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~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~32 mag arcsec^-2,
from which we determine a projected exponential scalelength of 0.70'+-0.01'
(i.e. 0.69+-0.01 kpc). The composite profile, ranging from the cored centre out
to R=7', is best fit by an EFF profile which gives a half-light radius of
1.41'+-0.04' (i.e. 1.39+-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 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 the gas and stars.
