Diffuse extraplanar dust in NGC 891

Abstract

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 ≈0.20.25 kpc and the other with a scale height of ≈1.22.0 kpc. The central face-on optical depth of the geometrically thick disk is found to be τthickB≈ 0.30.5 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. © 2014. The American Astronomical Society. All rights reserved. Printed in the U.S.A.