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

Abstract

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 less than or similar to z less than or similar to 6.4 we examine the spatial correlation between this sample and Ly alpha/Ly beta transmission fluctuations in a Keck ESI spectrum of the QSO. Interpreting the statistical II I proximity effect as arising from faint galaxies clustered around the LBGs, we translate the observed mean Lyra, transmitted flux into a constraint on the mean escape fraction < f(esc)> >= 0.08 at z similar or equal to 6. We also report individual transverse II I proximity effect for a z = 6.177 luminous LBG via a Ly beta transmission spike and two broad Ly alpha 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 similar or equal to 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.