The role of galaxies and agn in reionizing the IGM-I. Keck spectroscopy of 5 < z < 7 galaxies in the QSO field J1148+5251

Koki Kakiichi, Richard S. Ellis, Nicolas Laporte, Adi Zitrin, Anna Christina Eilers, Emma Ryan-Weber, Romain A. Meyer, Brant Robertson, Daniel P. Stark, Sarah E.I. Bosman

Research output: Contribution to journalArticlepeer-review

40 Scopus citations


We introduce a new method for determining the influence of galaxies and active galactic nuclei (AGN) on the intergalactic medium (IGM) at high redshift and illustrate its potential via a first application to the field of the z = 6.42 QSO J1148+5251. Correlating spatial positions Lyman break galaxies (LBGs) with the Lyman alpha forest seen in the spectrum of a background QSO, we provide a statistical measure of the typical escape fraction of Lyman continuum photons. Using Keck DEIMOS spectroscopy to locate seven colour-selected LBGs in the range 5.3≲z≲6.4 we examine the spatial correlation between this sample and Lyα/Lyβ transmission fluctuations in a Keck ESI spectrum of the QSO. Interpreting the statistical HI proximity effect as arising from faint galaxies clustered around the LBGs, we translate the observed mean Lya transmitted flux into a constraint on the mean escape fraction (fesc) ≥ 0.08 at z ≃ 6. We also report individual transverse HI proximity effect for a z = 6.177 luminous LBG via a Lyβ transmission spike and two broad Lyα transmission spikes around the z = 5.701 AGN. We discuss the origin of such associations which suggest that while faint galaxies are primarily driving reionization, luminous galaxies and AGN may provide important contributions to the UV background or thermal fluctuations of the IGM at z≃6. Although a limited sample, our results demonstrate the potential of making progress using this method in resolving one of the most challenging aspects of the contribution of galaxies and AGN to cosmic reionization.

Original languageEnglish
Pages (from-to)43-63
Number of pages21
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
StatePublished - 1 Sep 2018


  • Cosmology: observations
  • Dark ages, reionization, first stars
  • Galaxies: formation
  • Galaxies: high
  • Intergalactic medium
  • Quasars: absorption lines
  • Redshift

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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