Baryonic Processes in the Intergalactic Medium
Summary
The intergalactic medium (IGM) constitutes the diffuse baryonic component that fills the space between galaxies and underpins the cosmic web. Following the epoch of reionisation, a substantial fraction of baryons resides in a warm–hot phase (the warm–hot intergalactic medium, WHIM) at temperatures of 105–107 K, heated primarily by shock fronts induced by gravitational collapse of large-scale structure. The interplay of processes such as accretion flows along filaments, galactic and active-galactic-nucleus (AGN) feedback, and radiative cooling shapes the density, temperature and chemical enrichment of the IGM. Observational probes span the electromagnetic spectrum, from absorption features in quasar and gamma-ray-burst spectra to thermal Sunyaev–Zel’dovich (tSZ) distortions of the cosmic microwave background and soft X-ray emission from filaments. Hydrodynamic simulations predict that feedback from stellar winds and AGN jets redistributes baryons between the IGM and the circumgalactic medium (CGM), while also influencing galaxy evolution. Precise accounting of baryons in these diffuse phases is essential to resolve the “missing baryon” problem at low redshift, to constrain cosmological parameters and to understand the thermal and chemical history of the universe.
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Baryonic Processes in the Intergalactic Medium publication trend
The graph below shows the total number of articles in baryonic processes in the intergalactic medium across all publications each year (not limited to Nature Index journals).
Technical terms
Intergalactic Medium (IGM): The diffuse gas filling the space between galaxies, primarily composed of ionised hydrogen and helium.
Warm–Hot Intergalactic Medium (WHIM): A phase of the IGM at temperatures 105–107 K, heated by shocks during structure formation.
Circumgalactic Medium (CGM): The gaseous halo surrounding individual galaxies, extending beyond the stellar disc but within the virial radius.
Thermal Sunyaev–Zel’dovich (tSZ) effect: Distortion of the cosmic microwave background spectrum caused by inverse-Compton scattering off hot electrons in diffuse gas.
Dispersion Measure (DM): The integrated electron column density along the line of sight to a transient radio source, expressed in pc cm−3, used to trace baryons in the IGM.
References
- Cross Correlation between the Thermal Sunyaev–Zeldovich Effect and the Integrated Sachs–Wolfe Effect. The Astrophysical Journal Supplement Series (2024).
- FLIMFLAM DR1: The First Constraints on the Cosmic Baryon Distribution from Eight Fast Radio Burst Sight Lines. The Astrophysical Journal (2024).
- Baryons in the Warm-Hot Intergalactic Medium. The Astrophysical Journal (2001).
- Probing the physical properties of the intergalactic medium using gamma-ray bursts. Monthly Notices of the Royal Astronomical Society (2021).
- First detection of stacked X-ray emission from cosmic web filaments. Astronomy & Astrophysics (2020).
- The cosmic baryon partition between the IGM and CGM in the SIMBA simulations. Monthly Notices of the Royal Astronomical Society (2024).
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