 } M _ { \sun } yr ^ { -1 } .
The metallicity of the detected stars spans a wide range , and have lower than solar abundance .
A possible scenario is that all four dwarf galaxies were formed from material in the metal-poor outer part of the giant spiral galaxy M 81 after the tidal interaction between M 81 , M 82 , and NGC 3077 \sim 200 Myr ago .
While we do not directly detect pronounced old stellar populations , the photometric limits of our data are such that the presence of such a population is not entirely ruled out .