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

Abstract

Previous studies have noted difficulties in modeling the highest opacities of the z > 5.5 Lyα forest, epitomized by the extreme Lyα trough observed towards quasar ULAS J0148+0600. One possibility is that the most opaque regions at these redshifts contain significant amounts of neutral hydrogen. This explanation, which abandons the common assumption that reionization ended before z = 6, also reconciles evidence from independent observations of a significantly neutral Universe at z = 7.5. Here we explore a model in which the neutral fraction is still ≈ 10% at z = 5.5. We confirm that this model can account for the observed scatter in Lyα forest opacities, as well as the observed Lyβ transmission in the J0148 trough. We contrast the model with a competing "earlier" reionization scenario characterized by a short mean free path and large fluctuations in the post-reionization ionizing background. We consider Lyα and Lyβ effective optical depths, their correlations, trough size distributions, dark pixel fractions, the IGM thermal history, and spatial distributions of Lyman-α emitters around forest sight lines. We find that the models are broadly similar in almost all of these statistics, suggesting that it may be difficult to distinguish between them definitively. We argue that improved constraints on the mean free path and the thermal history at z > 5 could go a long way towards diagnosing the origin of the z > 5.5 opacity fluctuations.

The mean free path of ionizing photons at 5 < z < 6: evidence for rapid evolution near reionization

“…A short mean free path at z = 6.0 and a rapid increase from z = 6 to 5 are qualitatively consistent with models wherein reionization ends at z ∼ 6, or even later (e.g. Kulkarni et al 2019;Keating et al 2020a, b;Nasir & D'Aloisio 2020), but disfavour models wherein reionization ended early enough that the IGM has had time to fully relax by z ∼ 6 (see D 'Aloisio et al 2020).…”

“…3 This form is motivated by analytic models of the IGM opacity (Miralda-Escudé, Haehnelt & Rees 2000; Furlanetto & Oh 2005), as well as radiative transfer simulations of Lyman limit systems (McQuinn, Oh & Faucher-Giguère 2011). These studies suggest values of ξ ∼ 2/3 at z > 5, which has been adopted in recent models of the Ly α forest opacity fluctuations at these redshifts (Davies & Furlanetto 2016;D'Aloisio et al 2018;Nasir & D'Aloisio 2020). The uniform opacity model used by Worseck et al (2014) corresponds to ξ = 0.…”

A Significantly Neutral Intergalactic Medium Around the Luminous z = 7 Quasar J0252–0503

“…Observations of the Lyman series forests in z6 quasars indicate that the IGM is already highly ionized by z∼6 (e.g., Fan et al 2006;Bosman et al 2018;Eilers et al 2018Eilers et al , 2019Yang et al 2020), although the final completion of reionization might extend down to z∼5.5 (e.g., Becker et al 2015;Davies et al 2018a;Kulkarni et al 2019;Keating et al 2020;Nasir & D'Aloisio 2020). However, the Lyman series forests are very sensitive to neutral hydrogen and saturate even at low IGM neutral fraction (i.e., á ñ - x 10 H 4…”

The detectability of strong 21 centimetre forest absorbers from the diffuse intergalactic medium in late reionisation models

“…Furthermore, there is now growing evidence that reionisation ended rather late, and possibly even extended to redshifts as late as 5.3 (Kulkarni et al 2019;Nasir & D'Aloisio 2020;Qin et al 2021). This picture is motivated by the large spatial fluctuations observed in the Ly forest transmission at 5.5 (Becker et al 2015;Eilers et al 2018).…”