SIMBAD references

We revisit the proximity effect produced by quasars (QSOs) at redshifts 2.1-3.3 applying the From Lines to Overdensities (FLO) approach to a sample of ∼6300 Lyα lines fitted in 21 high resolution, high signal-to-noise spectra. This new technique allows to recover the hydrogen-density field from the Hi column densities of the lines in the Lyα forest, on the basis of simple assumptions on the physical state of the gas. To minimize the systematic uncertainties that could affect the density recovering in the QSO vicinity, we carefully determined the redshifts of the QSOs in our sample and modelled in detail their spectra to compute the corresponding ionizing fluxes. The mean density field obtained from the observed spectra shows a significant overdensity in the region within 4 proper Mpc from the QSO position, confirming that QSOs are hosted in high-density peaks. The absolute value of ρ/<ρ> for the peak is uncertain by a factor of ∼3, depending on the assumed QSO spectral slope and the minimum Hi column density detectable in the spectra. We do not confirm the presence of a significant overdensity extending to separations of ∼15 proper Mpc from the QSO, claimed in previous works at redshifts <z> ≃ 2.5 and 3.8. Our best guess for the ultraviolet background (UVB) ionization rate based on the intergalactic medium (IGM) mean density recovered by FLO is Γ_UVB≃ 10^–12/s. However, values of Γ_UVB≃ 3x10^–12/s could be viable if an inverted temperature-density relation with index α ≃ -0.5 is adopted.