The effect of (strong) discrete absorption systems on the Lyman α forest flux power spectrum

We demonstrate that the Lyman a forest flux power spectrum of 'randomized' quasi-stellar object (QSO) absorption spectra is comparable in shape and amplitude to the flux power spectrum of the original observed spectra. In the randomized spectra a random shift in wavelength has been added to the observed absorption lines as identified and fitted with VPFIT. At 0.03 s km -1 15 contribute at large scales, k < 0.03 s km -1 . We further show that a fraction of ≥ 15 per cent of the mean flux decrement is contributed by strong absorbers at z ≥ 2.1. Analysis of the flux power spectrum which use numerical simulations with too few strong absorption systems calibrated with the observed mean flux may underestimate the inferred rms fluctuation amplitude and the slope of the initial dark matter power spectrum.