2008ApJ...676L.113S

We present a study of the M81 companion dwarf irregular galaxy Holmberg IX using deep images taken with the Wide Field Channel of the Advanced Camera for Surveys on board the Hubble Space Telescope. Based on color-magnitude diagrams the resolved stellar population toward Holmberg IX contains numerous stars with ages of ≲200 Myr as well as older red giant stars. By charting the spatial distribution of the red giant stars and considering their inferred metallicities, we conclude that most of these older stars are associated with M81 or its tidal debris. At least 20% of the stellar mass in Holmberg IX was produced in the last ∼200 Myr, giving it the youngest stellar population of any nearby galaxy. The location of Holmberg IX, its high gas content, and its youthful stellar population suggest that it is a tidal dwarf galaxy, perhaps formed during the last close passage of M82 around M81.