THE IMPACT OF TEMPERATURE FLUCTUATIONS ON THE Lyα FOREST POWER SPECTRUM

We explore the impact of spatial fluctuations in the intergalactic medium (IGM) temperature on the Lyα forest flux power spectrum near z ∼ 3. We develop a semi-analytic model to examine temperature fluctuations resulting from inhomogeneous HI and incomplete HeII reionizations. There are two main motivations for this investigation. First, detection of these fluctuations might provide insight into the reionization histories of hydrogen and helium. Second, these fluctuations, neglected in previous analyses, could bias constraints on cosmological parameters from the Lyα forest. The inhomogeneous reionization of HI results in temperature fluctuations, since different regions in the IGM, reionized at different cosmic times, have different temperatures at lower redshift. However, we find that these fluctuations are likely to be very small, with an r.m.s. amplitude of . 5%, σT0/〈T0〉 . 0.05. Temperature fluctuations that arise from incomplete HeII reionization are larger, owing to the large temperature differences between the hot HeII reionized regions and the cooler surrounding IGM, in which HI/HeI are reionized at high redshift, but HeII has yet to reionize. These fluctuations might plausibly be as large as 50%, σT0/〈T0〉 ∼ 0.5. In practice, however, these temperature fluctuations have only a small effect on flux power spectrum predictions. The smallness of the effect is possibly due to density fluctuations dominating over temperature fluctuations on the scales probed by current measurements. Quantitatively, temperature fluctuations have a negligible effect on the flux power spectrum on intermediate scales: . 2% at k ∼ 0.01 s/km (∼ 1 hMpc). On the largest scales currently probed, k ∼ 0.001 s/km (∼ 0.1 hMpc), the effect may be as large as ∼ 10% in extreme models. The effect is larger on small scales, up to ∼ 20% at k = 0.1 s/km, due to thermal broadening. Our results suggest that the omission of temperature fluctuations effects from previous analyses does not significantly bias constraints on cosmological parameters. Subject headings: cosmology: theory – intergalactic medium – large scale structure of universe; quasars – absorption lines