Exploring the Cosmic Dawn through the 21 cm Forest and High-redshift Radio Sources with the SKA
Junsong Cang, Benedetta Ciardi, Barun Maity, Atsushi J. Nishizawa, Qi Niu, Yue Shao, Abinash Kumar Shaw, Hayato Shimabukuro, Tomáš Šoltinský, Tian-Yang Sun, Tsutomu T. Takeuchi, Yidong Xu, Kohji Yoshikawa, Bin Yue, Xin Zhang
astro-ph.CO
Submitted: 2026-06-23
Comments: Published in Advancing Astrophysics with the SKA II (AASKAII), 2026 (arXiv:2606.20366). Report-No:AASKAII/Cang01. Advancing Astrophysics with the SKA II (AASKAII) outlines the transformative scientific advances that will be enabled by the SKA telescopes
License: http://creativecommons.org/licenses/by-nc-sa/4.0/
The gist: The 21 cm forest, seen as absorption features in the spectra of distant radio sources, is produced by intervening neutral hydrogen and offers a direct probe of the neutral intergalactic medium during
Terminology
Abstract
The 21 cm forest, seen as absorption features in the spectra of distant radio sources, is produced by intervening neutral hydrogen and offers a direct probe of the neutral intergalactic medium during the epoch of reionization (EoR). Because it is sensitive to small-scale structure and gas temperature, it can constrain the thermal history of the early Universe and physics that affects structure formation. Detecting individual absorption lines is challenging, mainly because of their weakness and the scarcity of high-redshift radio-bright sources. Recent progress, however, has made 21 cm forest studies increasingly feasible: new statistical observables can improve sensitivity within realistic observing times, updated radio-source counts have revised expectations for suitable background quasars, and deep-learning methods can extract physical information more efficiently. In addition, new approaches have been developed to separate astrophysical effects from early galaxies from fundamental-physics effects on small-scale structure. With the Square Kilometre Array (SKA), the 21 cm forest will therefore provide a promising route to study early heating, possible exotic energy injection, dark matter properties, neutrino mass, the running spectral index, and baryon--dark-matter relative velocity. This chapter reviews recent developments in 21 cm forest research and discusses observational strategies and prospects for constraining the first galaxies and fundamental physics with SKA-Low.
Sources
- JWST ASPIRE: How Did Galaxies Complete Reionization? Evidence for Excess IGM Transmission around ${\rm [O\,{\scriptstyle III}]}$ Emitters during Reionization
- The radio properties of the JWST-discovered AGN
- Asking Fast Radio Bursts for More than Reionization History
- Probing small-scale dark matter clumping with the large-scale 21-cm power spectrum
- Wide-Field InfrarRed Survey Telescope-Astrophysics Focused Telescope Assets WFIRST-AFTA 2015 Report
- Formation, Evolution, and Revolution of Galaxies by SKA: Activities of SKA-Japan Galaxy Evolution Sub-SWG
- Dispersion Measures of Fast Radio Bursts through the Epoch of Reionization
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