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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 = ne0(1 + r2/rc2)–3/2β, rc being the core radius and ne0 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, ne0, 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 = μmHσz2/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.

The title of this keynote address is ‘Offshore Foundation Safety’. This is a wide subject and could be controversial, however nothing I am going to say is particularly new rather I am going to present old information with a new emphasis. I will be concentrating on the UK offshore sector but the principles that I will be elaborating on are the same worldwide.

Reliable detections of the Sunyaev-Zel’dovich effect exist for only a few clusters of galaxies although many observations exhibit random errors < 0.5 mK. Several observational techniques are in use and particular promise for future improvements in the measurements are shown by sub-millimetre work with single dishes and centimetre-wavelength observations with small, dedicated interferometers. The three best-observed clusters display effects which are clearly associated with the X-ray emitting intracluster gas, and which can be used to investigate the properties of this gas and to estimate the value of the Hubble constant.

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.

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 Ho, 10 kms−1 Mpc−1 < Ho < 160 kms−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.