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 \sim 25.2 , i.e .
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 } \sim 32 mag arcsec ^ { -2 } , from which we determine a projected exponential scalelength of 0.70 \arcmin \pm 0.01 \arcmin ( i.e . 0.69 \pm 0.01 kpc ) .
The composite profile , ranging from the cored centre out to R=7 \arcmin , is best fit by an EFF profile which gives a half-light radius of 1.41 \arcmin \pm 0.04 \arcmin ( i.e . 1.39 \pm 0.04 kpc ) , and an absolute magnitude M _ { V } = - 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 the gas and stars .