The nature of the UV halo around the spiral galaxy NGC 3628
  Thanks to deep UV observations with GALEX and Swift, diffuse UV haloes have
recently been discovered around galaxies. Based on UV-optical colours, it has
been advocated that the UV haloes around spiral galaxies are due to UV
radiation emitted from the disc and scattered off dust grains at high
latitudes. Detailed UV radiative transfer models that take into account
scattering and absorption can explain the morphology of the UV haloes, and they
require the presence of an additional thick dust disc next the to traditional
thin disc for half of the galaxies in their sample. We test whether such an
additional thick dust disc agrees with the observed infrared emission in NGC
3628, an edge-on galaxy with a clear signature of a thick dust disc. We extend
the far-ultraviolet radiative transfer models to full-scale panchromatic
models. Our model, which contains no fine-tuning, can almost perfectly
reproduce the observed spectral energy distribution from UV to mm wavelengths.
These results corroborate the interpretation of the extended UV emission in NGC
3628 as scattering off dust grains, and hence of the presence of a substantial
amount of diffuse extra-planar dust. A significant caveat, however, is the
geometrical simplicity and non-uniqueness of our model: other models with a
different geometrical setting could lead to a similar spectral energy
distribution. More detailed radiative transfer simulations that compare the
model results to images from UV to submm wavelengths are a way to break this
degeneracy, as are UV polarisation measurements.
