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 7 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 H I proximity effect as arising from faint galaxies clustered around the LBGs, we translate the observed mean Lyα transmitted flux into a constraint on the mean escape fraction f esc ≥ 0.08 at z 6. We also report individual transverse H I 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 reionisation, 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 reionisation.

Observing the tail of reionization: neutral islands in the z = 5.5 Lyman-α forest

“…due to an earlier reionization. The correlation between forest transmission and galaxies/LAEs is an informative window into the nature of IGM fluctuations at these redshifts Kakiichi et al 2018;Kashino et al 2019).…”

Lyman continuum escape fraction in Ly α emitters at z ≃ 3.1

“…However, LyC escape fraction can not be directly measured in galaxies beyond z = 4.5 due to the high opacity of the IGM to LyC ionising photons (e.g., Vanzella et al 2018). Thus we have to infer the LyC escape in the galaxies at the epoch of the reioisation based on either directly measuring LyC escape fraction in galaxies at lower redshift or correlating galaxy spatial positions with the Lyman alpha forest at z > 6 (e.g., Kakiichi et al 2018). In the last decade, people have conducted extensive studies of LyC escape fraction in galaxies at z < 4 using a number of different approaches, including the rest-frame ultraviolet spectroscopy (e.g., Leitet et al 2013;Leitherer et al 2016;Izotov et al 2016b,a;Steidel et al 2001Steidel et al , 2018Shapley et al 2006Shapley et al , 2016Nestor et al 2013) and narrow/intermediate/broad-band UV imaging (e.g., Siana et al 2007Siana et al , 2015Vanzella et al 2010Vanzella et al , 2016Nestor et al 2011;Cooke et al 2014;Rutkowski et al 2016Rutkowski et al , 2017Vasei et al 2016;Bian et al 2017;Japelj et al 2017;Naidu et al 2017;Fletcher et al 2018;Ji et al 2019).…”

Modelling a bright z = 6 galaxy at the faint end of the AGN luminosity function

“…In this picture, quasars mainly maintain the post-reionization UV background (e.g. Becker & Bolton 2013;Kulkarni et al 2019), with some additional role in determining the patchiness of the end of the process of reionization (Chardin et al 2015(Chardin et al , 2017Kakiichi et al 2018). This is for instance supported by observations of faint lensed galaxies behind clusters, indicating that the faint-end of the galaxy UV luminosity function (LF) might be steep, with no sign of any turn-over brighter than M UV −15 (e.g.…”