Ultraviolet Light from Old Stellar Populations

We consider the general theoretical problem of ultraviolet light from old stellar populations (t > 2 Gyr), and the interpretation of galaxy spectra at short wavelengths (lamda < 3200A) The sources believed to be responsible for the observed `ultraviolet upturn phenomenon' (UVX) are Post-AGB (P-AGB), extreme horizontal branch (EHB) and AGB-Manque (AGBM) stars. The production of EHB stars depends on mass loss on the red giant branch, which is poorly understood, making the far-UV flux problematical as an indicator of the gross properties, such as the age, of a given galaxy. We discuss the current state of the comparison between theory and observation for the UVX phenomenon, and revisit the interpretation of the well-known UVX-Mg2 correlation. In particular, we draw attention to the fact that the UVX appears not to be correlated with indices that measure the iron abundance, which has implications for models that explain the UV-Mg2 correlation as an abundance-driven effect. Finally, we note the potential utility of ultraviolet observations in distinguishing age and metallicity from galaxy spectral energy distributions.