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.

The ultraviolet upturn in brightest cluster galaxies

“… We find significant differences between the UV‐upturn distributions between BCGs and normal ellipticals in the same mass range in that the BCGs have higher UV upturns and are less scattered than the normal ellipticals. We do not find strong evidence in our data to support the predictions made from the models of Peng & Nagai (2009) that the UV‐upturn phenomena should be most pronounced in BCGs hosted by high‐mass, dynamically relaxed, cooling flow systems as a result of helium sedimentation. We find no clear correlation between the BCG velocity dispersion (mass) and the UV upturn, but we find the UV upturn of the BCG sample to be much less scattered with velocity dispersion than the non‐BCGs over the same mass range. We also find no clear correlation between the UV upturn and luminosity, but the BCGs are less scattered than the normal ellipticals. We do not find the UV‐upturn–Mg correlation reported for some samples of ordinary elliptical galaxies (Burstein et al 1988; Donas et al 2007), but we do find that the values are much less scattered than those of the normal elliptical sample that we use as reference.…”

“…We pay particular attention to the model of Peng & Nagai (2009). They pointed out that the helium sedimentation process can greatly increase the helium abundance in BCGs making them conducive to the formation of UV‐bright HB stars.…”

“…It has been suggested that helium sedimentation occurs in the intracluster medium (ICM) of galaxy clusters (Abramopoulos, Chanan & Ku 1981; Gilfanov & Syunyaev 1984; Qin & Wu 2000; Chuzhoy & Nusser 2003; Ettori & Fabian 2006; Peng & Nagai 2009). Under the influence of gravity, the heavier helium nuclei accumulate at the centres of massive galaxy clusters and can produce significant amounts of He abundance (Δ Y > 0.25) in the central regions of hot, massive clusters.…”

Outskirts of Galaxy Clusters

“…More details can be found, e.g., in Ettori & Fabian (2006);Markevitch (2007); Peng & Nagai (2009);Bulbul et al (2011), and references therein.…”

The current status on the UV upturn

“…Theories must now explain why only a small fraction of ellipticals show a UV upturn. An interesting scenario based on helium sedimentation preferentially occuring in brightest cluster galaxies has recently been suggested (Peng & Nagai 2009). While this scenario is not without problems, it poses an interesting possibility.…”