2010ApJ...714L..12M

Recent panoramic observations of the dominant spiral galaxies of the Local Group have revolutionized our view of how these galaxies assemble their mass. However, it remains completely unclear whether the properties of the outer regions of the Local Group large spirals are typical. Here, we present the first panoramic view of a spiral galaxy beyond the Local Group, based on the largest, contiguous, ground-based imaging survey to date resolving the stellar halo of the nearest prime analog of the Milky Way, NGC 891 (D ≈ 10 Mpc). The low surface brightness outskirts of this galaxy are populated by multiple, coherent, and vast substructures over the ∼90 kpc x90 kpc extent of the survey. These include a giant stream, the first to be resolved into stars beyond the Local Group using ground-based facilities, that loops around the parent galaxy up to distances of ∼50 kpc. The bulge and the disk of the galaxy are found to be surrounded by a previously undetected large, flat, and thick cocoon-like stellar structure at vertical and radial distances of up to ∼15 kpc and ∼40 kpc, respectively.