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Summary A theory is proposed for the correlation between radio continuum (RC) and thermal far-infrared (FIR) radiation for normal spiral galaxies. In this theory, we assume global energy outputs of the FIR and cosmic rays to be proportional to the star formation rate, and adopt energy equipartition between cosmic rays and the interstellar magnetic field so as to keep dynamical stability and steady star formation in the galaxies. It is shown that the tight correlation between the RC and FIR emission results from a steady star formation process in the galaxies rather than both of them being simply from the same source. An important feature of the model is that, in the majority of galaxies, most of the cosmic ray electrons responsible for the radio emission at the observed frequencies escape. An approximate solution to the problem of electron propagation with a free escape boundary condition is derived and the emissivity of synchrotron radio emission is calculated. Direct evidence for the theory is the prediction of a non-unity slope for the correlation between RC and FIR, and this is observed. From small and less luminous galaxies to large and more luminous ones, we predict that there is a turning point above which most of the electrons will be trapped in the galaxies rather than escaping, and correspondingly the slope of the correlation will become unity. From a sample of data, we find this trend.

The imminent launch of the NASA Gamma Ray Observaton, has focused on the need to provide u p b d a t e estimates O i the energy density oiintersteiiar radia. tion away from the galactic plane, this parameter being a prerecpiisite for calculation of the flux of gamma rays coming from the inverse Compton interactions of cosmic ray electrons. W e have made such calculations, using recent information on stellar distributions and on the extinction properties of dust in the interstellar medium. The ensuing energy densities are probably accurate to about 30%, the uncertainty being due t o lark of precise knowledge of the input parametas rather than appmximations in the calculations.

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The world data on extensive air showers (U) produced by cosmic rays of energy above lOI9 eV have been examined in detail, the individual shower characteristics being re-analysed where the basic data are available. We consider that our preVious claim for an enhancemenl of intensity in the galacric plane is strengthened, at least in the outer galaxy; in panicular we find new evidence for the presence of specific clusters of EAS both near the galactic plane and elsewhere. The clustem may correspond to the presence of specific sources or groups of sources.