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arxiv: 2606.24665 · v1 · pith:YO2IPMXHnew · submitted 2026-06-23 · 🌌 astro-ph.CO

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
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Tom\'a\v{s} \v{S}oltinsk\'y Tian-Yang Sun Tsutomu T. Takeuchi Yidong Xu Kohji Yoshikawa Bin Yue Xin Zhang
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Pith reviewed 2026-06-25 23:07 UTC · model grok-4.3

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keywords 21 cm forestepoch of reionizationSquare Kilometre Arraydark matterneutrino massintergalactic mediumhigh-redshift radio sourcescosmic dawn
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The pith

The 21 cm forest observed with the Square Kilometre Array can constrain early heating, dark matter properties, neutrino mass and related physics through absorption in high-redshift radio sources.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

This review establishes that absorption features from neutral hydrogen in the spectra of distant radio sources, known as the 21 cm forest, can directly probe the neutral intergalactic medium during reionization. Recent updates to source counts, new statistical observables, and deep-learning extraction methods are presented as making individual line detection and statistical studies feasible within realistic SKA observing times. The central argument is that these observations separate astrophysical heating from fundamental physics effects on small scales, opening access to early thermal history and parameters such as dark matter particle properties, neutrino mass, spectral index running, and baryon-dark matter velocity offsets.

Core claim

With the Square Kilometre Array, 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.

What carries the argument

The 21 cm forest: absorption lines produced by intervening neutral hydrogen in the spectra of high-redshift radio sources, sensitive to gas temperature and small-scale density fluctuations.

If this is right

  • Individual absorption lines or their statistical distribution become measurable, tightening bounds on the thermal history of the intergalactic medium.
  • Astrophysical signals from the first galaxies can be separated from small-scale modifications due to dark matter or neutrinos.
  • Joint constraints on neutrino mass and the running of the primordial spectral index become available alongside reionization studies.
  • Baryon-dark-matter relative velocity effects can be tested directly through their imprint on the absorption pattern.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same SKA data could be cross-correlated with other 21 cm observables to break degeneracies between heating sources and fundamental physics.
  • If the method succeeds, it would supply an independent check on dark matter models already constrained by Lyman-alpha forest or Milky Way satellite counts.
  • Non-detection after full SKA operations would force revision of either the expected source population or the assumed level of small-scale power.

Load-bearing premise

That revised radio-source counts combined with statistical observables and deep-learning methods will deliver enough sensitivity to extract the weak absorption signal in practical SKA integration times.

What would settle it

A deep SKA survey that finds far fewer bright high-redshift radio sources than current counts predict, or that yields no detectable 21 cm absorption features after the expected integration time, would show the proposed constraints cannot be reached.

Figures

Figures reproduced from arXiv: 2606.24665 by Abinash Kumar Shaw, Atsushi J. Nishizawa, Barun Maity, Benedetta Ciardi, Bin Yue, Hayato Shimabukuro, Junsong Cang, Kohji Yoshikawa, Qi Niu, Tian-Yang Sun, Tom\'a\v{s} \v{S}oltinsk\'y, Tsutomu T. Takeuchi, Xin Zhang, Yidong Xu, Yue Shao.

