The amplitude of the ionizing background that pervades the intergalactic
medium (IGM) at the end of the epoch of reionization provides a valuable
constraint on the emissivity of the sources which reionized the Universe. While
measurements of the ionizing background at lower redshifts rely on a
simulation-calibrated mapping between the photoionization rate and the mean
transmission of the Ly$\alpha$ forest, at $z\gtrsim6$ the IGM becomes
increasingly opaque, and transmission arises solely in narrow spikes separated
by saturated Gunn-Peterson troughs. In this regime, the traditional approach of
measuring the average transmission over large $50$ Mpc$/h$ regions is less
sensitive and sub-optimal. Additionally, the five times smaller oscillator
strength of the Ly$\beta$ transition implies the Ly$\beta$ forest is
considerably more transparent at $z\gtrsim6$, even in the presence of
contamination by foreground $z5$ Ly$\alpha$ forest absorption. In this
work we present a novel statistical approach to analyze the joint distribution
of transmission spikes in the co-spatial $z6$ Ly$\alpha$ and Ly$\beta$
forests. Our method relies on Approximate Bayesian Computation (ABC), which
circumvents the necessity of computing the intractable likelihood function
describing the highly correlated Ly$\alpha$ and Ly$\beta$ transmission. We
apply ABC to mock data generated from a large-volume hydrodynamical simulation
combined with a state-of-the-art model of ionizing background fluctuations in
the post-reionization IGM, and show that it is sensitive to higher IGM neutral
hydrogen fractions than previous techniques. As a proof of concept, we apply
this methodology to a real spectrum of a $z=6.54$ quasar and measure the
ionizing background from $5.4z 6.4$ along this sightline with
$\sim0.2$ dex statistical uncertainties.
Description
[1703.10174] A New Method to Measure the Post-Reionization Ionizing Background from the Joint Distribution of Lyman-$\alpha$ and Lyman-$\beta$ Forest Transmission
%0 Generic
%1 davies2017method
%A Davies, Frederick B.
%A Hennawi, Joseph F.
%A Eilers, Anna-Christina
%A Lukić, Zarija
%D 2017
%K IGM reionization transmission
%T A New Method to Measure the Post-Reionization Ionizing Background from
the Joint Distribution of Lyman-$\alpha$ and Lyman-$\beta$ Forest
Transmission
%U http://arxiv.org/abs/1703.10174
%X The amplitude of the ionizing background that pervades the intergalactic
medium (IGM) at the end of the epoch of reionization provides a valuable
constraint on the emissivity of the sources which reionized the Universe. While
measurements of the ionizing background at lower redshifts rely on a
simulation-calibrated mapping between the photoionization rate and the mean
transmission of the Ly$\alpha$ forest, at $z\gtrsim6$ the IGM becomes
increasingly opaque, and transmission arises solely in narrow spikes separated
by saturated Gunn-Peterson troughs. In this regime, the traditional approach of
measuring the average transmission over large $50$ Mpc$/h$ regions is less
sensitive and sub-optimal. Additionally, the five times smaller oscillator
strength of the Ly$\beta$ transition implies the Ly$\beta$ forest is
considerably more transparent at $z\gtrsim6$, even in the presence of
contamination by foreground $z5$ Ly$\alpha$ forest absorption. In this
work we present a novel statistical approach to analyze the joint distribution
of transmission spikes in the co-spatial $z6$ Ly$\alpha$ and Ly$\beta$
forests. Our method relies on Approximate Bayesian Computation (ABC), which
circumvents the necessity of computing the intractable likelihood function
describing the highly correlated Ly$\alpha$ and Ly$\beta$ transmission. We
apply ABC to mock data generated from a large-volume hydrodynamical simulation
combined with a state-of-the-art model of ionizing background fluctuations in
the post-reionization IGM, and show that it is sensitive to higher IGM neutral
hydrogen fractions than previous techniques. As a proof of concept, we apply
this methodology to a real spectrum of a $z=6.54$ quasar and measure the
ionizing background from $5.4z 6.4$ along this sightline with
$\sim0.2$ dex statistical uncertainties.
@misc{davies2017method,
abstract = {The amplitude of the ionizing background that pervades the intergalactic
medium (IGM) at the end of the epoch of reionization provides a valuable
constraint on the emissivity of the sources which reionized the Universe. While
measurements of the ionizing background at lower redshifts rely on a
simulation-calibrated mapping between the photoionization rate and the mean
transmission of the Ly$\alpha$ forest, at $z\gtrsim6$ the IGM becomes
increasingly opaque, and transmission arises solely in narrow spikes separated
by saturated Gunn-Peterson troughs. In this regime, the traditional approach of
measuring the average transmission over large $\sim 50$ Mpc$/h$ regions is less
sensitive and sub-optimal. Additionally, the five times smaller oscillator
strength of the Ly$\beta$ transition implies the Ly$\beta$ forest is
considerably more transparent at $z\gtrsim6$, even in the presence of
contamination by foreground $z\sim 5$ Ly$\alpha$ forest absorption. In this
work we present a novel statistical approach to analyze the joint distribution
of transmission spikes in the co-spatial $z\sim 6$ Ly$\alpha$ and Ly$\beta$
forests. Our method relies on Approximate Bayesian Computation (ABC), which
circumvents the necessity of computing the intractable likelihood function
describing the highly correlated Ly$\alpha$ and Ly$\beta$ transmission. We
apply ABC to mock data generated from a large-volume hydrodynamical simulation
combined with a state-of-the-art model of ionizing background fluctuations in
the post-reionization IGM, and show that it is sensitive to higher IGM neutral
hydrogen fractions than previous techniques. As a proof of concept, we apply
this methodology to a real spectrum of a $z=6.54$ quasar and measure the
ionizing background from $5.4\leq z \leq 6.4$ along this sightline with
$\sim0.2$ dex statistical uncertainties.},
added-at = {2017-03-31T10:04:32.000+0200},
author = {Davies, Frederick B. and Hennawi, Joseph F. and Eilers, Anna-Christina and Lukić, Zarija},
biburl = {https://www.bibsonomy.org/bibtex/2152ebf3f65b0121e4149214d150feae9/miki},
description = {[1703.10174] A New Method to Measure the Post-Reionization Ionizing Background from the Joint Distribution of Lyman-$\alpha$ and Lyman-$\beta$ Forest Transmission},
interhash = {554e120d4967d3b335f0d2369b511cfc},
intrahash = {152ebf3f65b0121e4149214d150feae9},
keywords = {IGM reionization transmission},
note = {cite arxiv:1703.10174Comment: 15 pages, 18 figures, ApJ submitted},
timestamp = {2017-03-31T10:04:32.000+0200},
title = {A New Method to Measure the Post-Reionization Ionizing Background from
the Joint Distribution of Lyman-$\alpha$ and Lyman-$\beta$ Forest
Transmission},
url = {http://arxiv.org/abs/1703.10174},
year = 2017
}