Diffuse Extraplanar Dust in NGC 891
We report the detection of vertically extended far-ultraviolet (FUV) 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 scaleheight of $\approx$ 0.2 - 0.25 kpc and the other with a scaleheight of $\approx$ 1.2 - 2.0 kpc. The central face-on optical depth of the geometrically thick disk is found to be $\tau_{thick} \approx$ 0.3 - 0.5 at B-band. The results indicate that the dust mass at |z| > 2 kpc is $\approx$ 3 - 5% of the total dust mass, which accord well with the recent Herschel sub-millimeter observation. Our results, together with the recent discovery of the UV halos in other edge-on galaxies, suggest the widespread existence of the geometrically thick dust layer above the galactic plane in spirals.