Fluctuations of the intergalactic ionization field at redshift z ~ 2

TL;DR

This study investigates the variability of the intergalactic ionization field at redshift around 2 by analyzing metal-line absorption systems in quasar spectra, revealing significant fluctuations in the spectral energy distribution of the ionizing background radiation.

Contribution

It introduces a novel method for reconstructing the EUV spectral energy distribution of the ionizing background from metal-line absorption systems at z ~ 2.

Findings

The ionizing background radiation fluctuates significantly at z ~ 2.

The scale of fluctuations in the ionization field is at least Delta_z ~ 0.004.

These fluctuations are consistent with previous estimates from Lyman-alpha forest data.

Abstract

(Abridged) Aims. 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. Methods. The spectrum of a bright quasar HS1103+6416 (zem = 2.19) contains five successive metal-line absorption systems at zabs = 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 (1 Ryd 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 zabs = 1.8916 system, the analysis also involves the HeI resonance lines of the Lyman series and the HeI 504 A 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 ~ 2 fluctuates at the scale of at least Delta_z ~ 0.004. This is consistent with Delta_z ~ 0.01 estimated from HeII and HI Lyman-alpha forest measurements between the redshifts 2 and 3.