Structure and Population of the NGC55 Stellar Halo from a
  Subaru/Suprime-Cam Survey
  As part of our survey of galactic stellar halos, we investigate the structure
and stellar populations of the northern outer part of the stellar halo in
NGC55, a member galaxy of the Sculptor Group, using deep and wide-field V- and
I-band images taken with Subaru/Suprime-Cam. Based on the analysis of the
color-magnitude diagrams (CMDs) for red-giant-branch (RGB) stars, we derive a
tip of RGB (TRGB)-based distance modulus to the galaxy of (m-M)_0 = 26.58 +/-
0.11 (d = 2.1 +/- 0.1 Mpc). From the stellar density maps, we detect the
asymmetrically disturbed, thick disk structure and two metal-poor overdense
substructures in the north region of NGC55, which may correspond to merger
remnants associated with hierarchical formation of NGC55's halo. In addition,
we identify a diffuse metal-poor halo extended out to at least z ~ 16 kpc from
the galactic plane. The surface-brightness profiles toward the z-direction
perpendicular to the galactic plane suggest that the stellar density
distribution in the northern outer part of NGC55 is described by a locally
isothermal disk at z <~ 6 kpc and a likely diffuse metal-poor halo with V-band
surface brightness of mu_V >~ 32 mag arcsec^{-2}, where old RGB stars dominate.
We derive the metallicity distributions (MDs) of these structures on the basis
of the photometric comparison of RGB stars with the theoretical stellar
evolutionary models. The MDs of the thick disk structures show the peak and
mean metallicity of [Fe/H]peak ~ -1.4 and [Fe/H]mean ~ -1.7, respectively,
while the outer substructures show more metal-poor features than the thick disk
structure. Combined with the current results with our previous study for M31's
halo, we discuss the possible difference in the formation process of stellar
halos among different Hubble types.
