Hopkins Ultraviolet Telescope Observations of Six Elliptical and S0 Galaxies
Thomas M. Brown
Department of Physics and Astronomy, Johns Hopkins University
Charles and 34th Streets, Baltimore, MD 21218
Henry C. Ferguson
Hubble Fellow, Space Telescope Science Institute
3700 San Martin Drive, Baltimore, MD 21218
and
Arthur F. Davidsen
Department of Physics and Astronomy, Johns Hopkins University
Charles and 34th Streets, Baltimore, MD 21218
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 M 49, M 60, M 87, M 89, 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 Endeavor in March 1995, and
cover the spectral range of 820 Å 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 
, 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 Mg
absorption line strength.
Subject headings: galaxies: evolution --- galaxies: stellar content --- ultraviolet: galaxies --- ultraviolet: stars
tbrown@pha.jhu.edu