Evolution of the UV Excess in Early-Type Galaxies

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  • 31 July 2008
Abstract
We examine the evolution of the UV emission from luminous early-type galaxies as a function of redshift. We perform a stacking analysis using Galaxy Evolution Explorer (GALEX) images of galaxies in the NOAO Deep Wide Field Survey (NDWFS) Bootes field and examine the evolution in the UV colors of the average galaxy. Our sample, selected to have limited ongoing star formation based on the optical to mid-IR SEDs of the galaxies, includes 1843 galaxies spanning the redshift range 0.05<z<0.65. We find evidence that the strength of the UV excess decreases, on average, with redshift, and our measurements also show moderate disagreement with previous models of the UV excess. Our results show little evolution in the shape of the UV excess with redshift, consistent either with the binary model for the formation of Extreme Horizontal Branch (EHB) stars or with no evolution in EHB morphology with look-back time. Finally, we see no significant influence of a galaxy's environment on the strength of its UV excess.

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