The stellar halo of the edge-on galaxy NGC 891



Isaac Roberts Fellow.


Using the Advanced Camera for Surveys on board the Hubble Space Telescope, we have resolved individual red giant branch stars in the outskirts of the edge-on galaxy NGC 891, down to ∼2 mag fainter than the tip of the red giant branch. The data set allows investigation of the stellar populations in the outer regions of an analogue galaxy to the Milky Way. We discuss the properties of the stellar populations located at ∼10 kpc above the galaxy disc. We find that these extraplanar stellar populations exhibit a broad red giant branch star colour distribution, suggestive of a large spread in heavy element abundance. The mean stellar metallicity at this height from the galaxy disc, derived using the mean colours of red giant branch stars, is [Fe/H]≈−0.9, a full ∼0.5 dex more metal rich than the mean metallicity of the non-rotationally supported stars in the inner haloes of the Galaxy and Andromeda. This suggests that metal-poor stars are not the dominant stellar component in the outskirts of massive spiral galaxies.