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

Research output: Contribution to journalArticle

  • Gabor Worseck
  • J. Xavier Prochaska
  • John M. O'Meara
  • George D. Becker
  • Sara L. Ellison
  • Sebastian Lopez
  • Avery Meiksin
  • Brice Menard
  • Michael T. Murphy
  • Michele Fumagalli

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Original languageEnglish
Pages (from-to) 1745-1760
Number of pages16
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
Early online date14 Oct 2014
Publication statusPublished - 1 Dec 2014


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.

    Research areas

  • galaxies: formation, intergalactic medium, quasars: absorption lines, dark ages, reionization, first stars , diffuse radiation

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