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

We demonstrate that the Lyman alpha 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 wave-length has been added to the observed absorption lines as identified and fitted with VPFIT. At 0.03 s km(-1) < k < 0.1 s km(-1) the '3D' power spectrum of the randomized flux agrees with that of observed spectra within the errors. At larger scales it still approaches 50 per cent of that of the observed spectra. At smaller scales the flux power spectrum is dominated by metal lines. Lines of increasing column density contribute to the '3D' flux power spectrum at increasingly larger scales. Lines with 13 < log(N-HI/cm(-2)) < 15 dominate at the peak of the '3D' power spectrum while strong absorbers with log(NHI/cm-2) > 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.