▪ 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.

Far-Ultraviolet Radiation from Elliptical Galaxies

It has long been realised that valuable information on the evolution and origin of galaxies lies within reach of our large telescopes, yet after 50 years we have little conclusive evidence of galaxy evolution Most of the early work was motivated by the search for a value for q0 — evolution was regarded simply as a further ‘correction’ in terms of the Hubble diagram The suggestion that luminosity evolution effects might dominate magnitude — redshift and number magnitude diagrams was made by Beatrice Tinsley in the late 1960’s (see Tinsley 1976 for a summary) q0 does, however, have an important part to play in these diagrams via the linking of cosmological timescales (redshifts and luminosity distances) and galactic timescales (years)

Evolution of Faint Galaxies

In BL Lac objects the absence of broad emission lines may be due either to a lack of ionizing continuum or to a lack of cool gas (T ≲ 104K)1. UV observations of BL Lac objects are interesting in that they provide a measurement of the continuum, and they include the wavelength range of Lyα, which is in principle the most favourable line for detection. We report here several spectra of the BL Lac object PKS2155–304 in the 1,150–3,200 A band taken with the IUE when the object was in a bright phase. The UV flux connects smoothly with the optical and IR observations of the source in its brightest state and its extrapolation matches the soft X-ray flux, implying a change in spectral slope around 1015Hz.

Far UV observations of PKS2155–304

I predict the evolution of the X-ray luminosity function of clusters of galaxies. Predominantly, I treat the assumption that galaxies form first, then cluster purely due to gravitation. I show that the richness distribution of Abell clusters favors this scenario, rather than the protocluster hypothesis. The luminosity function is produced by combining a generalized (for all Ω) Press-Schechter evolutionary mass function for clusters (derived herein) with a power law X-ray luminosity-mass relation; a power law relation is supported by observations of low-redshift clusters. I find very steep evolution in the luminosity function, and thus in the source counts, for large Ω, and moderate evolution for small Ω. For a variety of models for the gas supply rate to the intracluster medium, the evolution of the luminosity function does not vary greatly. Thus it appears that the Ω, dependence will dominate and that number counts of X-ray clusters will yield cosmological information. The power of a test of Ω with an evolving luminosity function is considerably enhanced relative to a test which involves solely global cosmological effects on a non-evolving population. This occurs because of the well-known result that, at late times, clustering tends to proceed slowly for universes of small Ω and rapidly for large Ω.

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Evolution of the Cluster X-Ray Luminosity Function

Two-color diagrams including magnitudes from several photometric systems are investigated with respect to the possibility of determining from the integrated photometry the galaxy type and its redshift. The color indices of galaxies are calculated using the energy distribution curves of Colemanet al. (1980). It is shown that redshift determination at low values ofz requires an ultraviolet magnitude with λ0 shorter than 300 nm. At redshiftsz>0.6, the infrared magnitude photometry becomes important. Different sources of errors and uncertainties are discussed.

Photometric Classification of Normal Galaxies and Their Redshift Determination

Observed spectra are reproduced in the 1000-3400 wavelength range; line identifications are tabulated, and spectrophotometry of parts of the continua is given. The flux density of a few line emissions can be estimated from their equivalent widths. The strongest line emissions differ in the two nuclei, and they do not fit spectral patterns that are observed in the sun, hot stars, gaseous nebulae, or quasi-stellar objects. However, the absorption-line features are rather similar to those of the sun.

H. M. Johnson

Papers

IUE spectra of the nuclei of M31 and M32.