Constraints on the Stellar Populations of Elliptical Galaxies from Ultraviolet Spectra

Abstract

We present preliminary results from spectral synthesis models of old stellar populations for the spectral range 912-4000 A with ~ 10A resolution, which can be used to investigate the UVX phenomenon and to assess ages and abundances. Model spectra incorporating extreme horizontal branch (EHB) and Post-Asymptotic Giant Branch (P-AGB) populations give good matches to the far-UV spectra of galaxies. These models indicate an EHB fraction which is < 10% of the total HB population in all but the most extreme examples of the UVX phenomenon, where the EHB fraction is still <= 20. Once the hot component that gives rise to the UVX phenomenon is accounted for, the mid-UV wavelength range (2200 < lambda < 3300 A) provides information about the age and metallicity of the underlying stellar population. The flux in this spectral range arises mainly from stars close to the main sequence turnoff. We compare models with the spectrum of M31 and discuss UV features which should be useful as population diagnostics.