Diffuse Extraplanar Dust in NGC 891

We report the detection of vertically extended far-ultraviolet and near-UV emissions in an edge-on spiral galaxy NGC 891, which we interpret as being due to dust-scattered starlight. Three-dimensional radiative transfer models are used to investigate the content of the extraplanar dust that is required to explain the UV emission. The UV halos are well reproduced by a radiative transfer model with two exponential dust disks, one with a scale height of ≈0.2–0.25 kpc and the other with a scale height of ≈1.2–2.0 kpc. The central face-on optical depth of the geometrically thick disk is found to be at the B band. The results indicate that the dust mass at | z | > 2 kpc is ≈3%–5% of the total dust mass, which is in good accordance with the recent Herschel submillimeter observation. Our results, together with the recent discovery of the UV halos in other edge-on galaxies, suggest the widespread existence of a geometrically thick dust layer above the galactic plane in spirals.