Reionization and its sources

TL;DR

This paper reviews the epoch of reionization, detailing its physical processes, sources like stars and black holes, and observational methods such as CMB and 21cm experiments to understand this key phase in cosmic history.

Contribution

It provides a comprehensive overview of reionization, integrating physical concepts, sources, stages, and observational techniques, highlighting recent advances and challenges.

Findings

Reionization involved ionized bubble formation and overlap.

Multiple observational probes are used to study reionization.

Understanding reionization informs cosmic structure formation.

Abstract

Reionization represents an important phase in the history of our Universe when ultraviolet radiation from the first luminous sources, primarily stars and accreting black holes, ionized the neutral hydrogen atoms in the intergalactic medium (IGM). This process follows the ``Dark Ages'', a period with no luminous sources, and is initiated by the formation of the first sources, marking the ``Cosmic Dawn''. Reionization proceeds through multiple stages: initially, ionized bubbles form around galaxies, then expand and overlap across the IGM, culminating in a fully ionized state, with neutral hydrogen remaining only in dense regions. Understanding reionization involves a diverse range of physical concepts, from large-scale structure formation and star formation to radiation propagation through the IGM. Observationally, reionization can be explored using the cosmic microwave background (CMB), Lyman-$\alpha$ absorption, high-redshift galaxy surveys, and emerging 21~cm experiments, which together offer invaluable insights into this transformative epoch.