Proximity Effect for Metal Absorption Systems

Abstract

Using our photoionization code CIRRUS, we show that the study of proximity effect on metal elements provides a possibility of placing constraints on the spectral shape of the metagalactic UV background (MUVB). From theoretical point of view, the most optimum indicator of the spectral shape of the MUVB is the ratio OIV/OIII. Unlike previous thought, the ratio SiIV/CIV is not a good indicator of the spectral shape because of its strong dependence on an ionization parameter as well. We also find that the observed excess of CIV systems in luminous QSOs may be a photoionization effect, but not a gravitational lensing effect.