[Paper Review] Dark-ages reionization and galaxy formation simulation -- XXI. Constraining the evolution of the ionizing escape fraction

Simon J. Mutch, Bradley Greig+3 more|arXiv (Cornell University)|Mar 13, 2023

Galaxies: Formation, Evolution, Phenomena4 citations

TL;DR

This study uses the Meraxes semi-analytic galaxy formation model with a Bayesian framework to jointly constrain the redshift- and halo mass-dependent ionizing photon escape fraction from high-redshift galaxies. It finds that low-mass haloes ($M_\text{halo} \lesssim 10^9\,M_\odot$) have a high escape fraction of $18\pm5\%$, while more massive haloes ($M_\text{halo} \gtrsim 10^9\,M_\odot$) have a lower escape fraction of $5\pm2\%$, with a transition at stellar mass $M_\star \sim 10^7\,M_\odot$ that is nearly redshift-independent.

ABSTRACT

The fraction of ionizing photons that escape their host galaxies to ionize hydrogen in the inter-galactic medium (IGM) is a critical parameter in analyses of the reionization era. In this paper we use the Meraxes semi-analytic galaxy formation model to infer the mean ionizing photon escape fraction and its dependence on galaxy properties through joint modelling of the observed high redshift galaxy population and existing constraints on the reionization history. Using a Bayesian framework, and under the assumption that escape fraction is primarily related to halo mass, we find that the joint constraints of the UV luminosity function, CMB optical depth, and the Ly$α$ forest require an escape fraction of $(18\pm5)\%$ for galaxies within haloes of $M\lesssim10^{9}$M$_\odot$ and $(5\pm2)\%$ for more massive haloes. In terms of galaxy properties, this transition in escape fraction occurs at stellar masses of $M_\star\sim10^7$M$_\odot$, nearly independent of redshift. As a function of redshift, reionization is dominated by the smaller $M_\star\lesssim10^7$M$_\odot$ galaxies with high escape fractions at $z\gtrsim6$ and by the larger $M_\star\gtrsim10^7$M$_\odot$ galaxies with lower escape fractions at $z\lesssim6$. Galaxies with star formation rates of $10^{-2.5}$M$_\odot$yr$^{-1}$ to $10^{-1.5}$M$_\odot$yr$^{-1}$ provide the dominant source of ionizing photons throughout reionization. Our results are consistent with recent direct measurements of a $\sim5\%$ escape fraction from massive galaxies at the end of reionization and support the picture of low mass galaxies being the dominant sources of ionizing photons during reionization.

Motivation & Objective

• To constrain the evolution of the ionizing photon escape fraction from galaxies during the Epoch of Reionization.
• To determine how the escape fraction depends on galaxy properties such as halo mass, stellar mass, and star formation rate.
• To reconcile observed high-redshift galaxy populations with constraints on the reionization history using a semi-analytic model.
• To use a Bayesian framework to infer the most probable escape fraction values and their uncertainties under physical assumptions.

Proposed method

• The Meraxes semi-analytic galaxy formation model is used to simulate galaxy populations across redshifts $z \sim 4-8$, including star formation, feedback, and halo growth.
• The model couples galaxy evolution to a semi-numerical reionization calculation, tracking the ionization state of the intergalactic medium (IGM) over time.
• A flexible, halo mass-dependent parametrization is assumed for the escape fraction, with free parameters constrained via nested sampling in a Bayesian framework.
• The model is calibrated against three key observational constraints: the high-redshift UV luminosity function, the CMB optical depth from Planck, and the Lyα forest transmission.
• The simulation uses the Tiamat N-body dark matter halo catalog, with limited resolution but minimal impact on stellar mass budget ($<35\%$ loss at $z \lesssim 12$).
• Statistical inference is performed using the ultranest sampler to explore the full posterior distribution of escape fraction parameters.

Experimental results

Research questions

• RQ1 What is the mean ionizing photon escape fraction from high-redshift galaxies, and how does it vary with halo mass?
• RQ2 At what stellar mass does the escape fraction transition from high to low values, and is this transition redshift-dependent?
• RQ3 Which galaxy populations—by mass or star formation rate—dominate the ionizing photon budget during reionization?
• RQ4 How well do the joint constraints of the UV luminosity function, CMB optical depth, and Lyα forest reproduce the observed reionization history?
• RQ5 Is the inferred escape fraction consistent with recent direct measurements from massive galaxies at $z \sim 6$?

Key findings

• The escape fraction is constrained to $18\pm5\%$ for galaxies in haloes with mass $M_\text{halo} \lesssim 10^9\,M_\odot$, indicating efficient ionizing radiation escape in low-mass systems.
• For more massive haloes ($M_\text{halo} \gtrsim 10^9\,M_\odot$), the escape fraction is $5\pm2\%$, significantly lower than in low-mass systems.
• The transition between high and low escape fractions occurs at a stellar mass of $M_\star \sim 10^7\,M_\odot$, and this transition point is nearly independent of redshift.
• Reionization is dominated by low-mass galaxies ($M_\star \lesssim 10^7\,M_\odot$) with high escape fractions at $z \gtrsim 6$, while more massive galaxies dominate at $z \lesssim 6$.
• Galaxies with star formation rates between $10^{-2.5}$ and $10^{-1.5}\,M_\odot\,\text{yr}^{-1}$ provide the dominant source of ionizing photons throughout reionization.
• The results are consistent with recent direct measurements of $\sim5\%$ escape fraction in massive galaxies at the end of reionization, supporting the role of low-mass galaxies as primary ionizers.