1996/09/06 by Francesco Haardt, Piero Madau, Haardt, Francesco +1
Physics and Astronomy · #Astrophysical Phenomena and Observations #Astrophysics (astro-ph) #Astrophysics and Cosmic Phenomena #FOS: Physical sciences #Galaxies: Formation, Evolution, Phenomena #astro-ph
paper · pdf · doi:10.48550/arxiv.astro-ph/9609057
2 pages, PASP latex [11pt,paspconf,epsf], + 1.ps figure To be published in the proceedings of 163 IAU Conference. Also available at http://fy.chalmers.se/~haardt/personal/curr.html
arxiv created 1996/09/06 · openalex publication_date 1996/09/06 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
The detection of a HeII absorption trough in the spectra of three high-redshift quasars provides new constraints on the spectral shape of the UV extragalactic background. The relative strengths of the observed flux decrements at the rest-frame wavelengths of 1216 and 304 A require a relatively soft radiation field at 4 ryd compared to 1 ryd. If the ionizing metagalactic flux is dominated by the integrated light from QSOs, and the HeII reionization of the universe was completed well before z~3, then the UV/soft X-ray spectrum of individual quasars at these epochs must, once the cosmological ``filtering'' through material along the line of sight is taken into account, satisfy similar constraints on the average. We model the propagation of AGN-like ionizing radiation through the intergalactic medium using CUBA, a numerical code developed in our prior work. We show that, in order to explain the reported HeII absorption, any thermal component responsible for the ``soft X-ray excess'' observed in the spectra of AGNs at z<0.3 must, at z~3, have a typical temperature >80 eV, and a luminosity not exceeding 20% of that of the ``UV bump''.