The Physics of the Intergalactic Medium

TL;DR

This paper reviews the current understanding of the intergalactic medium (IGM), highlighting its physical properties, role in cosmic structure formation, and its connection to galaxy evolution, based on observations and simulations.

Contribution

It provides a comprehensive synthesis of observational data and numerical simulations to elucidate the structure, composition, and significance of the IGM in cosmology.

Findings

IGM contains a residual neutral fraction detectable in QSO spectra.

Simulations reproduce the fluctuation statistics of the IGM.

The IGM is the main baryon reservoir and was enriched early by stars.

Abstract

Intergalactic space is filled with a pervasive medium of ionized gas, the Intergalactic Medium (IGM). A residual neutral fraction is detected in the spectra of Quasi-Stellar Objects at both low and high redshifts, revealing a highly fluctuating medium with temperatures characteristic of photoionized gas. The statistics of the fluctuations are well-reproduced by numerical gravity-hydrodynamics simulations within the context of standard cosmological structure formation scenarios. As such, the study of the IGM offers an opportunity to probe the nature of the primordial density fluctuations on scales unavailable to other methods. The simulations also suggest the IGM is the dominant reservoir of baryons produced by the Big Bang, and so the principal source of the matter from which galaxies formed. The detection of metal systems within the IGM shows that it was enriched by evolved stars early in its history, demonstrating an intimate connection between galaxy formation and the IGM. The author presents a comprehensive review of the current understanding of the structure and physical properties of the IGM and its relation to galaxies, concluding with comments on prospects for furthering the study of the IGM using future ground-based facilities and space-based experiments.