The growth lengths of Kelvin-Helmholtz instabilities are calculated for three vortex-sheet bounded beams with v = 0.9c. These beams survive the action of the instability for distances of several hundred beam radii, but no prediction of a single dominant mode of breakup can be made.

Observational data on the Sunyaev-Zel'dovich effect for three clusters are used to obtain limits to the value of the Hubble constant. Only a wide bound on the value of H0, 10 km s−1 Mpc−1 < H0 < 160 km s−1 Mpc−1, can be derived, mostly because of the large uncertainty in the measured temperature of the intracluster medium in each cluster. Better information on this temperature and on the distribution of the intracluster medium are needed for this method to yield a good measurement of H0.

Prior to ROSAT, separation of X-ray components in radio galaxies has been limited to a few well-known sources, e.g., M 87 and Cen A. Now, from ROSAT PSPC measurements of the first six objects in our study of low-power radio galaxies, we find that both resolved (thermal) and unresolved X-ray emission in a single source is typical (Worrall & Birkinshaw 1993; ApJ, submitted). The angular size of, and fraction of luminosity in, the resolved X-ray emission varies between objects. There is evidence to relate the unresolved X-ray emission with the inner radio jet.

The generalisation from circular to elliptical mass distributions acting as gravitational lenses produces a number of interesting effects. These effects include an extra indeterminacy in predictions of the time delay between the two brightest images, if only the relative brightness and separation of these images is known. This indeterminacy is removed if further information about the system (such as a measurement of the astigmatism of the lens) is available.

Line strength gradients in luminous ellipticals show changes in metallicity of roughly a factor of two out to an effective radius (Worthey, Faber & Gonzalez, 1992; Davies, Sadler & Peletier (DSP), 1993; Gonzalez, 1993). The observed decline in Mg2 line strength with increasing radius, while Hβ remains roughly constant, has been interpreted to indicate an age gradient with the central parts of ellipticals being younger than the outer regions.

If a galaxy cluster's X-ray gas distribution follows an isothermal polytropic β model, we may write the electron radial density distribution as; ne = ne 0(1 + r 2/rc 2)–3/2β , rc being the core radius and ne 0 the central electron density. This may be related to both an X-ray surface brightness distribution and a Sunyaev-Zel'dovich effect distribution (Sarazin 1986). Fitting to observational data then enables us to constrain the value of β. The normalisation value, ne 0, to obtain a total mass estimate is calculated via the relationship between the X-ray and S-Z distribution normalisation constants, and the gas temperature and spectral emissivity parameters from fits to the X-ray spectrum. We are then in a position to evaluate ne (r) and its integral; the total electron gas mass. If we can further assume that there exists a simple ratio between the electron and proton number densities within the gas, we may straightforwardly posit a value for the total gas mass. An additional method of determining the polytropic gas index exists, with optical constraints on the galactic velocity dispersion, through the relation; β = μm Hσz 2/kBTe . Studies at optical, as well as X-ray and radio wavelengths are thus useful as a corroborative measure in determining the total gas mass.

Recent data taken with the 40-m telescope of the Owens Valley Radio Observatory at 20 GHz have confirmed the detections of the Sunyaev-Zel’dovich effect towards three clusters of galaxies, 0016+16, Abell 665 and Abell 2218. Rough measurements of the angular scale of the effect have also been made, and a comparison of these with the X-ray observations supports an isothermal model for the cluster gas.

Recent observations of Cygnus A with the Hat Creek interferometer at 86.2 GHz limit the spectral curvature of the hotspots and show that the diffuse lobe emission has a spectral index of about 1.5. The central component is, at most, weakly variable and its spectrum shows a distinct break to a spectral index near 0.65 at about 20 GHz.

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.

We have undertaken a Spitzer campaign to measure the IR structures and spectra of low-redshift 3CRR radio galaxies. The results show that the 3.6 – 160 μm infrared properties vary systematically with integrated source power, and so demonstrate that contemporary core activity is characteristic of the behaviour of sources over their lifetimes. IR synchrotron emission is seen from jets and hotspots in some cases. Thermal emission is found from a jet/gas interaction in NGC7385. Most of the near-IR integrated colours of the low-redshift 3CRR radio galaxies are similar to those of passive galaxies, so that IR colours are poor indicators of radio activity.

We have completed a Chandra snapshot survey of 54 radio jets that are extended on arcsec scales. These are associated with flat spectrum radio quasars spanning a redshift range z=0.3 to 2.1. X-ray emission is detected from the jet of approximately 60% of the sample objects. We assume minimum energy and apply conditions consistent with the original Felten-Morrison calculations in order to estimate the Lorentz factors and the apparent Doppler factors. This allows estimates of the enthalpy fluxes, which turn out to be comparable to the radiative luminosities.

Most X-ray studies of radio-mode feedback have concentrated on locally-abundant low-power radio sources in relatively rich cluster environments. But the scaling found between mechanical and radiative power, when combined with the radio luminosity function, means that half of the heating in the local Universe is expected from higher-power sources, which lie within a factor of about three of the FRI/II transition, and these sources encounter a wide range of atmosphere properties. We summarize what is observed at FRI/II transition powers from a complete sample observed with modest Chandra exposure times. We then discuss two systems with deep Chandra data. In one we find that the work done in driving shocks exceeds that in evacuating cavities. This source also displays a remarkable jet-cloud interaction, and revealing hotspot X-ray emission. In the second we find evidence of radio-emitting plasma running along boundaries between gas of different temperature, apparently lubricating the gas flows and inhibiting heat transfer, and itself being heavily structured by the process.

Division VIII gathers astronomers engaged in the study of the visible and invisible matter in the Universe at large, from Local Group galaxies via distant galaxies and galaxy clusters to the large-scale structure of the Universe and the cosmic background radiation.

We investigate the variations of the magnetic field, Doppler factor, and relativistic particle density along the jet of a quasar at z=0.72. We chose 4C 19.44 for this study because of its length and straight morphology. The 18 arcsec length of the jet provides many independent resolution elements in the Chandra X-ray image. The straightness suggests that geometry factors, although uncertain, are almost constant along the jet. We assume the X-ray emission is from inverse Compton scattering of the cosmic microwave background. With the aid of assumptions about jet alignment, equipartition between magnetic-field and relativistic-particle energy, and filling factors, we find that the jet is in bulk relativistic motion with a Doppler factor ≈ 6 at an angle no more than 10∘ to the line of sight over de-projected distances ≈ 150–600 kpc from the quasar, and with a magnetic field ≈10 μGauss.

As documented by the reports of the Presidents of Commission 28 and Commission 47 the fields of extra-galactic research and cosmology have experienced a higher and higher development leading to a vast harvest of results and discoveries. They range from the description of the overall structure of the universe to that of the individual properties of galaxies. The availability of very large telescopes and the coverage of regions of the sky with deep surveys, on the observational side, and the wide use of sophisticated numerical simulations on the theoretical one are starting to produce a satisfactory understanding of the physical processes taking place during the evolution of galaxies. Very often there is an profitable interplay between the subjects of the two Commissions without clearcut boundaries. This makes Division VIII, which is one of the largest of the IAU, counting 1373 members, very well balanced and deserving to remain without modifications for the future.