QUASAR METALLIC ABSORPTION SYSTEMS IRRADIATED BY A STEEP-SPECTRUM UV BACKGROUND

We re-examine the spectral shape of the UV background at z similar or equal to 2 - 3 using the following observations: the ''proximity effec t'' of a Lyman a forest; the He II Gunn-Peterson optical depth, tentat ively detected by Jakobsen et al. (1994, Nature 370, 35); the soft X-r ay background at the present epoch; and the observed ionization states of the Lyman-limit systems (LLSs) along the Line of sight to quasar H S 1700 + 6416. We conclude that the spectral shape (I-nu proportional to nu(-alpha)) of the optimum ionizing radiation should have the follo wing three features: 1) a break at the He I ionization edge, by a fact or similar to 5, 2) a steep (alpha similar or equal to 3) spectrum up to the HeII ionization edge, and 3) a hat (\alpha\ less than or simila r to 1) spectrum in the energy-band range from far UV to soft X-ray. W hen we consider the spectral modification due to the intervening absor ption by Lyman a! clouds and LLSs, the spectral shape of the integrate d UV radiation from QSOs is roughly similar to that of our optimum ion izing background, but with a somewhat smaller break at the He II edge, and a negligible break at the He I edge. A combination of the UV abso rption feature and (soft) X-ray constraints is crucial to conclude whe ther or not photoionization is the main ionization mechanism of highly ionized metal ions observed in the QSO metallic absorption systems. M oreover, the LLSs at the relevant redshifts may originate in gaseous h alos, having a typical gas mass of 10(10) M. with a metallicity of abo ut (1 - 3) x 10(-2) solar value and a radius of 50 - 100 kpc.