The Universe Is Reionizing at z ∼ 7: Bayesian Inference of the IGM Neutral Fraction Using Lyα Emission from Galaxies

Abstract

We present a new flexible Bayesian framework for directly inferring the fraction of neutral hydrogen in the intergalactic medium (IGM) during the Epoch of Reionization (EoR, z ∼ 6 − 10) from detections and non-detections of Lyman Alpha (Lyα) emission from Lyman Break galaxies (LBGs). Our framework combines sophisticated reionization simulations with empirical models of the interstellar medium (ISM) radiative transfer effects on Lyα. We assert that the Lyα line profile emerging from the ISM has an important impact on the resulting transmission of photons through the IGM, and that these line profiles depend on galaxy properties. We model this effect by considering the peak velocity offset of Lyα lines from host galaxies' systemic redshifts, which are empirically correlated with UV luminosity and redshift (or halo mass at fixed redshift). We use our framework on the sample of LBGs presented in Pentericci et al. (2014) and infer a global neutral fraction at z ∼ 7 of x hi = 0.59 +0.11 −0.15 , consistent with other robust probes of the EoR and confirming reionization is on-going ∼ 700 Myr after the Big Bang. We show that using the full distribution of Lyα equivalent width detections and upper limits from LBGs places tighter constraints on the evolving IGM than the standard Lyα emitter fraction, and that larger samples are within reach of deep spectroscopic surveys of gravitationally lensed fields and JWST NIRSpec.

Long troughs in the Lyman-α forest below redshift 6 due to islands of neutral hydrogen

“…Another compilation of effective optical depths is presented in Eilers et al (2018), however due to systematic differences between the two datasets it is not possible to find a single reionization history that fits both measurements at once. We therefore limit our study here to a comparison with the , quasar damping wing studies (Greig et al 2017(Greig et al , 2019Davies et al 2018) and estimates from the fraction of Lyman break galaxies that show Lyα emission (Mason et al 2018(Mason et al , 2019. Bosman et al (2018) compilation.…”

The redshift evolution of rest-UV spectroscopic properties to z ∼ 5

“…Measuring the rest-UV spectra of distant galaxies requires optical instrumentation and long integrations. Based on observations with instruments such as the Low Resolution Imager and Spectrometer (LRIS) on the Keck I telescope (Oke et al 1995;Steidel et al 2004), there have been several analyses of the relationships between rest-UV spectral features and other galaxy properties out to z ∼ 4 (e.g., Du et al 2018;Jones et al 2012;Marchi et al 2019). Du et al (2018) (hereafter, D18) performed a comprehensive evolu-tionary analysis of rest-UV spectral trends of star forming galaxies from z ∼ 2-4.…”

The Imprint of Cosmic Reionization on the Luminosity Function of Galaxies

“…An alternative probe of the ionization state of the IGM comes from the spectra of QSOs: the absence of a Gunn-Peterson trough in the absorption spectra of quasars at z6 (e.g., Fan et al 2000), and its presence in spectra at z6 (e.g., Becker et al 2001;Bolton et al 2011), suggest that the universe completed the transition from neutral to ionized at around that time. A third source of evidence is the decline in Lyα emission 1 observed from galaxies at z>6, attributed to absorption by intervening H I gas (Stark & Ellis 2006;Fontana et al 2010;Caruana et al 2012Caruana et al , 2014Treu et al 2013;Pentericci et al 2014;Schenker et al 2014;Tilvi et al 2014;Mason et al 2018).…”