Figure 1
Figure 1. Figure 1: Schematic view of 21 cm forest. Credit: SKAO, Sherwood-relics, International Gemini Observa￾tory, Tomáš Šoltinský. which thereby offers a global view of the IGM during cosmic dawn and the Epoch of Reionization, the 21 cm forest provides a high-resolution, line-of-sight probe of the intervening gas, revealing detailed information about small-scale structures and local physical conditions. Together, these ap… view at source ↗
Figure 2
Figure 2. Figure 2: The differential number density of synthetic 21 cm forest absorption features at 𝑧 = 9 (top row panels), 7.5 (middle row panels) and 6 (bottom row panels). Each panel shows cases in which the X-ray efficiencies, 𝑓X = 0.01 (blue curves), 𝑓X = 0.1 (fuchsia curves) and 𝑓X = 1 (orange curves) is assumed. From left to right various effects on the 21 cm forest are tested, namely strength of Ly𝛼 coupling of the 𝑇… view at source ↗
Figure 3
Figure 3. Figure 3: The expected 1D power spectrum of the 21 cm forest for different 𝑓X (left panel) and 𝑚WDM (right panel), with 𝑆150 = 10 mJy. This figure is taken from Shao et al. (2023). 1 kHz, and 100 observed neutral segments. Because heating and dark matter effects leave distinct imprints on the 21 cm forest 1D power spectrum, the two can be disentangled, allowing us to simultaneously probe the thermal history of the U… view at source ↗
Figure 4
Figure 4. Figure 4: (left) first-order WST coefficient for CDM model, CDM + 𝑓X = 0.1, 0.3 models and Warm Dark Matter(WDM) models with different WDM mass. (right) second-order WST coefficient for CDM model. This figure is taken from Shimabukuro et al. (2025). averaging, 𝑆 𝑗1 1 = D 𝛿𝑇𝑏 ∗ 𝜓𝑗1 (𝑥) [PITH_FULL_IMAGE:figures/full_fig_p012_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: A schematic description of the high-dimensional principal component analysis (PCA) of the 21 cm forest. Left panel: Structure of the HDLSS data and their analysis. Even if we have significant important features, because of the existence of a huge noise sphere, they are completely embedded in the observed data. The high-dimensional PCA can subtract the noise sphere and enables to extract the desired feature… view at source ↗
Figure 6
Figure 6. Figure 6: The posterior distributions of log10 𝑓X and ⟨𝑥HI⟩ from a Bayesian statistics analyses based on 1D power spectrum of 21 cm forest corresponding to 1 and 2𝜎 confidence regions. True values of the log10 𝑓X and ⟨𝑥HI⟩ are indicated by the crosses with corresponding colour to the contours. This figure was taken from Šoltinský et al. (2025) and modified. robust extraction of such features, which will be important… view at source ↗
Figure 7
Figure 7. Figure 7: The posterior distributions for 𝑚WDM and 𝑇𝐾 with the inference normalizing flow and Fisher matrix under different heating levels. The figure is taken from Sun et al. (2025). 5 High-redshift radio sources 5.1 General high-z sources 5.1.1 Fast Radio Bursts as an independent probe Fast Radio Bursts (FRBs) are highly energetic transients of cosmic origin that are widely distributed across the whole sky(e.g. Ca… view at source ↗
Figure 8
Figure 8. Figure 8: Predicted redshift distribution of radio-loud quasars observable with one year of SKA-Low observations. The “21 cm Forest Survey” refers to high-spectral-resolution observations suitable for 21 cm forest analysis. The figure is taken from Niu et al. (2025). numerous point sources detected in radio observations (e.g., Liu et al., 2021; Yang et al., 2023; Euclid Collaboration et al., 2024) . Therefore, futur… view at source ↗
read the original 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.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

0 major / 2 minor

Summary. This manuscript is a review chapter summarizing the 21 cm forest—absorption features in high-redshift radio source spectra produced by neutral intergalactic hydrogen—as a probe of the epoch of reionization. It covers detection challenges arising from weak lines and scarce bright sources, recent progress in statistical observables, revised radio-source counts, deep-learning extraction methods, and techniques to separate astrophysical effects from fundamental physics influences on small-scale structure. The central claim is that SKA-Low observations will make the 21 cm forest a promising route to constrain early heating, exotic energy injection, dark matter properties, neutrino mass, the running spectral index, and baryon–dark-matter relative velocity, while reviewing observational strategies.

Significance. As a review synthesizing cited literature on statistical methods and SKA prospects, the chapter can provide a useful overview for the community if the summaries of external works are accurate. It explicitly credits advances in source counts and deep-learning approaches as enabling factors, which strengthens its utility for guiding future observations of cosmic dawn physics.

minor comments (2)
  1. [Abstract] Abstract: the statement that 'new statistical observables can improve sensitivity within realistic observing times' would be strengthened by citing at least one concrete example of an observable and the associated integration time or sensitivity gain from the referenced literature.
  2. [Conclusion] The manuscript would benefit from a short table or bullet list in the concluding section that maps each listed science target (early heating, dark matter properties, etc.) to the specific statistical observable or separation method discussed earlier.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for their positive assessment of the manuscript and for recommending acceptance. We are pleased that the review is viewed as a useful synthesis of recent developments in 21 cm forest studies and SKA prospects.

Circularity Check

0 steps flagged

No significant circularity: review summarizes external literature without internal derivations

full rationale

The manuscript is a review chapter summarizing cited literature on statistical observables, source counts, and deep-learning techniques. It presents no new quantitative predictions, equations, or derivations that could reduce to self-referential fits or self-citations. The central claim that SKA observations will provide a promising route inherits support entirely from referenced external works. No load-bearing steps match any of the enumerated circularity patterns.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

As a review, the text rests on the body of cited 21 cm cosmology and radio astronomy literature rather than introducing new derivations, free parameters, or entities.

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