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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