Fluctuations of the intergalactic ionization field at redshift z ∼ 2

Aims. To probe the spectral energy distribution (SED) of the ionizing background radiation at z less than or similar to 2 and to specify the sources contributing to the intergalactic radiation field. Methods. The spectrum of a bright quasar HS 1103+6416 (z(em) = 2.19) contains five successive metal-line absorption systems at z(abs) = 1.1923, 1.7193, 1.8873, 1.8916, and 1.9410. The systems are optically thin and reveal multiple lines of different metal ions with the ionization potentials lying in the extreme ultraviolet (EUV) range (similar to 1 Ryd to similar to 0.2 keV). For each system, the EUV SED of the underlying ionization field is reconstructed by means of a special technique developed for solving the inverse problem in spectroscopy. For the z(abs) = 1.8916 system, the analysis also involves the He I resonance lines of the Lyman series and the He I lambda 504 angstrom continuum, which are seen for the first time in any cosmic object except the Sun. Results. From one system to another, the SED of the ionizing continuum changes significantly, indicating that the intergalactic ionization field at z less than or similar to 2 fluctuates at the scale of at least Delta(z) similar to 0.004. This is consistent with Delta(z) less than or similar to 0.01 estimated from He II and H I Lyman-alpha forest measurements between the redshifts 2 and 3. A radiation intensity break by approximately an order of magnitude at E = 4 Ryd in SEDs restored for the z(abs) = 1.1923, 1.8873, 1.8916, and 1.9410 systems points to quasars as the main sources of the ionizing radiation. The SED variability is mostly caused by a small number of objects contributing at any given redshift to the ionizing background; at scales Delta(z) greater than or similar to 0.05, the influence of local radiation sources becomes significant. A remarkable SED restored for the z(abs) = 1.7193 system, with a sharp break shifted to E similar to 3.5 Ryd and a subsequent intensity decrease by similar to 1.5 dex, indicates a source with comparable inputs of both hard (active galactic nuclei, AGN) and soft (stellar) radiation components. Such a continuum can be emitted by (ultra) luminous infrared galaxies, many of which reveal both a strong AGN activity and intense star formation in the circumnuclear regions.