The metagalactic UV background at z < 3


The high ionization state of the intergalactic medium (IGM) is maintained by the metagalactic UV background radiation. This ionizing radiation field is supposed to be the integrated radiation of quasars and young star-forming galaxies, which is reprocessed and attenuated by the intergalactic gas. Since the UV background is inaccessible to direct observations, it is impossible to test theoretical predictions of its spectral energy distribution in great detail. However, constraints can be derived from studies of metal absorption systems, the HI Lyman alpha opacity, or the HeII Lyman alpha forest, respectively. Observations of the HeII/HI column density ratio eta particularly probe the variations of the UV background. Since unobscured quasars permitting observation of HeII absorption are rare, there are only two lines of sight, towards the quasars HE2347-4342 and HS1700+6416, where the HeII Lyman alpha forest can be resolved. This work presents a study of the UV background at redshifts z < 3 investigating the HeII absorption towards the quasars HS1700+6416 (z = 2.72), which provides the second line of sight where the HeII Lyman alpha forest is resolved. The numerous metal line systems detected in the spectra of this QSO are studied in order to avoid biases due to unrecognized metal line absorption in the HeII forest. In this connection, several shapes of the ionizing radiation are tested for each system. The majority of the considered systems, in particular those at z > 2, is best reproduced with an ionizing radiation similar to the UV background of Haardt & Madau (2001), where the HeII break, formally located at 4 Ryd, is shifted to lower energies (~ 3 Ryd), probably due to the opacity of the higher HeII Lyman series lines ...
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