THE HOT STELLAR COMPONENT IN ELLIPTIC GALAXIES AND SPIRAL BULGES .1. THE FAR-ULTRAVIOLET SPECTRUM OF THE BULGE OF M31

We present a spectrum of the bulge of M31 from 1850 angstrom to the Lyman limit measured through a 9'' x 116'' aperture by the Hopkins Ultraviolet Telescope (HUT) during the Astro-1 space shuttle mission in 1990 December. Apart from airglow, no significant emission features are present in the spectrum. A search for stellar absorption features reveals several significant detections, but uncertainties in the interstellar-medium contribution and the temperatures of the stars contributing to the spectrum preclude any definite conclusions about the metallicity of the stellar population. We compare the HUT spectra of the M31 bulge and of NGC 1399, the central giant elliptical in the Fornax Cluster. We find significant differences that are relatively insensitive to the assumptions about extinction. Evidently the stars producing the far-UV upturn in UV-bright galaxies such as NGC 1399 are not just more of the same stars producing the emission in UV-fainter galaxies like M31. We investigate the possibility that the two spectral-energy distributions (SEDs) differ only in the fractional contribution from classical post asymptotic giant branch (PAGB) stars and in the amount of extinction. We find an acceptable fit to the M31 spectrum for E(B - V) = 0.11 if approximately 65% of the flux (at approximately 1400 angstrom) comes from PAGB stars and the rest comes from stars of the type producing the far-UV emission in NGC 1399. The known PAGB population from optical observations (central stars of planetary nebulae) can account for less than 1% of the total far-UV flux. The pointlike sources recently detected at 1750 angstrom by the HST Faint Object Camera are probably PAGB stars with masses lower than planetary-nebula central stars (thought to be approximately 0.6 M.). These sources too appear to account for only a small fraction (approximately 15%) of the flux observed by HUT at 1750 angstrom. These results suggest that PAGB stars with still lower masses, which would be fainter than the FOC detection limit for point sources, contribute to the M31 SED. We examine the hypothesis that the UV continuum in both galaxies is dominated by stars in post-horizontal branch (post-HB) phases of evolution, with a distribution of post-HB masses governed by the mean metallicity and metallicity spread of the population. The number of UV photons produced per star increases toward lower mass, while the characteristic temperature of the UV emitting population decreases. In this scenario, the far-UV flux of NGC 1399 is produced mostly by low-mass extreme horizontal branch, post-early-AGB, and/or AGB-Manque stars, while in M31 there is a large contribution from classical PAGB stars as well. We can thus qualitatively explain both the relative fluxes of the UV rising branches in NGC 1399 and M31, and the shapes of their SEDs. This scenario makes a prediction, which we confirm from existing surveys, that the number of planetary nebulae per unit luminosity is anticorrelated with the strength of the UV upturn. It also makes testable predictions for the far-UV SEDs of low-metallicity ellipticals and the spectral evolution of giant ellipticals.