The role of galaxies and AGN in reionising the IGM -- III : IGM-galaxy cross-correlations at z~6 from 8 quasar fields with DEIMOS and MUSE

TL;DR

This study measures the correlation between galaxies and IGM transmission at z~6, revealing an excess of Lyman-alpha transmission near galaxies and constraining the escape fraction of ionizing photons, suggesting faint galaxies significantly contributed to reionisation.

Contribution

It provides the first detection of Lyman-alpha transmission excess at large scales around galaxies at z~6 and models the galaxy-IGM cross-correlations including multiple physical effects.

Findings

Detected excess Lyman-alpha transmission spikes at 10-60 cMpc from galaxies.

Constrained the average escape fraction of ionizing photons to about 0.14-0.23.

Faint galaxies are necessary contributors to reionisation, possibly dominating over bright galaxies.

Abstract

We present improved results of the measurement of the correlation between galaxies and the intergalactic medium (IGM) transmission at the end of reionisation. We have gathered a sample of $13$ spectroscopically confirmed Lyman-break galaxies (LBGs) and $21$ Lyman-$\alpha$ emitters (LAEs) at angular separations $20'' \lesssim \theta \lesssim 10'$ ($\sim 0.1-4$ pMpc at $z\sim 6$) from the sightlines to $8$ background $z\gtrsim 6$ quasars. We report for the first time the detection of an excess of Lyman-$\alpha$ transmission spikes at $\sim 10-60$ cMpc from LAEs ($3.2\sigma$) and LBGs ($1.9\sigma$). We interpret the data with an improved model of the galaxy-Lyman-$\alpha$ transmission and two-point cross-correlations which includes the enhanced photoionisation due to clustered faint sources, enhanced gas densities around the central bright objects and spatial variations of the mean free path. The observed LAE(LBG)-Lyman-$\alpha$ transmission spike two-point cross-correlation function (2PCCF) constrains the luminosity-averaged escape fraction of all galaxies contributing to reionisation to $\langle f_{\rm esc} \rangle_{M_{\rm UV}<-12} = 0.14_{-0.05}^{+0.28}\,(0.23_{-0.12}^{+0.46})$. We investigate if the 2PCCF measurement can determine whether bright or faint galaxies are the dominant contributors to reionisation. Our results show that a contribution from faint galaxies ($M_{\rm UV} > -20 \, (2\sigma)$) is necessary to reproduce the observed 2PCCF and that reionisation might be driven by different sub-populations around LBGs and LAEs at $z\sim 6$.