HOT GALACTIC HALOES AS THE SOURCE OF THE IONIZING-RADIATION OF LYMAN LIMIT SYSTEMS OF QUASARS

The absorption-line systems observed in quasar spectra are thought to arise from clouds in the haloes of intervening galaxies. The ionizatio n state of the absorption-line systems is generally explained by a uni versal ionizing radiation field. Recent spectroscopic observations wit h the Hubble Space Telescope, however, have allowed the derivation, fo r Lyman limit systems (LLSs) of the QSO HS 1700 + 6416, of column dens ities for C, N and O in several ionization stages (O III to O VI, C II to C IV, N II to N v) as well as of He I. These new data severely con strain the shape of the ionizing radiation spectrum, posing serious di fficulties for a metagalactic origin of the radiation. Here we explore an entirely different possibility for the source of the ionizing radi ation. Since the LLSs are probably located in galactic haloes, we assu me that the origin of the ionizing radiation is local, arising from th e extreme-ultraviolet (EUV) and soft X-ray emission produced by the ho t gas in the galactic halo. A model for the evolution of elliptical ga laxies predicts a large X-ray luminosity during the early phases, whic h decreases with time at later epochs. The ionizing radiation needed t o account for the line ratios of the absorption-line systems of the QS O HS 1700 + 6416 is naturally produced by the hot gas in the galaxy, a nd has the optimal shape to explain simultaneously the presence of He I as well as the ratios of several ionization stages of O.