The effects of ultraviolet background correlations on Lyα forest flux statistics

We examine the possible effects of ultraviolet (UV) background fluctuations on the pixel flux power spectrum and autocorrelation function of the Lyalpha forest due to a finite number of sources in an attenuating medium. We consider scenarios in which quasi-stellar object (QSO) sources dominate the contribution to the UV background. To estimate their contribution, we use the QSO luminosity functions from the Two-Degree Field and the Sloan Digital Sky Survey. We estimate self-consistent values for the attenuation length at the Lyman edge through the intergalactic medium using particle mesh simulations of the Lya forest, normalized by the measured mean Lyalpha flux. It is necessary to add the contribution from Lyman-limit systems based on their measured statistical properties because the simulations are unable to reproduce the measured abundances of these systems, suggesting that their formation may involve more complex hydrodynamical processes than simple collapse into dark matter haloes. We examine the convergence properties of the flux power spectrum and autocorrelation function under differing assumptions regarding pressure smoothing. On the basis of our tests, we recommend that simulations used to estimate the flux power spectrum at high redshifts (z > 3) be performed in a box of comoving size at least 25 h(-1) Mpc on a side and a comoving force mesh as fine as 30-60 h(-1) kpc. The requirements become more severe at lower redshifts. Even with these specifications, we find that the flux autocorrelation function does not converge to better than 10 per cent on scales exceeding 3 per cent of the box size without added pressure smoothing. Much better convergence properties are obtained for the relative effects of the UV background fluctuations on the flux power spectrum. The background fluctuations increase the large-scale power and suppress the power at intermediate scales. At z less than or equal to 4, the effect is only at the few per cent level. By z = 5.5, however, the large-scale power is boosted by more than 10 per cent, growing rapidly at higher redshifts, and suggesting that the flux power spectrum may serve as a useful tool for distinguishing QSO-dominated UV background scenarios from those in which more abundant sources like galaxies dominate.