Annu. Rev. Astron. Astrophys. 1999. 37: 603-648 Copyright © 1999 by Annual Reviews. All rights reserved

1. INTRODUCTION

Far-ultraviolet radiation was first detected from early-type galaxies by the Orbiting Astronomical Observatory-2 in 1969. This was a major surprise because it had been expected that such old stellar populations would be entirely dark in the far-UV. To the contrary, not only did elliptical galaxies and the bulges of early-type spirals contain bright UV sources, but their energy distributions actually increased to shorter wavelengths over the range 2000 to 1200 Å, resembling the Rayleigh-Jeans tail of a hot thermal source with T_e 2000K. The effect was therefore called the "UV-upturn," the "UV rising-branch," or, more simply, the "UVX." It was only the second new phenomenon (after X-rays from the active galaxy M87) discovered by space astronomy outside our Galaxy.

Controversy flourished over the interpretation of the UVX for the next 20 years because of the slow accumulation of high quality UV data. More recent evidence has winnowed the alternatives and strongly supports the idea that the UVX is a stellar phenomenon (as opposed to nuclear activity, for example) associated with the old, dominant, metal-rich population of early-type galaxies. It is the most variable photometric feature of old stellar populations. It appears to be produced mainly by low-mass, helium-burning stars in extreme (high temperature) horizontal branch and subsequent phases of evolution. Such objects have very thin envelopes (M_ENV 0.05 M) overlying their cores. On both theoretical and observational grounds, the lifetime UV outputs of these stars are exquisitely sensitive to their physical properties. They depend strongly, for instance, on helium abundance; the UV spectrum is the only observable in the integrated light of old populations with the potential to constrain their He abundances. More remarkably, changes of only a few 0.01 M in the mean envelope mass of an extreme horizontal branch population can significantly affect the UV spectrum of an elliptical galaxy.

If this interpretation is correct, then far-UV observations become a uniquely delicate probe of the star formation and chemical enrichment histories of elliptical galaxies. They do, that is, once we understand the basic astrophysics of these advanced evolutionary phases and their production by their parent populations. However, this is one of the last underexplored corners of normal stellar evolution, and a complete interpretation is not yet at hand, even for nearby systems such as globular clusters where full color-magnitude diagram information is available. The key physical process involved in producing the small-envelope stars is mass loss during low-gravity phases on the red giant branch and subsequent asymptotic giant branch. Serious modeling of mass loss has only recently begun, and we so far have little intuition for the effects of population characteristics such as metal abundance. Although the interpretation of the integrated light of galaxies has heretofore relied on astrophysics established and tested in the context of local stars, it may be that the UVX problem will be the first where observations of galaxies will act as strong diagnostics of stellar evolution theory. At any rate, it is clear that to understand the controlling mechanisms of the UVX in galaxies we must conjoin integrated light observations of distant galaxies with the stellar astrophysics of globular clusters and hot field stars in our own and nearby galaxies.

There are broader ramifications of this interpretation as well. UV light acts as a tracer for stellar mass loss. As the primary source of fresh interstellar gas and dust in old populations, stellar mass loss is directly linked to a diverse set of other important phenomena, including gas recycling into young generations of stars, galactic winds, X-ray cooling flows, far-infrared interstellar emission, dust in galaxy cores, and gas-accretion fueling of nuclear black holes. The UV light also traces the production of low-mass stellar remnants. The hot UVX stars, regardless of their origin, are important distributed contributors to the interstellar ionizing radiation field of old populations. It is possible that the UVX is influenced by, and therefore reflects, galaxy dynamics. Finally, characterization of the UV light of nearby ellipticals, its separation into young or old stellar sources, and its predicted evolution is also basic to the development of realistic "K-corrections" for cosmological applications to high redshift galaxies and to interpretation of the cosmic background light.

There has been excellent progress over the last decade in understanding the UVX phenomenon, but the first question that might occur to the reader is why it took 30 years simply to identify its source. The answer lies in our historically limited capability for extragalactic UV observations, a subject we discuss in the next section. Following that, we describe the discovery of far-UV light from old populations and its basic observational characteristics, the lively debate over the leading alternative interpretations, and the confluence of theory and new observations that has led to the currently accepted interpretation. We also discuss several of the other observational opportunities presented by the generally faint UV background in galaxies. By "early-type" galaxies in this paper, we mean ellipticals, S0s, and the large bulges of spirals of types Sa and Sb, although most of the detailed analysis to date has concentrated on Es and S0s.