Neutral Hydrogen Dynamics in Galaxy Formation
Summary
Neutral hydrogen (H i) underpins our understanding of how galaxies assemble and evolve. As the dominant component of the cool interstellar medium, H i serves as the primary reservoir of baryonic fuel for star formation. Observations of its 21 cm hyperfine transition enable the mapping of gas distributions across cosmic time, from local discs to the most distant systems detectable with modern radio facilities. The spatial and kinematic structure of H i reveals the influence of processes such as cold accretion, galactic inflows and outflows, environmental stripping and tidal interactions. These dynamics regulate the balance between atomic and molecular phases, setting the pace of star formation and the morphological transformation of galaxies. Numerical simulations, calibrated against H i surveys, capture the interplay between feedback, gas cooling and dark-matter potentials, predicting how H i is accreted from the intergalactic medium, self-shields to form denser phases and is redistributed by mergers or close encounters. Integrating observations and theory offers a panoramic view of cosmic gas cycles, illuminating the mechanisms that drive galaxy growth, quench star formation and shape large-scale structure.
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Neutral Hydrogen Dynamics in Galaxy Formation publication trend
The graph below shows the total number of articles in neutral hydrogen dynamics in galaxy formation across all publications each year (not limited to Nature Index journals).
Technical terms
Neutral hydrogen (H i): Atomic hydrogen in its ground state, observable via the 21 cm hyperfine transition.
21 cm line: The radio emission arising from the spin-flip transition of neutral hydrogen, used to trace cold gas.
Intensity mapping: A technique that measures integrated 21 cm emission over large volumes, correlating fluctuations with galaxy tracers.
Redshift-space distortions: Apparent anisotropies in clustering due to line-of-sight velocity components of galaxies or gas.
Molecular-to-atomic gas ratio (Rₘₒₗ): The mass ratio of molecular hydrogen (H₂) to atomic hydrogen (H i), indicative of the balance between cold gas phases.
References
- The Most Distant H i Galaxies Discovered by the 500 m Dish FAST. The Astrophysical Journal Letters (2024).
- Detection of Cosmological 21 cm Emission with the Canadian Hydrogen Intensity Mapping Experiment. The Astrophysical Journal (2023).
- xGASS: total cold gas scaling relations and molecular-to-atomic gas ratios of galaxies in the local Universe. Monthly Notices of the Royal Astronomical Society (2018).
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