The Ultraviolet Upturn in Elliptical Galaxies as an Age Indicator

Abstract

The UV upturn phenomenon in elliptical galaxies, although challenging because of its complexity, is attractive for its potential value as an age indicator of old stellar systems. This work represents the combined efforts of two population synthesis groups with substantially different views to work together to minimize uncertainties in modeling and analysis. Unfortunately, this study, using the currently available data, cannot determine the metallicity of the dominant UV sources, one of the most outstanding problems related to the UV upturn phenomenon, as some input parameters need to be constrained better. We have found, however, that it is feasible to select a more likely model empirically because different models predict substantially different UV-to-V flux ratios as functions of redshift: metal-rich solutions predict a much steeper decline in the UV-to-V flux ratio than metal-poor solutions. We show that such differences in model predictions are quite independent of cosmology and are detectable using current and upcoming space UV facilities. The various alternatives suggest significantly different ages for the present-epoch giant ellipticals: the metal-rich solutions suggest 30%-50% smaller ages than the metal-poor solutions. Thus, an empirical fitting would not only reveal the origin of the UV upturn but yield independent age estimations for ellipticals. We show that this may effectively constrain some of the cosmological parameters that predict a unique age for the present-epoch galaxies. If we use the most recent estimations of H0 and Ω0, the younger, metal-rich models would have no conflict with a cosmology of a negligibly small Λ0, whereas the older, metal-poor models unavoidably suggest a substantially large value of Λ0 (i.e., Λ00.63 for zfor=∞) in the context of an inflationary universe.