The highly photoionized stage of the intergalactic medium after reionization is maintained by the intergalactic UV background. This radiation field is produced by the radiation of quasars and galaxies reprocessed by the IGM itself. While the spectral hardness of the background radiation can be probed by observations of intergalactic HeII absorption, more detailed constraints on the spectral energy distribution can be obtained by studying metal absorption systems. We will present a new approach to constrain the UV background by the investigation of metal-line systems at various redshifts aiming to probe the spectrum and evolution of the ionizing radiation.
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