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

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

Research output: Contribution to journalArticlepeer-review


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.
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


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

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