The spectral shape of the ionizing extragalactic background radiation at z ~ 0

Type Journal Article
Names Bart Wakker
Publication HST Proposal
Pages 13893
Journal Abbreviation HST Proposal
Date October 1, 2014
Library Catalog
Abstract The intensity and spectral shape of the cosmic ionizing UV background radiation is important for understanding the structure, formation, evolution and physical state of the low-density intergalactic medium {IGM}. The UV background radiation is produced by the integrated radiation from QSOs and galaxies, which is then reprocessed by the clumpy IGM. Despite its importance, the UV background is still poorly known both observationally and theoretically, especially at low redshifts. We propose to constrain the spectral shape of the ionizing UV background radiation at z 0 in the energy range 1 < E < 6 ryd using 16 optically thin metal absorbers which are in photoionization equilibrium with the extragalactic UV background. The selected absorbers have C III and C IV detections often accompanied by Si II, Si III and Si IV, which are clearly associated with each other and with H I at the same redshift. Comparing observations to metal column density predictions based on the photoionization code CLOUDY, we propose the first study of the spectral shape, fluctuation and dominant ionizing source of the UV background radiation at z 0.
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