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Temperature fluctuations in the intergalactic medium

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The temperature of the low-density intergalactic medium (IGM) is set by the balance between adiabatic cooling resulting from the expansion of the Universe , and photoheating by the ultraviolet (UV) background. We have analysed the Lyα forest of 11 high-resolution quasar spectra using wavelets, and find strong evidence of a marked jump in the temperature at the mean density, T0, of 60±14 per cent over the redshift interval z=[3.5,3.1] , which we attribute to reionization of He ii. The jump can be seen in all three of our spectra that straddle redshift 3.3, at a significance of 99 per cent. Below z∼3.1 , our results are consistent with a smooth cooling down of the universe, as expected when adiabatic expansion dominates over photoheating by a UV background from quasi-stellar objects (QSOs) and galaxies. We find no evidence of thermal fluctuations on scales 5000 km s−1 larger than 50 per cent, which could be detected by our method, suggesting that the IGM follows a reasonably well-defined temperature–density relation. We demonstrate that the mean wavelet amplitude 〈A〉∝1/T0 , and calibrate the relation with hydrodynamical simulations. We find T01.2×104 K at z3.6 . Such high temperatures suggest that H ireionization occurred relatively recently.
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Keywords: cosmology: theory; hydrodynamics; intergalactic medium; large-scale structure of Universe; quasars: absorption lines

Document Type: Research Article

Affiliations: 1: 1Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, 2: 2Max-Planck Institut für Astrophysik, Postfach 123, 85740 Garching, Germany and 3: 3European Southern Observatory, Karl-Schwarzschild-Straße 2, D-85748 Garching bei München, Germany

Publication date: May 1, 2002

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