Before the advent of space astronomy, it was generally believed that elliptical galaxies would be dark in the UV. Optical spectra of ellipticals showed that their light is dominated by cool red stars whose UV output is negligible. Such stars are very old, and the absence of young blue stars indicated that ellipticals had a quiescent stellar population dating back about 10 billion years. In contrast, star formation in spiral galaxies (including the Milky Way) and irregular galaxies, continues quite vigorously to this day. The first UV observations of galaxies made with the OAO-2 satellite (Code & Welch 1979) and subsequently with the Astronomical Netherlands Satellite (ANS) and IUE showed, however, that elliptical galaxies become brighter below 2000 Å, with the flux continuing to rise toward the shortest wavelength observed (1250 Å). The strength of this UV upturn varies greatly among different galaxies, and Burstein et al. (1988) showed that galaxies with higher heavy-element abundances have stronger UV upturns.
These surprising results suggested the possibility that star formation is continuing in giant ellipticals at a low enough rate to be undetectable in the visible spectrum and yet to be dominant in the UV. A young stellar population would be expected to include a certain proportion of massive, very hot O and B stars whose spectral flux distributions could explain the UV upturn. Further support for this idea came from X-ray observations that revealed hot gas in some ellipticals that might cool and condense to form new stars.
Alternatively, the old stellar population might provide enough UV light from dying stars that have shed their cool outer layers in the post-red-giant phase of their evolution, revealing their small, hot cores. Such stars are well known in our Galaxy. They include the so-called horizontal-branch stars in globular clusters and the Milky Way halo, the central stars of planetary nebulae, and similar stars that eventually cool down to become white dwarfs. Until recently, both of these very different ideas-young stars or dying stars-remained as potential explanations of the UV light in elliptical galaxies, though neither could claim compelling agreement with the observational facts (Greggio & Renzini 1990).
This problem was originally one of the major motivations for the development of HUT. For example, its focal ratio and large apertures make it more sensitive than HST to the diffuse flux expected from large, nearby galaxies, and its wavelength coverage down to the Lyman limit provides an excellent discriminator of the temperature of the stars responsible for the UV radiation. During the Astro-1 mission, HUT obtained the first detailed far-UV spectrum of an elliptical galaxy (Ferguson et al. 1991), NGC 1399 in the Fornax cluster, which is known to have one of the strongest UV upturns of any normal elliptical studied with IUE (Burstein et al. 1988). From the absence of any clear absorption or emission feature at CIV (1550 Å), which is strong in hot, young O stars in our galaxy, Ferguson et al. (1991) were able to exclude such stars as the principal source of the UV light in NGC 1399. The decrease in flux observed in the spectrum below 1050 Å also argues against stars with temperatures greater than about 25,000 K, further proof that O stars are not present. Such stars would be expected if a low level of normal star formation were continuing today, though a burst of star formation that ended about years ago could possibly explain the data. A population of this age would contain only stars of spectral type B and later ( K), because the hotter, more massive O stars would all have evolved to produce supernovae by now, leaving behind, as remnants, faint neutron stars and possibly black holes.
Ferguson et al. (1991) also found that theoretical models based on the evolution of post-asymptotic giant-branch (PAGB) stars (such as central stars of planetary nebulae) do not fit the data obtained with HUT. This is basically because stars that follow the evolutionary paths calculated for this phase spend a large fraction of their time at very high temperatures, K, whereas the HUT data indicate that the principal contribution to the UV radiation comes from stars with about 25,000 K. Although the hotter PAGB stars must be present in the old stellar population of ellipticals, they can contribute only a small fraction of the UV light seen in NGC 1399 if the evolutionary tracks and stellar atmospheres used to model them are correct.
Thus, both of the most popular explanations of the UV light in ellipticals appear to be excluded by the HUT results. Recently, however, there have been new calculations of the late stages of evolution of low-mass, high-metallicity stars (Brocato et al. 1990; Castellani & Tornambè 1991; Horch, Demarque & Pinsonneault 1992). These calculations have identified two new types of evolutionary behavior-the post-early-AGB stars (PEAGB) and the AGB-manqué stars. The PEAGB stars evolve part of the way up the AGB but depart from it at an early stage, before the onset of the thermal pulses that are experienced by normal PAGB stars. They then evolve to higher temperatures at lower luminosity than PAGB stars, with a longer timescale. They maintain two shell sources of nuclear burning (one hydrogen and one helium) during this evolution and consequently emit more UV light over their lifetimes than do the more luminous PAGB stars, making them an excellent candidate to account for the strong UV upturns in ellipticals. The AGB-manqué stars begin on the blue end of the horizontal branch and skip the evolution up the AGB altogether. They evolve slowly toward higher luminosities, before becoming even hotter and bluer in their final stages, and then settle on the white dwarf cooling sequence. These stars also provide an interesting, new possible source for the UV light in ellipticals.
We have computed an integrated spectrum of a population of stars evolving along one of these newly identified evolutionary tracks, namely, the one with the largest integrated output in the HUT wavelength band. When the result is compared with the HUT spectrum of NGC 1399 (Figure 2), the agreement is excellent. Therefore, it appears likely that these new theoretical insights, in combination with the HUT observations, have at last provided a viable explanation for the unexpectedly strong UV light in elliptical galaxies. Observations of a few more elliptical galaxies, including some with lower metallicity and weaker UV upturns, can be made on Astro-2. Such observations will then place important constraints on the theoretical calculations of these late stages of stellar evolution.