| Description: | NGC 891 is an edge-on galaxy that is referred to in the literature as the Milky Way analogue. I will give a summary of recent investigations of the thick disk and halo of this galaxy based on deep Hubble Space Telescope ACS observations and compare them with what we know about our own galaxy.
Using the deep ACS imaging observations of three fields in this galaxy, we resolved the stellar content in NGC891. From the star counts we detected the presence of a thick disk component with vertical scaleheight h_Z = 1.44 ± 0.03kpc and radial scalelength h_R = 4.8 ± 0.1kpc, only slightly longer than that of the thin disc. The stellar spheroid, probing the halo population, presents a modest chemical gradient, with the median reaching [Fe/H] ~ -1.3 at r ~ 20kpc. Within the distance of the solar-like radius the metallicity distribution is significantly different with respect to that of the Milky Way. I will argue that we found evidence for significant small-scale variations in the median colour and density in NGC 891 halo, that are most likely due to variations in the stellar metallicity. Their presence suggests that the halo of this galaxy is composed of a large number of incompletely mixed sub-populations, testifying to its past accretion history. This is now being confirmed with our new wide field images from Subaru that indicate a turbulent past accretion history in NGC 891. |
