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The Giant Gemini GMOS survey of zem > 4.4 quasars – I. Measuring the mean free path across cosmic time.

Worseck, G. and Prochaska, J.X. and O'Meara, J. and Becker, G. and Ellison, S. and Lopez, S. and Meiksin, A. and Ménard, B. and Murphy, M. and Fumagalli, M. (2014) 'The Giant Gemini GMOS survey of zem > 4.4 quasars – I. Measuring the mean free path across cosmic time.', Monthly notices of the Royal Astronomical Society., 445 (2). pp. 1745-1760.


We have obtained spectra of 163 quasars at zem > 4.4 with the Gemini Multi Object Spectrometers, the largest publicly available sample of high-quality, low-resolution spectra at these redshifts. From this data set, we generated stacked quasar spectra in three redshift intervals at z ∼ 5 to model the average rest-frame Lyman continuum flux and to assess the mean free path λ912mfp of the intergalactic medium to H I-ionizing radiation. At mean redshifts zq = (4.56, 4.86, 5.16), we measure λ912mfp=(22.2±2.3,15.1±1.8,10.3±1.6)h−170 proper Mpc with uncertainties dominated by sample variance. Combining our results with measurements from lower redshifts, the data are well modelled by a power law λ912mfp=A[(1+z)/5]η with A=(37±2)h−170 Mpc and η = −5.4 ± 0.4 at 2.3 < z < 5.5. This rapid evolution requires a physical mechanism – beyond cosmological expansion – which reduces the effective Lyman limit opacity. We speculate that the majority of H I Lyman limit opacity manifests in gas outside galactic dark matter haloes, tracing large-scale structures (e.g. filaments) whose average density and neutral fraction decreases with cosmic time. Our measurements of the mean free path shortly after H I reionization serve as a valuable boundary condition for numerical models thereof. Our measured λ912mfp≈10 Mpc at z = 5.2 confirms that the intergalactic medium is highly ionized without evidence for a break that would indicate a recent end to H I reionization.

Item Type:Article
Keywords:Galaxies: formation, Intergalactic medium, Quasars: absorption lines, Dark ages, Reionization, First stars, Diffuse radiation.
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Publisher statement:This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Date accepted:No date available
Date deposited:17 October 2014
Date of first online publication:December 2014
Date first made open access:No date available

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