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

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FAR-ULTRAVIOLET RADIATION FROM ELLIPTICAL GALAXIES

Robert W. O'Connell


Astronomy Department, University of Virginia, P.O. Box 3818, Charlottesville, Virginia 22903-0818


Abstract. Far-ultraviolet radiation is a ubiquitous, if unanticipated, phenomenon in elliptical galaxies and early-type spiral bulges. It is the most variable photometric feature associated with old stellar populations. Recent observational and theoretical evidence shows that it is produced mainly by low-mass, small-envelope, helium-burning stars in extreme horizontal branch and subsequent phases of evolution. These are probably descendants of the dominant, metal rich population of the galaxies. Their lifetime UV outputs are remarkably sensitive to their physical properties and hence to the age and the helium and metal abundances of their parents. UV spectra are therefore exceptionally promising diagnostics of old stellar populations, although their calibration requires a much improved understanding of giant branch mass loss, helium enrichment, and atmospheric diffusion.

Keywords: stellar populations, hot stars, mass loss, galaxy evolution


Table of Contents

INTRODUCTION

INSTRUMENTAL CONSIDERATIONS

DISCOCVERY AND ALTERNATIVE INTERPRETATIONS

SPATIAL STRUCTURE OF THE UVX
Evidence Against Young Stars
Structural Variations

SPECTRAL AND PHOTOMETRIC CHARACTERISTICS OF THE UVX
Incidence, Spectral Shape, and Line Features
Amplitude and Correlations with Other Properties

CANDIDATE LOW MASS UVX SOURCES
Globular Cluster-Type Populations
Single Star Candidates
Relationship to Global Characteristics of Parent Populations
Binaries and Dynamical Effects

HOT LOW MASS STARS IN ELLIPTICAL GALAXIES
Interpretation of UV Spectra and Colors
Inferences About Chemical Abundances and Ages
Resolved UV Star Populations
Cosmic Evolution of the UVX
Summary and Key Issues

OTHER FAR-UV PHENOMENA IN E GALAXIES

CONCLUSION

REFERENCES

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