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Session 62 - Elliptical Galaxies: Dynamics and the UV Upturn.
Oral session, Tuesday, January 14
We have analyzed the far-ultraviolet (FUV) spectra of six elliptical and S0 galaxies in order to characterize their hot stellar populations. The spectra were obtained using the Hopkins Ultraviolet Telescope (HUT) in March 1995. These data, together with the spectra of two galaxies observed with HUT in 1990, represent the only FUV spectra of early type galaxies that extend to the Lyman limit at 912 Å\ and therefore include the ``turnover'' in the spectral energy distribution below Lyman alpha. Using an extensive new grid of synthetic spectra which match the HUT resolution and cover the relevant parameter space of temperature and gravity, we have constructed synthetic spectral energy distributions by integrating over predicted stellar evolutionary tracks for horizontal branch stars and their progeny. When the models are compared with the HUT data, we find that those with supersolar metal abundances and helium best reproduce the flux across the entire HUT wavelength range, while those with subsolar Z amp; Y fit less well, partly because of a significant flux deficit shortward of 970 Å\ in the models. We find that AGB-Manqu\acutee evolution is required in all fits to the HUT spectra, suggesting that all of the galaxies have some subdwarf B star population. At any Z amp; Y, the models 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. We find that most absorption features in the spectra are consistent with Z = 0.1 Z_\sun, 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.
Program listing for Tuesday