Helium sedimentation and the ultraviolet upturn in brightest cluster galaxies

Abstract

Recent observations with the Galaxy Evolution Explorer (GALEX) show strong unexpected ultraviolet (UV) excess in the spectrum of brightest cluster galaxies (BCGs). It is believed that the excess UV signal is produced by old and evolved core-He burning stars, and the UV flux strength could be greatly enhanced if the progenitor stars have a high value of He abundance. In this work, we propose that the sedimentation process can greatly enhance the He abundance in BCGs. Our model predicts that the UV flux strength is stronger in more massive, low-redshift, and dynamically relaxed BCGs. These predictions are testable with the current generation of GALEX+SDSS observations. © 2009. The American Astronomical Society. All rights reserved.