The stellar population content of the thick disc and halo of the Milky Way analogue NGC 891

We present deep VI images obtained with the Advanced Camera for Surveys on board the Hubble Space Telescope, covering three fields in the north-east side of the edge-on disc galaxy NGC 891. The observed fields span a wide range of galactocentric distances along the eastern minor axis, extending from the plane of the disc to 12 kpc, and out to similar to 25 kpc along the major axis. The photometry of individual stars reaches similar to 2.5 mag below the tip of the red giant branch. We use the astrophotometric catalogue to probe the stellar content and metallicity distribution across the thick disc and spheroid of NGC 891. The colour-magnitude diagrams of thick disc and spheroid population are dominated by old red giant branch stars with a wide range of metallicities, from the sparsely populated metal-poor tail at [Fe/H] similar to -2.4 dex, up to about half-solar metallicity. The peak of the metallicity distribution function of the thick disc is at -0.9 dex. The inner parts of the thick disc, within similar to 14 kpc along the major axis show no vertical colour/metallicity gradient. In the outer parts, a mild vertical gradient of Delta(V - I)(0)/Delta vertical bar Z vertical bar = 0.1 +/- 0.05 kpc (1) or less than 0.1 dex kpc (1) is detected, with bluer colours or more metal-poor stars at larger distances from the plane. 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-disc 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 metallicity distribution functions of the thick-disc stars show no significant changes in both the radial and the vertical directions. The stellar populations situated within the solar-cylinder-like distances show strikingly different properties from those of the Galaxy populating similar distances. This suggests that the accretion histories of both galaxies have been different. The spheroid population, composed of the inner spheroid and the halo, 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, corresponding to similar to 13r(eff). Similar to the thick-disc stars, large variations around the mean relation are present.