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Detection of Ultraviolet Halos around Highly Inclined Galaxies

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(2014)cite arxiv:1401.4170Comment: 33 pages, 20 figures, 7 tables, submitted to ApJ.

Abstract

We report the discovery of diffuse ultraviolet light around late-type galaxies out to 5-20 kpc from the midplane using Swift and GALEX images. The emission is consistent with the stellar outskirts in the early-type galaxies but not in the late-type galaxies, where the emission is quite blue and consistent with a reflection nebula powered by light escaping from the galaxy and scattering off dust in the halo. Fitting a simple reflection nebula model to the halo SEDs points to SMC-type dust (lacking a UV bump), and the halo colors and luminosities are consistent with this scenario. Our results agree with expectations from halo dust discovered (at larger radii) in extinction by Menard et al. (2010) to within a few kpc of the disk and imply a comparable amount of hot and cold gas in galaxy halos (a few x10^8 Msun within 20 kpc) if the dust resides primarily in Mg II absorbers.

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