The Ultraviolet Spectra of Old Stellar Populations

Abstract

We investigate the utility of UV spectra of old stellar populations as diagnostics of galaxy properties. For λ < 3800 Å, the integrated light of old stellar populations is dominated by two components: the hot stars which give rise to the UV upturn phenomenon at far-UV wavelengths, and the stars (i.e. main sequence, and subgiants) that lie closest to the turnoff. The mid-UV radiation from the turnoff varies strongly with metallicity, and less so with age; mid-UV light must be correct for the effect of the independent far-UV upturn component, but this is straightforward. Population synthesis models that account for the flux from the UV upturn can therefore determine the characteristics of the underlying stellar content from the mid-UV spectral region. The age & metallicity dependence of the far-UV 1550 Å is not well understood (see Dorman, O’Connell & Rood 1995, ApJ 442,105) since the mass loss mechanism on the RGB that produces very blue HB stars has no physical model. In contrast mid-UV indicators derive from the turnoff population which best represents the quantities we wish to measure, and are in addition visible at favourable optical wavelengths for z ≲1.