Hopkins Ultraviolet Telescope Observations of Six Elliptical and S0 Galaxies

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Abstract
In order to better understand the hot stellar populations of E and S0 galaxies, we observed six objects using the Hopkins Ultraviolet Telescope. Through our 10'' × 56'' aperture, we observed the centers of M49, M60, M87, M89, NGC 3115, and NGC 3379 with varying exposure times and signal-to-noise. The far-ultraviolet (FUV) spectra were obtained during orbital night as part of the Astro-2 mission on the Space Shuttle Endeavour in 1995 March, and cover the spectral range of 820 to 1840 Å with a resolution of 2-4 Å. This sample quadruples the number of early-type galaxies studied to the Lyman limit. After correcting for geocoronal emission and interstellar extinction in our own Galaxy, all the spectra are similar, even though the "UV upturn" strength, as characterized by the parameter m1550 - V, varies over the range 2.04-3.86 mag for these galaxies. Comparison with models of evolved stellar populations confirms the conclusion from Astro-1 data that the FUV flux can be explained by the post horizontal branch evolution of stars with a narrow range of temperature and envelope mass on the extreme horizontal branch (EHB). These comparisons also put constraints on the FUV flux contribution from post-asymptotic giant branch (PAGB) stars. Furthermore, we find that the EHB stellar evolutionary flux deduced from the model fits to our data is positively correlated with the Mg2 absorption line strength.