SIMBAD references

We use two large surveys of quasars (QSOs), namely, 2QZ-6QZ and SDSS DR1, to estimate mean properties of the background ultraviolet (UV) radiation and the temperature of intergalactic gas at redshifts z~ 2-4. We show that the intensity of the UV background radiation as estimated by Haardt & Madau (1996) should be increased by ∼20-30 per cent. We also briefly discuss spatial variations of both the intensity of background UV radiation and the gas temperature. These variations can be studied in more detail with high-resolution observations of the proximity effects, Lyman α absorbers and metal systems. Using the minimal spanning tree technique, we analyse the spatial distribution of QSOs and show that it is quite similar to random (Poissonian).