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 approximate to 0.2-0.25 kpc and the other with a scale height of approximate to 1.2-2.0 kpc. The central face-on optical depth of the geometrically thick disk is found to be tau(thick)(B) approximate to 0.3-0.5 at the B band. The results indicate that the dust mass at vertical bar z vertical bar > 2 kpc is approximate to 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.