2013A&A...552A..83A

To probe the spectral energy distribution (SED) of the ionizing background radiation at z≲2 and to specify the sources contributing to the intergalactic radiation field. 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 (∼1Ryd to ∼0.2keV). 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 HeI resonance lines of the Lyman series and the HeI λ504 Å continuum, which are seen for the first time in any cosmic object except the Sun. From one system to another, the SED of the ionizing continuum changes significantly, indicating that the intergalactic ionization field at z≲2 fluctuates at the scale of at least Δ_z∼0.004. This is consistent with Δ_z≲0.01 estimated from HeII and HI Lyman-α forest measurements between the redshifts 2 and 3. A radiation intensity break by approximately an order of magnitude at E=4Ryd 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 Δ_z>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∼3.5Ryd and a subsequent intensity decrease by ∼1.5dex, 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.