A deep, wide-field study of Holmberg II with Suprime-Cam: evidence for ram pressure stripping

We present a deep, wide-field optical study of the M81 group dwarf galaxy Holmberg II (HoII) based on Subaru/Suprime-Cam imaging. Individual stars are resolved down to I similar to 25.2, that is, about 1.5?mag below the tip of the red giant branch (RGB). We use resolved star counts in the outskirts of the galaxy to measure the radial surface brightness profile down to mu V similar to 32?mag arcsec-2, from which we determine a projected exponential scalelength of 0.70 +/- 0.01?arcmin (i.e. 0.69 +/- 0.01?kpc). The composite profile, ranging from the cored centre out to R = 7?arcmin, is best fitted by an ElsonFallFreeman profile which gives a half-light radius of 1.41 +/- 0.04?arcmin (i.e. 1.39 +/- 0.04?kpc), and an absolute magnitude MV = -16.3. The low surface brightness stellar component of HoII is regular and symmetric and has an extent much smaller than the vast H?i cloud in which it is embedded. We compare the spatial distribution of the young, intermediate-age and old stellar populations, and find that the old RGB stars are significantly more centrally concentrated than the young stellar populations, contrary to what is observed in most dwarf galaxies of the local Universe. We discuss these properties in the context of the comet-like distribution of H?i gas around HoII, and argue for the presence of a hot intragroup medium in the vicinity of HoII to explain the contrasting morphologies of gas and stars.