Hubble Space Telescope Far-Ultraviolet Imaging of M31, M32, and NGC 205

Hubble Space Telescope Faint Object Camera (FOG) f/48 images of M31, M32, and NGC 205 (held of view 23'' x 23'' with 0''.45 pixel size) are analyzed as observed through the combined UV filters F150W and F130LP. The absolute calibration of the data and the internal disagreement between observed and expected count rates in the UV region lead us to suggest that the filter combination F150W + F130LP suffers from a 5 times degraded UV sensitivity. A corrected efficiency curve is constructed using the UV/optical spectral energy distributions of these three galaxies, which is consistent with all of the data analyzed here. Eighty-one individual stars are detected in M31, 10 stars in M32, and 78 stars in NGC 205. Comparisons with other UV images and optical images indicates that these stars are hot, UV-bright stars, even though our corrected efficiency curve suggests that flux from 1200-2450 Angstrom contributes only 7% of the counts in M32, 19% in M31, and 60% in NGC 205. The morphology of the galaxies in our images is consistent with existing data. The complex nucleus of M31 as seen by Lauer et al. (1993) is confirmed; M32 has a generally smooth appearance and NGC 205 is dominated by a UV-bright, somewhat resolved nucleus. Analysis of these data is done through the new, extensive stellar isochrones of Bertelli et al. (1994) and the population synthesis models of Bressan, Chiosi, and Fagotto (1994). This analysis shows that high-metal stars (Z > 0.05) evolve into UV-bright stars (P-EAGB, H-HB, and AGB-manque stars) that are less luminous and cooler but are significantly longer lived than the P-AGB stars produced by stars with Z < 0.05. Moreover, the proportion of P-EAGB, H-HB, and AGB-manque stars is also a function of age, with order stars of fixed mean metallicity having a higher proportion than younger stars. Hence, with either metallicity or age differences as an interpretation of the line-strength luminosity correlation for ellipticals, the high-metallicity ''tail'' of the stellar content of a galaxy can produce far-UV flux in much greater proportion than its actual proportion of galaxy mass. Separately, the UV-brightest stars in these stellar populations will be the shorter lived P-AGB stars and, hence, more readily observed in imaging observations such as ours. The resulting model of the sources of far-UV flux is inherently composite, with the total UV flux from a stellar population both rapidly increasing and changing its mean spectrum with increasing mean metallicity (or mean age). This model is consistent with five pieces of observational evidence: (1) the correlation of UV-optical color with metallicity documented by Burstein et al. (1988) for early-type galaxies, (2) the low absolute UV flux from M32, (3) the apparent composite nature of UV flux from giant E's and the bulge of M31, as seen by the Hopkins Ultraviolet Telescope (HUT) spectral observations of Ferguson and Davidsen (1993), (4) our FOC observations of P-AGB stars that contribute a minority UV flux in M31, and (5) the possibility that ellipticals might have correlations of either age or metallicity with absolute luminosity (Faber, Gonzalez, and Worthey 1992).