A Far‐Ultraviolet Analysis of the Stellar Populations in Six Elliptical and S0 Galaxies

Abstract

We have analyzed the far-ultraviolet (FUV) spectra of six elliptical and S0 galaxies in order to characterize the stellar population that produces the ultraviolet flux in these galaxies. The spectra were obtained using the Hopkins Ultraviolet Telescope (HUT) during the Astro-2 mission aboard the Space Shuttle Endeavour in 1995 March and cover the spectral range from 820 to 1840 A with a resolution of 3 A. These data, together with the spectra of two galaxies observed with HUT on the Astro-1 mission, represent the only FUV spectra of early-type galaxies that extend to the Lyman limit at 912 A and therefore include the turnover in the spectral energy distribution below Lyα. Using an extensive new grid of LTE and non-LTE synthetic spectra that match the HUT resolution and cover the relevant parameter space of temperature and gravity, we have constructed synthetic spectral energy distributions by integrating over various predicted stellar evolutionary tracks for horizontal-branch stars and their progeny. When the computed models are compared with the HUT data, we find that models with supersolar metal abundances and helium best reproduce the flux across the entire HUT wavelength range, while those with subsolar Z and Y fit less well, partly because of a significant flux deficit shortward of 970 A in the models. High-Z models are preferred because the contribution from the later, hotter, post-HB evolutionary stages makes up a higher fraction of the sub-Lyα flux in these tracks. We find that AGB-Manque evolution is required in all of the fits to the HUT spectra, which suggests that all of the galaxies have some subdwarf B-star population. At any Z and Y, the model spectra that best match 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 and Astro-2 data are also the first with the resolution and signal-to-noise ratio needed to detect and measure absorption lines in the FUV spectra of elliptical galaxies, which allows a direct estimate of the abundances 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 best-fitting spectral energy distributions. However, given the strong observational and theoretical evidence for diffusion processes in the atmospheres of evolved stars, the observed atmospheric abundances may not reflect the interior abundances in the population producing the ultraviolet flux in elliptical galaxies.