2012MNRAS.426.3490B

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 ∼ 25.2, that is, about 1.5mag 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 µ_V ∼ 32mag arcsec^–2, from which we determine a projected exponential scalelength of 0.70±0.01arcmin (i.e. 0.69±0.01kpc). The composite profile, ranging from the cored centre out to R = 7arcmin, is best fitted by an Elson–Fall–Freeman profile which gives a half-light radius of 1.41±0.04arcmin (i.e. 1.39±0.04kpc), 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 HI 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 HI 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.

Based on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.