[Paper Review] Reionization with star-forming galaxies: insights from the Low-z Lyman Continuum Survey

Maxime Trebitsch, Pratika Dayal+19 more|arXiv (Cornell University)|Dec 12, 2022

Galaxies: Formation, Evolution, Phenomena4 citations

TL;DR

This study uses the Low-z Lyman Continuum Survey (LzLCS) to calibrate a semi-analytical model (Delphi) that links UV slope to LyC escape fraction ($f_{\rm esc}$) in low-redshift star-forming galaxies. Applying this relation to high-redshift galaxies, it reproduces the observed reionization history and finds that galaxies with $-16 \lesssim M_{\rm UV} \lesssim -13.5$ dominate reionization, with population-averaged $f_{\rm esc} \approx 8\%, 10\%, 20\%$ at $z = 4.5, 6, 8$, respectively.

ABSTRACT

The fraction of ionizing photons escaping from galaxies, $f_{esc}$, is at the same time a crucial parameter in modelling reionization and a very poorly known quantity, especially at high redshift. Recent observations are starting to constrain the values of $f_{esc}$ in low-z star-forming galaxies, but the validity of this comparison remains to be verified. Applying at high-z the empirical relation between $f_{esc}$ and the UV slope trends derived from the Low-z Lyman Continuum Survey, we use the DELPHI semi-analytical galaxy formation model to estimate the global ionizing emissivity of high-z galaxies, which we use to compute the resulting reionization history. We find that both the global ionizing emissivity and reionization history match the observational constraints. Assuming that the low-z correlations hold during the epoch of reionization, we find that galaxies with $-16 \lesssim M_{UV} \lesssim -13.5$ are the main drivers of reionization. We derive a population-averaged $\langle f_{esc} angle \simeq 8\%, 10\%, 20\%$ at z=4.5, 6, 8.

Motivation & Objective

• To assess whether low-redshift LyC-emitting galaxies can serve as proxies for high-redshift sources during reionization.
• To determine if the empirical $f_{\rm esc}$-$\beta$ relation from the LzLCS can be extrapolated to high redshift to model global ionizing emissivity.
• To test whether such a model can reproduce the observed reionization history and constraints on the ionization fraction of the intergalactic medium.
• To identify the dominant galaxy population responsible for reionization based on UV magnitude and escape fraction.

Proposed method

• The Delphi semi-analytical galaxy formation model is used to simulate high-redshift galaxy populations.
• The $f_{\rm esc}$-$\beta$ relation derived from the LzLCS is applied to assign escape fractions to galaxies based on their UV slope ($\beta$).
• The model computes the global ionizing emissivity by summing over all galaxies, weighted by their UV luminosity and $f_{\rm esc}$.
• The reionization history is computed by solving the ionization equation using the derived emissivity, and compared to observational constraints.
• The luminosity function and halo mass function are used to weight contributions by galaxy magnitude and host halo mass.
• The model tracks the evolution of $f_{\rm esc}$ with redshift, accounting for photo-suppression in low-mass haloes as reionization proceeds.

Experimental results

Research questions

• RQ1 Can the $f_{\rm esc}$-$\beta$ relation measured in low-redshift LyC emitters be reliably extrapolated to the high-redshift epoch of reionization?
• RQ2 What is the contribution of different galaxy luminosity bins to the global ionizing emissivity during reionization?
• RQ3 Which galaxy population—based on UV magnitude—dominates the LyC output at $z \gtrsim 4$?
• RQ4 How does the population-averaged $f_{\rm esc}$ evolve with redshift in this model, and does it match observational constraints?
• RQ5 To what extent do low-mass, faint galaxies drive the early stages of reionization before being photo-suppressed?

Key findings

• The model reproduces the observed reionization history when the LzLCS $f_{\rm esc}$-$\beta$ relation is applied to high-redshift galaxies.
• Galaxies with $-16 \lesssim M_{\rm UV} \lesssim -13.5$ are the dominant drivers of reionization, while brighter galaxies ($M_{\rm UV} < -20$) are sub-dominant at all redshifts.
• The population-averaged $f_{\rm esc}$ decreases from $\approx 20\%$ at $z \gtrsim 8$ to $\approx 8\%$ at $z \approx 4.5$, consistent with high-resolution simulations and $z \gtrsim 3$ observations.
• Faintest galaxies in the lowest mass haloes dominate early reionization but become photo-suppressed by the growing UV background at $z \lesssim 9$, reducing their contribution.
• The model's predicted $f_{\rm esc}$ evolution qualitatively matches that of radiation hydrodynamics simulations, such as Rosdahl et al. (2018) and Trebitsch et al. (2021), despite differences in normalization.
• The results support the use of low-redshift LyC emitters as proxies for high-redshift sources, validating the LzLCS as a critical calibration tool for reionization modeling.