Continuum shapes used to approximate the SED in the EUV (1 Ryd–0.2 keV) range: a simple power law (blue dotted line), an AGN continuum by Mathews & Ferland (1987) (blue line), an ionizing spectrum of the metagalactic radiation field at redshift z ~ 2 by Haardt & Madau (1996) (black line), and its approximation by a broken power law (red line). All the SEDs are normalized so that J_ν(hν = 1 Ryd) = 1. The positions of ionization thresholds of different ions are indicated by tick marks.