UV-bright nearby early type galaxies observed in the mid-infrared:
  evidence for a multi-stage formation history by way of WISE and GALEX imaging
  In the local Universe, 10% of massive elliptical galaxies are observed to
exhibit a peculiar property: a substantial excess of ultraviolet emission (UVX)
over what is expected from their old, red stellar populations. Several origins
for the UVX have been proposed, including a population of hot young stars, or a
population of old, blue horizontal branch or extended horizontal branch (BHB or
EHB) stars that have undergone substantial mass loss from their outer
atmospheres. We explore the radial distribution of ultraviolet excess (UVX) in
a selection of 49 nearby E/S0-type galaxies by measuring the extended
photometry in the UV-midIR with GALEX, SDSS and WISE. We compare UV/optical and
UV/mid-IR colors with the Flexible Stellar Population Synthesis with EHB models
(Conroy & Gunn 2010). We find that combined WISE mid-IR and GALEX UV colors are
more effective in distinguishing models than optical colors, and that the
UV/mid-IR combination is sensitive to EHB fraction. There are strong color
gradients with the outer radii bluer than the inner half-light radii by ~1
magnitude. This color difference is easily accounted for with a BHB fraction
increase of 0.25 with radius. We estimated the average ages for the inner and
outer radii are 7.0+/-0.3 Gyr, and 6.2+/-0.2 Gyr, respectively, with the
implication that the outer regions are likely to have formed ~1 Gyr after the
inner regions. Additionally, we find that metallicity gradients are likely not
a significant factor in the color difference. The separation of color between
the inner and outer regions, which agrees with a specific stellar population
difference (e.g., higher EHB populations), and the ~0.5-2 Gyr age difference
suggests multi-stage formation. Our results are best explained by inside-out
formation: rapid star formation within the core at early epochs (>4 Gyr ago)
and at least one later stage starburst event coinciding with z~1
