Fitting the spectrum of the X-ray background: the effects of high metallicity absorption

Recent work by Risaliti et al.(1999) suggests that more than half of all Seyfert 2 galaxies in the local universe are Compton-thick (N_H > 10^24 cm^-2). This has implications for AGN synthesis models for the X-ray background (XRB), the flexibility of which for the inclusion of large numbers of high-z type 2 sources we examine here. We highlight the importance of Compton down-scattering in determining the individual source spectra and the fit to the XRB spectrum, and demonstrate how parameter space opens up considerably if a super-solar iron abundance is assumed for the absorbing material. This is illustrated with a model which satisfies the present constraints, but which predicts substantial numbers of type 2 source at the faint flux levels soon to be probed for the first time by the Chandra and XMM missions. We demonstrate also how a strong negative K-correction facilitates the detection of sources with 10^~24 < N_H < 10^25 cm^-2 out to the highest redshifts at which they could plausibly exist.