We present a structural analysis of NGC 891 , 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 h _ { Z } = 1.44 \pm 0.03 { kpc } and radial scale length h _ { R } = 4.8 \pm 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 r \sim 0.5 { kpc } to the edge of the survey at r \sim 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 r \sim 15 { kpc } is moderately metal-rich ( median { [ Fe / H ] \sim - 1.1 } ) and approximately uniform in median metallicity .
Beyond that distance a modest chemical gradient is detected , with the median reaching { [ Fe / H ] \sim - 1.3 } at r \sim 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 .