# 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

**Authors:** Romain A. Meyer, Koki Kakiichi, Sarah E. I. Bosman, Richard S. Ellis, Nicolas Laporte, Brant E. Robertson, Emma V. Ryan-Weber, Ken Mawatari, Adi Zitrin  
**arXiv:** [1912.04314](https://arxiv.org/abs/1912.04314)  
**Date:** 2019-12-09  
**Summary page:** https://www.summarizepaper.com/en/paper/1912.04314v1/

## Key points

- Researchers presented improved results on the correlation between galaxies and the intergalactic medium (IGM) transmission at the end of reionisation.
- The team gathered a sample of 13 spectroscopically confirmed Lyman-break galaxies (LBGs) and 21 Lyman-alpha emitters (LAEs).
- They detected an excess of Lyman-alpha transmission spikes at $\sim$10-60 cMpc from LAEs ($3.6\sigma$) and LBGs ($3.1\sigma$).
- The observed LAE(LBG)-Lyman-alpha transmission spike two-point cross-correlation function (2PCCF) constrained the luminosity-averaged escape fraction of all galaxies contributing to reionisation.
- The team investigated whether their 2PCCF measurement could determine whether bright or faint galaxies were dominant contributors to reionisation.
- A contribution from faint galaxies ($M_{UV}> -20$, $2\sigma$) was necessary to reproduce the observed 2PCCF, suggesting that reionisation might be driven by different subpopulations around LBGs and LAEs at $z \sim 6$.
- This study provides new insights into the role of galaxies and AGN in reionising the IGM.

## Summary

In a recent study, researchers presented improved results on the correlation between galaxies and the intergalactic medium (IGM) transmission at the end of reionisation. The team gathered a sample of 13 spectroscopically confirmed Lyman-break galaxies (LBGs) and 21 Lyman-alpha emitters (LAEs) at angular separations ranging from 20'' to 10' ($\sim$0.1-4 pMpc at $z\sim6$) from the sightlines to eight background $z\gtrsim6$ quasars. For the first time, they detected an excess of Lyman-alpha transmission spikes at $\sim$10-60 cMpc from LAEs ($3.6\sigma$) and LBGs ($3.1\sigma$). To interpret their data, the team used an improved model of galaxy-Lyman-alpha transmission and two-point cross-correlations that included enhanced photoionisation due to clustered faint sources, enhanced gas densities around central bright objects, and spatial variations in mean free path. The observed LAE(LBG)-Lyman-alpha transmission spike two-point cross-correlation function (2PCCF) constrained the luminosity-averaged escape fraction of all galaxies contributing to reionisation to $\langle f_{esc} \rangle_{M_{UV}<-12}=0.10^{+0.17}_{-0.05}$ for LBGs and $\langle f_{esc} \rangle_{M_{UV}<-12}=0.18^{+0.52}_{-0.06}$ for LAEs. The team also investigated whether their 2PCCF measurement could determine whether bright or faint galaxies were dominant contributors to reionisation. Their results showed that a contribution from faint galaxies ($M_{UV}> -20$, $2\sigma$) was necessary to reproduce the observed 2PCCF and that reionisation might be driven by different subpopulations around LBGs and LAEs at $z \sim 6$. Overall, this study provides new insights into the role of galaxies and AGN in reionising the IGM. The team's improved model and measurements could help refine our understanding of how galaxies contribute to reionisation and how it varies across different populations of galaxies.