Ionizing Photon Escape Fraction Of, Models Reionization History Consistent with Observations at Redshift

Understanding how the universe transitioned from a neutral state to its current ionized form, a period known as reionization, remains a fundamental challenge in cosmology, and recent research addresses the crucial role of faint galaxies in this process. Zewei Wu, Andrey Kravtsov, and Harley Katz from The University of Chicago investigate how the escape of ionizing photons from these galaxies impacts our understanding of reionization history. Their models demonstrate that accurately accounting for the contribution of faint galaxies, particularly when considering how the escape of these photons varies with galaxy luminosity, is essential for aligning theoretical predictions with current observational constraints. This work highlights the sensitivity of reionization models to assumptions about photon escape, revealing it as a primary source of uncertainty and offering crucial insights into the conditions that governed the early universe.

This work presents model calculations of the reionization history of hydrogen, utilising star formation histories computed with a galaxy formation model that reproduces properties of local dwarf galaxies and UV luminosity functions of galaxies at z=5. The team employs ionizing photon density functions predicted by the model, alongside different models for the escape fraction of ionizing photons, fesc, to investigate the effects of ionizing photons originating from faint galaxies. By varying assumptions about fesc, researchers study the evolution of the hydrogen ionized fraction with redshift, QHII(z), and its sensitivity to these parameters.

Early Universe Reionization Simulation and Modelling

Scientists have developed a sophisticated model to simulate the reionization of the universe, a period when the first stars and galaxies transformed neutral hydrogen into an ionized state. The team combined detailed simulations of galaxy formation with analytical calculations to predict the abundance of ionizing photons and how it changed over time. This approach allows researchers to understand the ionization state of the intergalactic medium and its impact on the early universe. The simulations accurately reproduce the characteristics of galaxies observed today and at high redshifts, providing a robust framework for studying reionization.

A key element of the model is a modified mathematical function used to describe the distribution of galaxy luminosities and the number of ionizing photons they emit. This function allows for a more accurate fit to the simulation data, improving the reliability of the predictions. The team presented extensive data, including parameters derived from the simulations and analytical calculations, to demonstrate the model’s accuracy and consistency with observations. These parameters describe how the properties of galaxies evolve with redshift, providing insights into the processes driving reionization.

The research incorporates key concepts such as reionization, luminosity functions, ionizing flux, and redshift, providing a comprehensive understanding of the processes involved. The team carefully considered the impact of various factors, including the distance to galaxies and the density of gas, to ensure the model’s accuracy. This work represents a significant step forward in understanding the complex processes that governed the reionization of the universe.

Faint Galaxies Drive Hydrogen Reionization History

Scientists have achieved a detailed understanding of the reionization history of hydrogen, utilising a galaxy formation model that accurately reproduces the properties of local dwarf galaxies and the UV luminosity functions of galaxies at high redshift. The research team calculated the evolution of hydrogen ionization with redshift, investigating the impact of faint galaxies and varying assumptions about the escape fraction of ionizing photons. Results demonstrate that incorporating the contribution of faint galaxies with UV luminosities brighter than -13 yields a hydrogen reionization history consistent with current observational constraints, provided a constant ionizing photon escape fraction of 0. 1 is assumed.

The study reveals a strong sensitivity of reionization to the assumed escape fraction of ionizing photons, particularly for galaxies of differing luminosities. A model assigning high escape fractions to faint galaxies resulted in early reionization inconsistent with observations. However, a model where the escape fraction correlates with the recent maximum specific star formation rate, informed by the SPHINX galaxy formation simulation, produced a reionization history in excellent agreement with existing data. This successful model predicts a sizeable ionized hydrogen fraction of 0. 15 to 0.

2 at redshifts between 8 and 12. Measurements confirm that assumptions regarding the escape fraction represent the largest source of uncertainty in modeling hydrogen reionization, exceeding the impact of uncertainties in the relative contribution of galaxies with different luminosities or the clumping factor of intergalactic gas. The team extended their modeling to redshifts as high as 16, demonstrating the model’s ability to reproduce estimates of the UV luminosity function relevant for robust reionization studies. These calculations provide a crucial step forward in understanding the complex processes that governed the reionization of the universe.

Faint Galaxies Dominate Early Reionization History

This research presents detailed calculations of the reionization history of hydrogen, a crucial period when the universe transitioned from neutral to ionized gas. By employing a galaxy formation model that accurately reproduces observed properties of both local dwarf galaxies and galaxies at high redshifts, the team investigated the contribution of faint galaxies to this process. Results demonstrate that accounting for the ionizing radiation from these faint galaxies, with a reasonable assumption about the escape of ionizing photons, yields a reionization history consistent with current observational constraints. The study highlights the significant role of the ionizing photon escape fraction as the dominant source of uncertainty in modeling reionization, exceeding the impact of uncertainties in galaxy luminosity contributions or gas clumping.

Specifically, a model assigning higher escape fractions to faint galaxies resulted in early reionization inconsistent with observations, while a model correlating escape fraction with star formation rate provided a history aligning well with existing data, even predicting a sizeable ionized hydrogen fraction at higher redshifts. The team acknowledges that assumptions regarding the escape fraction remain the largest unknown factor influencing theoretical predictions. Future work will likely focus on refining these assumptions and further constraining the properties of faint galaxies to reduce uncertainties in understanding this pivotal epoch in cosmic history.