Galaxies and the Intergalactic Medium Reveal Reionization’s Inside-Out Progression.

Analyses of 948 oxygen-emitting galaxies reveal evolving relationships between galaxy density and intergalactic medium transmission during reionization. Suppression occurs in overdense regions at high redshift, enhancing at lower redshifts. Signals suggest reionization progressed outwards from clustered galaxies, with background radiation becoming dominant.

Understanding the period following the ‘dark ages’ of the universe – when the first stars and galaxies began to illuminate the cosmos – remains a central challenge in cosmology. Recent research focuses on the interplay between early galaxies and the intergalactic medium (IGM), the diffuse gas filling the space between them, during the epoch of reionization. A detailed analysis of this relationship, utilising a substantial dataset known as EIGER, has revealed how the connection between galaxies and the IGM evolved in the final stages of reionization. This work, detailed in a new publication by Daichi Kashino (National Astronomical Observatory of Japan), Simon J. Lilly (ETH Zürich), Jorryt Matthee (Institute of Science and Technology Austria), Ruari Mackenzie (EPFL), and Anna-Christina Eilers, Rongmon Bordoloi, Robert A. Simcoe, Rohan P. Naidu, Minghao Yue, and Bin Liu (all from Massachusetts Institute of Technology), presents compelling evidence for an ‘inside-out’ progression of reionization, where ionized bubbles initially formed around dense concentrations of galaxies before expanding into the voids. The study, titled ‘EIGER VII. The evolving relationship between galaxies and the intergalactic medium in the final stages of reionization’, correlates the distribution of galaxies with variations in the transmission of light from distant quasars, providing insights into the evolving ionization state of the IGM.

Galaxy-IGM Connection During Reionization: A Multi-Sightline Analysis

This study details the relationship between galaxies and the intergalactic medium (IGM) during the epoch of reionization – the period when the universe transitioned from being predominantly neutral hydrogen to ionized hydrogen. Researchers analysed data from the EIGER survey, utilising 948 oxygen [\OIII]-emitting galaxies and mapping their distribution along six quasar sightlines. This methodology allows for detailed reconstruction of neutral hydrogen distribution, revealing the complex interplay between galaxies and the surrounding cosmic gas. The investigation centres on correlating galaxy density with the transmission of Lyman-alpha (\Lya) and Lyman-beta (\Lyb) light, a sensitive measure of the IGM’s ionization state.

Lyman-alpha and Lyman-beta photons are readily absorbed by neutral hydrogen. Therefore, the amount of transmitted light indicates the density of neutral hydrogen along the line of sight. Transmission curves reveal a redshift-dependent relationship, indicating a changing influence of galaxy density on IGM ionization. Specifically, the study demonstrates that galaxy density suppresses transmission in overdense regions at higher redshifts, but enhances it at lower redshifts. This suggests a dynamic interplay between galaxy distribution and IGM ionization.

Cross-correlation measurements demonstrate excess absorption within 3 comoving megaparsecs (cMpc) of galaxies at higher redshifts, while enhanced transmission extends to 5–20 cMpc. This provides a detailed picture of the spatial relationship between galaxies and ionized regions. Statistical tests confirm these correlations are not attributable to chance, strengthening the evidence for a genuine connection between galaxies and the IGM.

These findings indicate that, at earlier times, local radiation fields from galaxies dominate the ionization of surrounding gas, while the background radiation field increasingly influences the IGM as reionization progresses. The observed patterns strongly support a model of ‘inside-out’ reionization, where ionized regions originate around clustered, star-forming galaxies and gradually expand into underdense areas.

Analysing multiple quasar sightlines is crucial to mitigate the effects of cosmic variance – the inherent fluctuations in the distribution of matter in the universe. Cosmic variance can skew results derived from single sightlines. By combining data from multiple lines of sight, researchers obtain a more representative picture of the galaxy-IGM connection during this crucial epoch, allowing for more robust conclusions.

This research provides valuable insights into the early universe and the processes that shaped the cosmic structures we observe today. Future studies will continue to refine our understanding of the galaxy-IGM connection and the evolution of the universe.