Reconstructing the intergalactic UV background with QSO absorption lines

Abstract

We present a new approach to constrain the spectral energy distribution of the intergalactic UV background observationally by studying metal absorption systems. We study single-component metal line systems exhibiting various well-measured species. Among the observed transitions at least two ratios of ionization stages from the same element are required, e.g. CIII/CIV and SiIII/SiIV. For each system photoionization models are constructed varying the spectrum of the ionizing radiation. The spectral energy distribution can then be constrained by comparing the models with the observed column density ratios. Extensive tests with artificial absorbers show that the spectrum of the ionizing radiation cannot be reconstructed unambiguously, but it is possible to constrain the main characteristics of the spectrum. Furthermore, the resulting physical parameters of the absorber, such as ionization parameter, metallicity, and relative abundances, may depend strongly on the adopted ionizing spectrum. Even in case of well-fitting models the uncertainties can be as high as ~0.5 dex for the ionization parameter and up to ~1.5 dex for the metallicity. Therefore, it is essential to know the hardness of the UV background when estimating the metallicity of the intergalactic medium. Applying the procedure to a small sample of 3 observed single-component metal line systems yields a soft ionizing radiation at z > 2 and a slightly harder spectrum at z < 2. The resulting energy distributions exhibit strong HeII Ly alpha re-emission features suggesting that reprocessing by intergalactic HeII is important. Comparing to UV background spectra from the literature indicates that the recent model of Madau & Haardt (2009) including sawtooth modulation due to reprocessing by intergalactic HeII with delayed helium reionization fits the investigated systems very well.

accepted for publication by A&A

References in corpus (14)

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Cited by in corpus (13)

• The Evolution of the Intergalactic Medium
• New synthesis models of consistent extragalactic background light over cosmic time
• Metal-enriched, sub-kiloparsec gas clumps in the circumgalactic medium of a faint z = 2.5 galaxy
• Constraints on X-ray Emissions from the Reionization Era
• Early and Extended Helium Reionization Over More Than 600 Million Years of Cosmic Time
• The physical properties of z>2 Lyman limit systems: new constraints for feedback and accretion models
• Relationship between the Metallicity of the Circumgalactic Medium and Galaxy Orientation
• Constraints on QSO emissivity using H I and He II Lyman alpha forest
• Role of ionizing background on the statistics of metal absorbers in hydrodynamical simulations
• Low Mass Group Environments have no Substantial Impact on the Circumgalactic Medium Metallicity
• Fluctuations of the intergalactic ionization field at redshift z ~ 2
• Role of ionizing background on the non-thermal broadening inferred for the aligned absorbers
• Physical conditions in high$-$z optically thin C III absorbers: Origin of cloud sizes and associated correlations