The Ionization and Abundance of C and Si in QSO Absorbers

We have analyzed high resolution data of absorption lines of Si and C in theabsorption systems observed in the spectra of QSOs, in order to study theionization state and the overabundance of Si with respect to C in the absorbersand also to study the change in these properties with redshift. No correlationis found between column density ratios of Si IV to C IV of intervening systemsand redshift. The data do not provide any evidence for an abrupt change in thevalues of the ratio at any particular redshift unlike that for Lyman alphaforest clouds. We have compared the observed ratios of column densities of SiII and Si IV and of Si IV and C IV in different classes of absorption systemswith predictions of photo ionization models for different shapes of thebackground radiation field. Overabundance of Si over C can be ruled out inseveral of the intervening systems for any shape of the background radiation.For these systems we can also rule out any contribution from the stellarsources to the background, which is then entirely from the AGNs. Nooverabundance is needed in other intervening systems if the radiation fieldfrom stellar sources contributes significantly to the UV background.Overabundance is, however, present in Lyman alpha forest clouds at redshiftslarger than 3 and in systems associated with the QSOs. For all the interveningsystems a minimum of 10 % of the background is contributed by the AGNs.Comment: To be published in Pub.Astr.Soc.Japan, 3 figure