2 v 1 3 0 Se p 19 97 Recovery of the mass power spectrum from the Ly - α for - est : The effect of ionizing background fluctuations

Recovery of the mass power spectrum from the Ly-α forest: The effect of ionizing background fluctuations from discrete QSO sources. Abstract. In gravitational instability models, there is a close relationship between most Lyα absorption and the matter density along the line of sight. Croft et al. (1997) have shown that it is possible to use this relationship with some additional assumptions (such as Gaussianity of the initial density) to recover the power spectrum of mass fluctuations, P (k), from QSO absorption spectra. The relative uniformity of the ionizing radiation background on the scales of interest is an additional assumption required by the technique. Here we examine the fluctuations in Lyα spectra caused if the ionizing background radiation is generated by discrete QSO sources. We present our results alongside the preliminary application of the P (k) recovery technique to the QSO Q1422+231. We find the ionizing background fluctuations to have an effect orders of magnitude smaller than the matter fluctuations at the scales on which we measure P (k) (λ < 10 h −1 Mpc). In the " Fluctuating Gunn-Peterson Approximation " for the high-z Lyα forest (see the contribution by Weinberg in these proceedings), variations in the optical depth along the line of sight to a QSO are directly related to fluctuations both in the underlying matter density and in the intensity of the ionizing background radiation. Croft et al. (1997) ([1]) have developed a technique to recover P (k) for the mass, which assumes that only the matter fluctuations have a non-negligible effect. The results of an illustrative application to a Keck spectrum of QSO Q1422+231 (observation by [3]) are shown in Figure 1(a). In order to test at least qualitatively the effect of ionizing background flucuations, which were not included in [1] , we have created simulated QSO Lyα absorption spectra in a universe where the matter density is uniform and the only fluctuations present in absorption are those due to the sources of ionizing radiation being discrete. We have populated a 0.6 h −1 Gpc comoving cube (in a q 0 = 0.1, h 100 =0.5 universe) with QSOs distributed randomly and with luminosities randomly sampled from the (z = 3) luminosity function of [2]. We include a lower cutoff at M B = −23 and calculate the opacity of the IGM in Euclidean space in the optically thin limit. Five randomly chosen example spectra (each is …