HUBBLE-SPACE-TELESCOPE FAR-ULTRAVIOLET IMAGING OF M31, M32, AMD NGC-205

Hubble Space Telescope Faint Object Camera (FOG) f/48 images of M31, M 32, 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 be tween observed and expected count rates in the UV region lead us to su ggest that the filter combination F150W + F130LP suffers from a 5 time s degraded UV sensitivity. A corrected efficiency curve is constructed using the UV/optical spectral energy distributions of these three gal axies, which is consistent with all of the data analyzed here. Eighty- one individual stars are detected in M31, 10 stars in M32, and 78 star s in NGC 205. Comparisons with other UV images and optical images indi cates that these stars are hot, UV-bright stars, even though our corre cted efficiency curve suggests that flux from 1200-2450 Angstrom contr ibutes only 7% of the counts in M32, 19% in M31, and 60% in NGC 205. T he morphology of the galaxies in our images is consistent with existin g data. The complex nucleus of M31 as seen by Lauer et al. (1993) is c onfirmed; M32 has a generally smooth appearance and NGC 205 is dominat ed by a UV-bright, somewhat resolved nucleus. Analysis of these data i s done through the new, extensive stellar isochrones of Bertelli et al . (1994) and the population synthesis models of Bressan, Chiosi, and F agotto (1994). This analysis shows that high-metal stars (Z > 0.05) ev olve into UV-bright stars (P-EAGB, H-HB, and AGB-manque stars) that ar e 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 a n interpretation of the line-strength luminosity correlation for ellip ticals, the high-metallicity ''tail'' of the stellar content of a gala xy 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 resul ting model of the sources of far-UV flux is inherently composite, with the total UV flux from a stellar population both rapidly increasing a nd changing its mean spectrum with increasing mean metallicity (or mea n age). This model is consistent with five pieces of observational evi dence: (1) the correlation of UV-optical color with metallicity docume nted by Burstein et al. (1988) for early-type galaxies, (2) the low ab solute 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 Ultraviole t 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 co rrelations of either age or metallicity with absolute luminosity (Fabe r, Gonzalez, and Worthey 1992).