The Gunn–Peterson Effect: A Test for a Black-Hole-Induced Photoionization of the Intergalactic Medium

Abstract

There are many experimental evidences of the presence of an ionizing background radiation flux at large redshifts, whose nature is doubtful. A lot of information about the characteristics of such a background can be obtained both from the study of the Gunn–Peterson effect and from the so-called "proximity effect." In some previous works I suggested the possibility that this ionizing flux comes from the quantum evaporation of primordial black holes (PBH's); here, I discuss the constraints that the experimental measurements put upon the free parameters of this reionization model and I try to verify its reliability. In particular, the radiation intensity of the background at the hydrogen Lyman edge, as inferred from the proximity effect, enables me to determine an upper limit to the PBH's average relic mass; due to our poor knowledge of the ultimate fate of the evaporating black holes, this limit represents an important theoretical information. In the second part of this paper I study the absorption of the ionizing background due to Lyα clouds: in particular, I discuss this phenomenon in the presence of different absorption levels and I calculate the HI Gunn–Peterson optical depth τ GP (z); from a comparison with the experimental data of Giallongo et al.(τ GP, HI < 0.02 ± 0.03) I obtain a constraint on the intergalactic medium density parameter, namely Ω IGM < 0.020. A study of the characteristics of the absorbers is also performed: I determine the hydrogen gas density nH,c and the column density N HI for Lyα clouds; a satisfactory agreement with the available experimental data is obtained in the case of expanding, adiabatically cooled clouds. Finally, the same kind of analysis is performed for He II: in this case, the theoretical optical depth I obtain is smaller than the preliminary experimental lower limit of Jakobsen et al.(τ GP > 1.7).