The problem of the order of magnitude discrepancy in the expected and observed ratios of the Lyman and Balmer lines in quasars and active galaxies is reviewed. Whereas early photoionization models for the emission line regions predicted F(Lα)/F(Hβ) ≳ 40, the observations give values for this ratio in the range 3–8. Attempts at explaining the observations have involved dust, both external and internal to the emission line regions, and improved treatments of the collisional processes and radiative transfer effects in dense (Ne ~ 1010 cm−3), optically thick clouds. None of the effects considered is, by itself, able to explain all the observations, and a combination of several of them is probably required.

The International Ultraviolet Explorer (IUE) is an 18 inch (45 cm) space telescope for ultraviolet spectrophotometry in geosynchronous orbit and is a joint project of the European Space Agency, the American National Aeronautics and Space Administration and the British Science Research Council. (Boggess et al. 1978 ab). It is a small instrument to use for objects of truly high redshift and therefore IUE observations relevant to extragalactic astronomy in general will be covered here.

A program to search for luminosity evolution of giant elliptical galaxies at high redshift has been begun at 2.2μm. Observing at infrared wavelengths offers the possibility of avoiding large and uncertain K corrections at redshifts near 1. First results of this program are in agreement with conclusions drawn from much larger optical studies, and demonstrate that luminosity evolution may be present.

From infrared observation of a radio selected sample of QSOs, it is shown that optical identification programs produce samples biased against very red sources, even if identifications are made without regard to color.

The (V-K) colors of giant elliptical galaxies as a function of redshift are discussed. Present data are consistent with mild color evolution at z ~ 0.45. An infrared Hubble (redshift-magnitude) diagram is given. Cosmological models with qo = 0 and no luminosity evolution are clearly excluded by the present data. A wide variety of models including those with qo = 0 are permissible if luminosity evolution is included. Instrumental and programmatic implications of these results are summarized.

Low resolution spectra of a significant sample of quasars show that the Paschen α and Balmer line ratios do not agree with the radiative recombination case B result and vary widely within the quasars sampled. The range in Pα:Hβ ratios is a factor of ~ 6, while the range in Lyα:Hα ratios is a factor of ~ 5. For the Pα:Balmer series, the deviations from case B recombination are not consistent with reddening, but appear, within large dispersions, to be consistent with optical depth effects in the Balmer lines affecting the line ratios. The Lyα:Hα ratio is, however, correlated with the continuum spectral index, and can be explained as due to reddening affecting both the lines and continuum.

The spectrum of the microwave background radiation closely resembles that of a blackbody with a peak at 6 cm−1. Many experimenters have contributed to the determination of this simple but important fact. In this review we describe the experimental difficulties which have plagued background measurements in the high frequency Wien region. An evaluation of the present status of our knowledge of the entire spectrum will be given and prospects for future experiments will be mentioned.

Small angular scale fluctuations in the temperature of the relict radiation may provide crucial information regarding the evolution of large scale structure in the universe. Some aspects of the interface between theory and observation in this developing study are considered.

Evidence from X-ray observations of clusters of galaxies suggests that they contain intergalactic plasma at a temperature of approximately 108 K which emits thermal bremsstrahlung (cf. Lea et al. 1973). Although numerous models for the origin and distribution of the plasma have been presented (cf. Gould and Raphaeli 1978), the amount and distribution of the plasma is still not certain. Several years ago, Sunyaev and Zel-dovich (1972) pointed out another observable consequence of hot gas in clusters of galaxies: inverse Compton scattering by electrons in the plasma would increase the energy of photons of the microwave background as these photons pass through the cluster. The fractional change in intensity of the microwave background is:

Results for the decrements in the microwave background radiation towards the centres of 13 clusters of galaxies are presented. It is shown that these data imply central gas densities of about 2 × 10−24 kg m−3 and cluster masses of about 5 × 1045 kg.

Measurements of the large-angular-scale anisotropy of the cosmic background radiation made from the northern hemisphere are in essential agreement with each other and indicate a first order spherical harmonic component with an amplitude of approximately 3 mK. New data from the southern hemisphere support these previous results. This first order anisotropy is interpreted as resulting from the motion of the solar system relative to the cosmic background radiation. There is no evidence of any higher order anisotropy to the level of 1 mK.