OBSERVATIONAL TESTS FOR ULTRAVIOLET SOURCES IN ELLIPTIC GALAXIES

An understanding of the empirical correlation between the strength of the UV upturn and the metallicity of luminous elliptical galaxies (Bur stein and coworkers) is vital in the study of the evolution of ellipti cal galaxies. We support a modestly old (15-20 Gyr), metal-rich hypoth esis ([Fe/H] greater than or similar to 0) for the UV sources in UV up turn galaxies as an alternative to the old (greater than or similar to 20 Gyr), metal-poor ([Fe/H] less than or similar to -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 c omplete library of stellar evolution tracks, both of which are crucial in producing UV light in the metal-rich systems. The metal-rich hypot hesis has an advantage over the metal-poor one not only because it exp lains 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 met allicity of the UV sources.