Cosmological radiative transfer for the line-of-sight proximity effect

Abstract

We study the proximity effect in the Ly-a forest around high redshift quasars as a function of redshift and environment employing a set of 3D radiative transfer simulations. The analysis is based on dark matter only simulations at redshifts 3, 4, and 4.9 and, adopting an effective equation of state for the baryonic matter, we infer the HI densities and temperatures in the cosmological box. The UV background (UVB) and additional QSO radiation with Lyman limit flux of L_{νLL} = 1e31 and 1e32 erg / Hz s are implemented with a radiative transfer code until an equilibrium configuration is reached. We analyse mock spectra originating at the QSO in the most massive halo, in a random filament and in a void. The proximity effect is studied using flux transmission statistics, in particular with the normalised optical depth. Beyond a radius of r > 1 Mpc / h from the quasar, we measure a transmission profile consistent with geometric dilution of the QSO ionising radiation. A departure from geometric dilution is only seen, when strong absorbers intervene the line-of-sight. The cosmic density distribution around the QSO causes a large scatter in the normalised optical depth. The scatter decreases with increasing redshift and increasing QSO luminosity. The mean proximity effect provides an average signal that is biased by random large scale density enhancements on scales up to r \approx 15 Mpc / h. The distribution of the proximity effect strength provides a measure of the proximity effect along individual lines of sight. It shows a clear maximum almost without an environmental bias. Therefore it can be used for an unbiased estimate of the UVB. Differing spectral energy distributions between the QSO and the UVB modify the profile which can be reasonably well corrected analytically. A few Lyman limit systems have been identified that prevent the detection of the proximity effect due to shadowing.