# The Role of Galaxies and AGN in Reionising the IGM - I: Keck Spectroscopy of 5 < z < 7 Galaxies in the QSO Field J1148+5251

**Authors:** 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  
**arXiv:** [1803.02981](https://arxiv.org/abs/1803.02981)  
**Date:** 2018-03-08  
**Summary page:** https://www.summarizepaper.com/en/paper/1803.02981v1/

## Key points

- A new method for determining the impact of galaxies and active galactic nuclei (AGN) on the physical state of the intergalactic medium (IGM) at high redshift is introduced
- The method is applied to the field of the $z=6.42$ QSO J1148+5251
- Keck DEIMOS spectroscopy is used to locate 7 colour-selected Lyman break galaxies (LBGs) in the redshift range $5.3\lesssim z\lesssim 6.4$, and a faint $z=5.701$ AGN is confirmed
- The researchers examine the spatial correlation between this sample and Ly$\alpha$/Ly$\beta$ transmission fluctuations in a Keck ESI spectrum of the QSO, providing a statistical measure of the typical escape fraction of Lyman continuum photons close to the end of cosmic reionisation
- Observed mean Ly$\alpha$ transmitted flux around an average detected LBG is translated into a constraint on mean escape fraction $\langle f_{\rm esc}\rangle\geq0.08$ at $z\simeq6$
- Evidence is reported for individual transverse HI proximity effects from 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
- Faint galaxies are primarily driving reionisation, but luminous galaxies and AGN may provide important contributions to UV background or thermal fluctuations of the IGM at $z\simeq6$
- This method has potential in resolving one of the most challenging aspects of understanding how galaxies and AGN contribute to cosmic reionisation

## Summary

This study introduces a new method for determining the impact of galaxies and active galactic nuclei (AGN) on the physical state of the intergalactic medium (IGM) at high redshift. The researchers demonstrate the potential of this method by applying it to the field of the $z=6.42$ QSO J1148+5251. Keck DEIMOS spectroscopy is used to locate 7 colour-selected Lyman break galaxies (LBGs) in the redshift range $5.3\lesssim z\lesssim 6.4$, and a faint $z=5.701$ AGN is confirmed. The researchers then examine the spatial correlation between this sample and Ly$\alpha$/Ly$\beta$ transmission fluctuations in a Keck ESI spectrum of the QSO, providing a statistical measure of the typical escape fraction of Lyman continuum photons close to the end of cosmic reionisation. Interpreting the statistical HI proximity effect as arising from faint galaxies clustered around detected LBGs, they translate observed mean Ly$\alpha$ transmitted flux around an average detected LBG into a constraint on mean escape fraction $\langle f_{\rm esc}\rangle\geq0.08$ at $z\simeq6$. Evidence is also reported for individual transverse HI proximity effects from 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, suggesting that while faint galaxies are primarily driving reionisation, luminous galaxies and AGN may provide important contributions to UV background or thermal fluctuations of the IGM at $z\simeq6$. These results demonstrate the potential of this method in resolving one of the most challenging aspects of understanding how galaxies and AGN contribute to cosmic reionisation.