The ultraviolet upturn in elliptical galaxies as an age indicator

The UV upturn phenomenon in elliptical galaxies, although challenging becau se of its complexity, is attractive for its potential value as an age indic ator of old stellar systems. This work represents the combined efforts of t wo population synthesis groups with substantially different views to work t ogether to minimize uncertainties in modeling and analysis. Unfortunately, this study, using the currently available data, cannot determine the metall icity of the dominant UV sources, one of the most outstanding problems rela ted to the UV upturn phenomenon, as some input parameters need to be constr ained better. We have found, however, that it is feasible to select a more likely model empirically because different models predict substantially dif ferent UV-to-V flux ratios as functions of redshift: metal-rich solutions p redict a much steeper decline in the UV-to-V flux ratio than metal-poor sol utions. We show that such differences in model predictions are quite indepe ndent of cosmology and are detectable using current and upcoming space UV f acilities. The various alternatives suggest significantly different ages fo r 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 ag e estimations for ellipticals. We show that this may effectively constrain some of the cosmological parameters that predict a unique age for the prese nt-epoch galaxies. If we use the most recent estimations of H-0 and Omega(0 ), the younger, metal-rich models would have no conflict with a cosmology o f a negligibly small Lambda(0),, whereas the older, metal-poor models unavo idably suggest a substantially large value of A, (i.e., Lambda(0) greater t han or similar to 0.63 for z(for) = infinity) in the context of an inflatio nary universe.