An HST/ACS investigation of the spatial and chemical structure and
  sub-structure of NGC 891, a Milky Way analogue
  We present a structural analysis of NGC891, an edge-on galaxy that has long
been considered to be an analogue of the Milky Way. Using starcounts derived
from deep HST/ACS images, we detect the presence of a thick disk component in
this galaxy with vertical scale height 1.44+/-0.03 kpc and radial scale length
4.8+/-0.1 kpc, only slightly longer than that of the thin disk. A stellar
spheroid with a de Vaucouleurs-like profile is detected from a radial distance
of 0.5 kpc to the edge of the survey at 25 kpc; the structure appears to become
more flattened with distance, reaching q = 0.50 in the outermost halo region
probed. The halo inside of 15 kpc is moderately metal-rich (median [Fe/H] ~
-1.1) and approximately uniform in median metallicity. Beyond that distance a
modest chemical gradient is detected, with the median reaching [Fe/H] ~ -1.3 at
20 kpc. We find evidence for subtle, but very significant, small-scale
variations in the median colour and density over the halo survey area. We argue
that the colour variations are unlikely to be due to internal extinction or
foreground extinction, and reflect instead variations in the stellar
metallicity. Their presence suggests a startling conclusion: that the halo of
this galaxy is composed of a large number of incompletely-mixed
sub-populations, testifying to its origin in a deluge of small accretions.
