The stellar population content of the thick disk and halo of the Milky
  Way analogue NGC 891
  We present deep ACS images of 3 fields in the edge-on disk galaxy NGC 891,
which extend from the plane of the disk to 12 kpc, and out to 25 kpc along the
major axis. The photometry of individual stars reaches 2.5 magnitudes below the
tip of the RGB. We use the astrophotometric catalogue to probe the stellar
content and metallicity distribution across the thick disk and spheroid of NGC
891. The CMDs of thick disk and spheroid population are dominated by old RGB
stars with a wide range of metallicities, from a metal-poor tail at [Fe/H] ~
-2.4 dex, up to about half-solar metallicity. The peak of the MDF of the thick
disk is at -0.9 dex. The inner parts of the thick disk, within 14 kpc along the
major axis show no vertical colour/metallicity gradient. In the outer parts, a
mild vertical gradient of Delta(V-I)/Delta|Z| = 0.1 +/- 0.05 kpc^-1 is
detected. This gradient is however accounted for by the mixing with the metal
poor halo stars. No metallicity gradient along the major axis is present for
thick disk stars, but strong variations of about 0.35 dex around the mean of
[Fe/H] = -1.13 dex are found. The properties of the asymmetric MDFs of the
thick disk stars show no significant changes in both the radial and the
vertical directions. The stellar populations at solar cylinder-like distances
show strikingly different properties from those of the Galaxy, suggesting that
the accretion histories of both galaxies have been different. The spheroid
population shows remarkably uniform stellar population properties. The median
metallicity of the halo stellar population shows a shallow gradient from about
-1.15 dex in the inner parts to -1.27 dex at 24 kpc distance from the centre.
Similar to the thick disk stars, large variations around the mean relation are
present.
