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 ionization state of the absorption-line systems is generally explained by a universal ionizing radiation field. Recent spectroscopic observations with the Hubble Space Telescope, however, have allowed the derivation, for Lyman limit systems (LLSs) of the QSO HS 1700 + 6416, of column densities 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 constrain the shape of the ionizing radiation spectrum, posing serious difficulties for a metagalactic origin of the radiation. Here we explore an entirely different possibility for the source of the ionizing radiation. Since the LLSs are probably located in galactic haloes, we assume that the origin of the ionizing radiation is local, arising from the extreme-ultraviolet (EUV) and soft X-ray emission produced by the hot gas in the galactic halo. A model for the evolution of elliptical galaxies predicts a large X-ray luminosity during the early phases, which decreases with time at later epochs. The ionizing radiation needed to account for the line ratios of the absorption-line systems of the QSO HS 1700 + 6416 is naturally produced by the hot gas in the galaxy, and has the optimal shape to explain simultaneously the presence of He I as well as the ratios of several ionization stages of O.