The apparent shape of the "Strömgren sphere'' around the highest-redshift QSOs with Gunn-Peterson troughs

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  • 4 November 2004
Abstract
Although the highest-redshift QSOs (z>6.1) are embedded in a significantly neutral background universe (mass-averaged neutral hydrogen fraction >1%) as suggested by the Gunn-Peterson absorption troughs in their spectra, the intergalactic medium in their vicinity is highly ionized. The highly ionized region is generally idealized as spherical and called the Str\"omgren sphere. In this paper, by combining the expected evolution of the Str\"omgren sphere with the rule that the light of speed is finite, we illustrate the apparent shape of the ionization fronts around the highest-redshift QSOs and its evolution, which depends on the age, the luminosity evolution and the environment of the QSO (e.g., the hydrogen re-ionization history). The apparent shape may systematically deviate from a spherical shape, unless the QSO age is significantly long compared to the hydrogen recombination process within the ionization front and the QSO luminosity evolution is significantly slow. The apparent shape of the "Str\"omgren sphere'' may be directly mapped by transmitted spectra of background sources behind or inside the ionized regions or by surveys of the hyper-fine transition (21cm) line emission of neutral hydrogen. The apparent shape could also be used to constrain the cosmological parameter \Omega_\Lambda by the Alcock-Paczy\'nski test.

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