Evolution of the UV Excess in Early-Type Galaxies

TL;DR

This study investigates how the UV excess in early-type galaxies evolves with redshift, revealing a decrease in UV excess strength over time and challenging existing models of its origin.

Contribution

It provides the first extensive analysis of UV excess evolution in early-type galaxies using GALEX data across a broad redshift range.

Findings

UV excess strength decreases with redshift

Little evolution in UV continuum shape with redshift

No significant environmental influence on UV excess

Abstract

We examine 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) Bo\"otes field and examine the evolution in the UV colors of the average galaxy. Our sample, selected to have minimal ongoing star formation based on the optical to mid-IR SEDs of the galaxies, includes 1843 galaxies spanning the redshift range $0.05\leq z\leq0.65$. 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 continuum 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. However, the binary formation model predicts that the strength of the UV excess should also be relatively constant, in contradiction with our measured results. Finally, we see no significant influence of a galaxy's environment on the strength of its UV excess.