IMPLICATIONS OF THE IONIZING CONTINUUM OF 3C263

We present ultraviolet IUE and soft X-ray EXOSAT observations of the quasar 3C263 which reveal the low-energy part of the ionizing continuum and allow the high-energy tail to be strongly constrained. The ionizing spectrum confirms that the pressure of the extra-nuclear emission-line gas discovered earlier by us around the quasar is greater than 10(6) cm-3 K at approximately-20 kpc. Plausible models of the spectrum imply that the pressure may be up to 10 times higher than this minimum. I- and R-band images of the quasar and the surrounding field confirm Yee & Green's findings that the quasar is embedded in a moderately rich cluster. The I-band image shows the structure of the nebulosity around the quasar. We interpret the high pressure of the emission-line gas as due to confinement by dense, cooling intracluster gas, qualitatively similar, but of a greater magnitude, to that found in nearby clusters of galaxies. The minimum equipartition pressure of the resolved radio emission from the quasar is in agreement with the pressure profile inferred from the optical data.