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 NGC 891, an edge-on galaxy that has long been considered to be an analogue of the Milky Way. Using star-counts derived from deep Hubble Space Telescope/Advanced Camera for Surveys (HST/ACS) images, we detect the presence of a thick disc component in this galaxy with vertical scaleheight hZ= 1.44 ± 0.03 kpc and radial scalelength hR= 4.8 ± 0.1 kpc, only slightly longer than that of the thin disc. A stellar spheroid with a de Vaucouleurs-like profile is detected from a radial distance of r∼ 0.5 kpc to the edge of the survey at r∼ 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∼ 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 r∼ 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.