Observational Tests for Ultraviolet Sources in Elliptical Galaxies

Abstract

An understanding of the empirical correlation between the strength of the UV upturn and the metallicity of luminous elliptical galaxies (Burstein and coworkers) is vital in the study of the evolution of elliptical galaxies. We support a modestly old (15-20 Gyr), metal-rich hypothesis ([Fe/H] 0) for the UV sources in UV upturn galaxies as an alternative to the old (20 Gyr), metal-poor ([Fe/H] -1.0) hypothesis of Park & Lee. This preliminary result of our population synthesis is based on a Gaussian mass distribution on the horizontal branch and a complete library of stellar evolution tracks, both of which are crucial in producing UV light in the metal-rich systems. The metal-rich hypothesis has an advantage over the metal-poor one not only because it explains the UV upturn-metallicity relation but also because it requires neither an uncomfortably large age nor a large age span for elliptical galaxies. We suggest several observational tests to constrain the metallicity of the UV sources.