A FAR-ULTRAVIOLET ANALYSIS OF THE STELLAR POPULATIONS IN 6 ELLIPTIC AND S0 GALAXIES

We have analyzed the far-ultraviolet (FUV) spectra of six elliptical a nd S0 galaxies in order to characterize the stellar population that pr oduces the ultraviolet flux in these galaxies. The spectra were obtain ed using the Hopkins Ultraviolet Telescope (HUT) during the Astro-2 mi ssion aboard the Space Shuttle Endeavour in 1995 March and cover the s pectral range from 820 to 1840 Angstrom with a resolution of 3 Angstro m. These data, together with the spectra of two galaxies observed with HUT on the Astro-1 mission, represent the only FUV spectra of early-t ype galaxies that extend to the Lyman limit at 912 Angstrom and theref ore include the ''turnover'' in the spectral energy distribution below Ly alpha. Using an extensive new grid of LTE and non-LTE synthetic sp ectra that match the HUT resolution and cover the relevant parameter s pace of temperature and gravity, we have constructed synthetic spectra l energy distributions by integrating over various predicted stellar e volutionary tracks for horizontal-branch stars and their progeny. When the computed models are compared with the HUT data, we find that mode ls with supersolar metal abundances and helium best reproduce the flux across the entire HUT wavelength range, while those with subsolar Z a nd Y fit less well, partly because of a significant flux deficit short ward of 970 Angstrom in the models. High-Z models are preferred becaus e the contribution from the later, hotter, post-HE evolutionary stages makes up a higher fraction of the sub-Ly alpha flux in these tracks. We find that AGB-Manque evolution is required in all of the fits to th e HUT spectra, which suggests that all of the galaxies have some subdw arf B-star population. At any Z and Y, the model spectra that best mat ch the HUT flux are dominated by stars evolving from a narrow range of envelope mass on the blue end of the horizontal branch. The Astro-1 a nd Astro-2 data are also the first with the resolution and signal-to-n oise ratio needed to detect and measure absorption lines in the FUV sp ectra of elliptical galaxies, which allows a direct estimate of the ab undances in the atmospheres of the stars that produce the UV flux. We find that most absorption features in the spectra are consistent with Z = 0.1 Z., significantly lower than the abundances implied by the bes t-fitting spectral energy distributions. However, given the strong obs ervational and theoretical evidence for diffusion processes in the atm ospheres of evolved stars, the observed atmospheric abundances may not reflect the interior abundances in the population producing the ultra violet flux in elliptical galaxies.