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 through mid-IR with GALEX , SDSS and WISE .
We compare UV/optical and UV/mid-IR colors with the Conroy & Gunn ( 2010 ) Flexible Stellar Population Synthesis models , which allow for the inclusion of EHB stars .
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 \sim 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 \pm 0.3 Gyr , and 6.2 \pm 0.2 Gyr , respectively , with the implication that the outer regions are likely to have formed \sim 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 \sim 0.5 to 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 \sim 1 .